1. Optimal Vitamin D Levels in Health and Disease: Current Understanding Based on IOM Guidelines

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1 1. Optimal Vitamin D Levels in Health and Disease: Current Understanding Based on IOM Guidelines Sue Shapses, PhD Clifford J. Rosen, MD Case 1-1 A 78-year-old white female presents with a chief complaint of low back pain. Her history is that she developed sudden lower back discomfort upon awakening approximately two weeks ago. The pain is localized to the lower end of the spine and does not radiate into the legs. It is worsened by movement, sneezing, and coughing and generally is described as knife-like. In the last two weeks the pain has been steady but not progressively worse. She is otherwise relatively healthy although she has noted some weakness in her legs when walking up stairs or squatting. There is no history of weight loss, fevers, fatigue, or malaise. Her only medication is hydrochlorthiazide for mild hypertension. The patient s sister has recently had a fracture of the wrist at age 72 and her mother suffered a hip fracture at age 88. The patient does not believe in supplements and does not routinely engage in exercise. The patient took estrogen for 5 years after menopause, at age 52, but no medications after that. Since her husband died three years ago she has stayed home virtually all the time and eats twice per day but only small meals. She also reports lactose intolerance. There is no history of renal stones. Her exam is remarkable for kyphosis of the thoracic spine and some mild proximal muscle weakness in the lower limbs only. Her body mass index (BMI) is 21 kg/m2 and she reports that her height, 4 ft 11 inches, is 2 inches less than what she thought. Laboratory studies revealed an erythrocyte sedimentation rate (ESR) of 10, a normal chemistry profile except for a slightly increased alkaline phosphatase, and normal complete blood count. Thyroid Acknowledgment: The authors thank D. Sukumar for assistance in reviewing and editing this manuscript. Translational Endocrinology & Metabolism, Volume 2, Number 3,

2 stimulating hormone (TSH) was Serum 25-hydroxyvitamin D [25(OH)D] was 15 ng/ml, intact parathyroid hormone (PTH) was 70 pg/ml, urinary calcium was 100 mg/24 hours, creatinine clearance was 45 cc/min (estimated). Serum and urinary protein electrophoresis were normal. Plain films of the spine revealed a compression fracture of uncertain age at L4 and two old compression fractures (20% compressed) of T7 and T8. The vertebrae appeared markedly osteopenic on visual inspection. Case Summary This older postmenopausal woman presented with very localized back pain and clinical findings of height loss, kyphosis, and radiographic evidence of previous vertebral fractures. The most likely diagnosis is osteoporosis, with a new compression fracture of L4. No bone mineral density (BMD) assessment was performed, but her radiographs, clinical history, and genetic background support the diagnosis of very low bone mass with enhanced skeletal fragility. Compression fractures that are not related to a clinical history of trauma are common in older postmenopausal women with low bone mass. Although compression fractures were once considered merely morphometric vertebral fractures with no clinical sequelae, several cohort studies have demonstrated that radiographic findings like this patient s are associated with higher rates of mortality and morbidity than are found in individuals with the same bone mass but no fractures (1). Since previous fracture is a very strong risk factor for subsequent fracture, individuals like this patient should be aggressively treated with an anti-osteoporosis drug, whether BMD measurement is obtained or not. However, before initiating osteoporosis therapy, a closer look for secondary causes of low bone mass and kyphosis is warranted, particularly because anti-osteoporosis drugs can be associated with significant but rare adverse events. The workup for secondary osteoporosis in this woman was unrevealing except for her low serum 25(OH)D and a modestly increased alkaline phosphatase. In patients presenting with osteoporosis (i.e., T-score < 2.5 or fractures), a secondary cause may be found. In this case, abnormal thyroid and parathyroid function were ruled out, and there was no evidence to suggest indolent multiple myeloma (i.e., normal ESR and protein electrophoresis) or an underlying malignancy. The slightly increased alkaline phosphatase could be secondary to fracture healing, high bone turnover from secondary 14 Translational Endocrinology & Metabolism: Vitamin D Update

3 hyperparathyroidism, an occult gastrointestinal disorder, such as non-tropical sprue (celiac disease or gluten-induced enteropathy), or liver disease. In this case, the patient s low vitamin D level might be contributing to the pathogenesis of her osteoporotic fractures. Thus it is imperative to consider a systematic approach to the evaluation and treatment of low serum 25(OH)D. There are several questions that a provider should ask when confronted with a low vitamin D level in an individual who presents with low bone mass. First, what constitutes vitamin D deficiency? Second what is the etiology of the low 25(OH)D level? Third, and most importantly, how does one treat a patient with low vitamin D levels who also has evidence of osteoporosis, and how do these patients differ from individuals with normal skeletal health? In this case of an elderly woman with a new osteoporotic fracture, her serum level of 25(OH)D was 15 ng/ml and her PTH levels were modestly increased, as was her alkaline phosphatase level. Serum 25(OH)D is an integrated measure of vitamin D sources, which are the sum of solar exposure and dietary intake. Few would argue that the patient should be considered vitamin D deficient with this level of 25(OH)D. Moreover, her increased PTH can be attributed to a decrease in optimal calcium absorption, in part because of the low serum 25(OH)D (2). However, many factors contribute to changes in PTH levels, including renal function, calcium intake, and body composition. The inverse relationship between PTH and 25(OH)D levels that occurs particularly between 20 and 30 ng/ml is not a simple threshold effect. In fact, Sai et al. recently reviewed 72 studies that examined the relationship of PTH to 25(OH)D and found there was no discrete threshold for low 25(OH)D and increased PTH across a wide range of serum vitamin D levels (3). Notwithstanding, this patient has mild secondary hyperparathyroidism, which can at least partially be attributed to her low serum 25(OH)D leading to impaired calcium absorption. She also has reduced renal function (glomerular filtration rate (GFR) = 45 cc/ min) which impairs 1-alpha-hydroxylation of vitamin D in the kidney (2, 3). Since 1,25-dihydroxyvitamin D [1,25(OH) 2 D] is the active hormone that promotes calcium absorption in the gut, the combination of reduced exposure (low serum 25(OH)D) and impaired activation results in a compensatory increase in PTH. Secondary hyperparathyroidism stimulates bone remodeling and in older individuals leads to uncoupling of bone turnover, so that bone resorption is markedly enhanced in the presence of either normal or reduced Optimal Vitamin D Levels in Health and Disease 15

4 bone formation. Clearly, these changes would be detrimental to this woman, who already has suffered an osteoporotic fracture and likely has lost significant bone mass. The modest increase in alkaline phosphatase (20 30% above the high normal) can be related to the higher bone turnover from increased PTH or in some cases it can be related to enhanced osteoblastic activity following fracture healing. What is the etiology of the patient s low serum 25(OH)D level and are there other deleterious effects of this deficiency state? The patient did have proximal muscle weakness, and this could be attributed to the low vitamin D levels. Indeed, if bone biopsy were undertaken, it might show signs of undermineralized osteoid, the hallmark of osteomalacia. Muscle weakness can lead to falls, and trauma is a major cause of osteoporotic fractures. The U.S. Preventive Services Task Force (USPSTF) recently issued a report on falls and noted that vitamin D replacement was significantly effective in reducing the risk of fall-related injuries (4). More debatable is whether there is a threshold level of 25(OH)D for fall prevention. Bischoff-Ferrari and others have suggested that serum 25(OH)D levels have to be at least 30 ng/ml to show a significant effect on fall rates and fractures (5). However the USPSTF was unable to find a threshold level of 25(OH)D that could prevent falls (4). The patient s low levels of 25(OH)D should be thoroughly investigated for an etiologic basis. The most frequent secondary cause of low serum 25(OH)D independent of diet, skin color, and solar exposure, is gastrointestinal malabsorption. This can be secondary to disorders like biliary cirrhosis, inflammatory bowel disease, gastric bypass, intestinal overgrowth, scleroderma, and gluten enteropathy. These diseases should always be considered when evaluating a patient with low vitamin D, particularly gluten enteropathy, since it is frequently associated with low bone mass and osteoporotic fractures. Gluten enteropathy is diagnosed by positive tissue transglutaminase antibodies and a small bowel biopsy revealing flattened intestinal villi. Drugs like phenytoin, barbiturates, and glucocorticoids can also cause low vitamin D levels by enhancing the catabolism of 25(OH)D. Enhanced skin pigmentation blunts UV-induced production of previtamin D, resulting in low serum 25(OH)D (particularly among African Americans). Obesity is often associated with levels of 25(OH)D in the range of ng/ml, 16 Translational Endocrinology & Metabolism: Vitamin D Update

5 as noted for this woman. The mechanism(s) responsible for obesity s effect is not clear, although it is well established that vitamin D is stored in fat cells, and there is a direct but inverse relationship between BMI and serum levels of 25(OH)D levels (6 8) and 1,25(OH) 2 D (9, 10). Nevertheless, in this patient s case, excess adiposity is not present and does not explain her low 25(OH)D. On the other hand, it is possible that her relatively low BMI (21 kg/m 2 ), as well as the absence of regular exercise, have contributed to her risk of fragility fractures (11). Finally, management of this patient requires consideration of both her low bone mass and her low serum 25(OH)D. First-line therapy for osteoporosis is the bisphosphonates, particularly in a woman with multiple fractures. However, low levels of 25(OH) D could predispose the patient to bisphosphonate-induced hypocalcemia, a rare but important side effect (12). Hence, initial supplementation with vitamin D is necessary, not only to protect against hypocalcemia during bisphosphonate treatment but also to treat osteomalacia. Oral vitamin D is nearly 100% absorbed from the gut and unless the patient has malabsorption, this would be the initial treatment of choice, prior to initiation of bisphosphonate therapy. In general, for patients with 25(OH)D levels below 20 ng/ml, every 100 IU of vitamin D administered increases serum 25(OH)D 1 ng/ml. However, many clinicians advocate more rapid replacement, with doses as high as 50,000 units once per week. Re-evaluation of the serum 25(OH)D would be appropriate after 12 weeks (13). The choice of vitamin D 2 versus vitamin D 3 remains controversial, although there is some evidence that D 2 has a somewhat shorter half-life than D 3. For the high-dose (50,000 units) vitamin D preparations, ergocalciferol (D2) is readily available and relatively inexpensive. Bisphosphonate therapy should be initiated once the 25(OH)D level reaches 20 ng/ml or higher. The proximal muscle weakness will improve with time, but patience is needed because improved muscle function often takes 8 12 months. In respect to muscle function and vitamin D, there continues to be debate about the direct vs. indirect effects of vitamin D. Recently, DeLuca and colleagues were unable to demonstrate the presence of vitamin D receptors (VDR) in adult human muscle tissue, implying that earlier reports of VDR in muscle were false positives related to the quality of the antibody (14). Therefore, the mechanism for muscle weakness in vitamin D deficiency is still not clear. Optimal Vitamin D Levels in Health and Disease 17

6 Vitamin D Background Vitamin D is a fat-soluble sterol and has two major forms, vitamin D 2 (ergocalciferol) and vitamin D 3 (cholecalciferol), which differ in their side-chain structure but which elicit similar responses in the body. Vitamin D (D 2 and D 3 ) undergoes enzymatic hydroxylation that converts it to 25(OH)D (calcidiol, half-life ~15 days) in the liver and then 1-alphahydroxylase (cytochrome P450 27B1, CYP27B1) converts it to its biologically active form, 1,25(OH) 2 D (calcitriol, half-life ~10-20 hours) in the kidney. 25(OH)D, the major circulating form of vitamin D, is used clinically as a marker of vitamin D status. It is bound to its carrier protein, vitamin D binding protein (DBP), which also transports vitamin D and calcitriol. The metabolism of vitamin D is under tight regulation by PTH and fibroblast growth factor-23 (FGF-23). Low serum phosphorus levels increase the synthesis of calcitriol. Calcitriol s actions are mediated though binding to VDR present in tissues throughout the body (and it is possible that other ligands also activate VDR). Calcitriol is therefore hypothesized to function at the cellular level in a variety of organs, physiological functions, and disease states, including skeletal health, physical performance, cardiovascular health and disease, hypertension, autoimmune disorders (type 1 diabetes), infectious diseases, cancer, pregnancy, and neuropsychological functioning (15). Blood 25(OH)D Levels and Change in Normal Range of Vitamin D Considerable controversy surrounds the definition of vitamin deficiency, despite the recent Institute of Medicine (IOM) report (15). Prior to 2000, most clinicians considered levels of serum 25(OH)D less than 20 ng/ml to be low. However, over the last decade, investigators, clinicians, and laboratories have introduced the concept of vitamin D insufficiency and have declared that patients in the range of ng/ml were insufficient in vitamin D. Subsequently, reference ranges were changed so that normal levels of 25(OH)D were defined as lying between 30 and 76 ng/ml. This resulted in declarations that 50 80% of the North American population was vitamin D insufficient or deficient (16). In support of that tenet, some observational and controlled trials pointed to an optimal vitamin D level (optimal to prevent falls, fractures, heart disease, and cancer) of at least 30 ng/ml (16, 17). However, as discussed below, the 30 ng/ml cutoff value has come under increased scrutiny and is currently being debated, in part because the IOM report suggested that 20 ng/ml was a sufficient level of 18 Translational Endocrinology & Metabolism: Vitamin D Update

7 TABLE 1-1. Definitions of serum 25(OH)D (15) At risk of vitamin D deficiency: <12 ng/ml (30 nmol/l) At risk of vitamin D inadequacy: ng/ml (30 49 nmol/l) Sufficient in vitamin D: ng/ml ( nmol/l) Possibly harmful vitamin D: >50 ng/ml (>125 nmol/l) serum 25(OH)D for skeletal health in most healthy individuals (15). One rationale for the IOM argument stems from data suggesting that there is no absolute threshold for serum 25(OH)D that causes an increase in PTH levels. At a serum level of 20 ng/ml, nearly all of the population would have levels consistent with bone health, whereas at 16 ng/ml, 50% would be assured bone health. Using the definitions in Table 1-1, the patient in the case study would be considered at risk of vitamin D deficiency. In the United States, 8% of the population is at risk of deficiency, having 25(OH)D levels of <12 ng/ml, 24% are at risk of vitamin D inadequacy (12 19 ng/ml), 67% are in the sufficient range (20 50 ng/ ml), and about 1% are in the harmful range (>50 ng/ml)(18). The discrepancy between the very low vitamin D intake (below the RDA) in the U. S. population and the serum levels found to be generally in the sufficient range is likely due to inaccuracies in reporting vitamin D food fortification, estimating supplement use, and sun exposure (19) (for discus sion of the discrepancy, see below). Change in the RDA Recommendations in the 2011 Report Before the 2011 IOM report, the last set of recommendations for calcium and vitamin D was in The updated set of recommendations arose because of new evidence from higher-quality studies and a re-evaluation of the literature (Table 1-2). The units used in this chapterare International Units (IU) (micrograms times 40 = IU). Dietary reference intakes (DRI) are determined using several reference values (20), including the estimated average requirement (EAR), which is expected to satisfy the needs of 50% of the people in that age group. The EAR for vitamin D (400 IU/day) is the same for all individuals, including children and adults. The recommended dietary allowance (RDA) is the daily dietary intake level of a nutrient considered sufficient to meet the requirements of nearly all (97.5%) healthy individuals in each life-stage and gender group. It is not a target to be met by all individuals, and intakes below the RDA cannot be assumed to Optimal Vitamin D Levels in Health and Disease 19

8 TABLE 1-2. Vitamin D: Recommended Intakes 1 4 Life Stage RDA 1 UL 2 Birth to 6 months 400 IU IU 6 to 12 months 400 IU IU Children 1 3 years 600 IU 2500 IU Children 4 8 years 600 IU 3000 IU Children 1 13 years 600 IU 4000 IU Teens years 600 IU 4000 IU Adults years 600 IU 4000 IU Adults 71 years and older 800 IU 4000 IU Pregnant & breastfeeding teens/women 600 IU 4000 IU 1. Recommended Dietary Allowance (RDA) intake that meets the needs of 97.5% of the North American population. 2. Tolerable Upper Intake Level (UL) above which there is risk of adverse events. The UL is not intended as a target intake due to no consistent evidence of greater benefit at intake levels above the RDA. 3. Adequate Intake (AI) reference value; no RDAs established for infants 0 to 12 months 4. For all ages and life stages, 16 ng/ml is the serum 25-hydroxyvitamin D (25OHD) level corresponding to the EAR and covering the needs of 50% of the North American population, and 20 ng/ml is the 25OHD level corresponding to the RDA set for 97.5% of the population. Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: The National Academies Press; 2011 (15) be inadequate, because the RDA by definition exceeds the actual requirements of all but 2% to 3% of the population. The RDA for vitamin D is 600 IU/day for all ages except individuals older than 70 years of age, for whom the RDA is 800 IU/day. Vitamin D intake of IU/day is consistent with maintaining serum levels of 25(OH)D >20 ng/ml during the winter months (15). Adequate intake (AI) is the amount recommended where no RDA has been established for a specific demographic group; AI is typically reserved for the inevitable uncertainties associated with specifying requirements for infants, but AI was used in the 1997 IOM guidelines for calcium recommendations for adults. The AI for vitamin D is 400 IU/day for infants. The tolerable upper intake level (UL) reflects the highest level of daily consumption where potential risk of adverse effects may increase. For vitamin D, intakes up to 10,000 IU/day have not been associated with hypercalcemia and therefore this value 20 Translational Endocrinology & Metabolism: Vitamin D Update

9 was used in the determination of the UL; however, additional observational data contributed to the decision to include a safety factor. The UL for vitamin D at all life stages is 4,000 IU/day. Importantly, ULs have often been misused as levels to be allowed in controlled clinical trials. ULs are not defined for this purpose, and higher levels in controlled research that includes monitoring and other safety precautions are both acceptable and necessary to advance the field. Study Design Considerations in Determining the Evidence for Vitamin D Intake Large ecological studies are often used as proof that treatment should be modified when there are no randomized controlled trials, especially when the studies are published in reputable journals. The studies can only be used to determine the direction of more carefully designed studies to build on the initial observations. Study designs to provide unbiased evidence, in decreasing order of reliability, are: randomized controlled trials, prospective cohort, retrospective cohort, case-control, cross-sectional, and lastly ecological studies. Ideally, studies will havea dose-response design, but the high cost and effort required for dose-response studies have limited their use. High-quality studies are important in light of previous observational studies purporting health benefits of other micronutrients (e.g., beta-carotene, vitamins C, E, folic acid, and selenium) that were not proven true in well-designed clinical trials, and some micronutrients were shown to confer higher disease risk (21, 22). In addition, the assumed benefits of hormone replacement therapy led to a change in medical practice for years prior to the findings in the Women s Health Initiative trials that showed both positive and negative effects on health outcomes (23). Therefore, in determining the vitamin D recommendations, the use of well-designed studies was considered important for assessing the longterm effects of higher vitamin D intake and higher serum 25(OH)D levels. Usual Vitamin D Intakes and Serum 25(OH)D Concentrations Vitamin D intake from food alone and from foods plus supplements for the various adult age groups in the U.S. population is shown in Figure 1-1 (see also chapter on children for intakes in these age groups). Median vitamin D intake is below the RDA for both males (272 to 396 IU/ day, depending upon life stage) and females (160 to 260 IU/day). All groups increased total intake when supplements were considered, and Optimal Vitamin D Levels in Health and Disease 21

10 the increase was most marked among older women, as is the case for calcium and many other supplemental nutrients. What does this mean with respect to food vs. supplemental intake? Most individuals do not get adequate dietary intake of vitamin D, but this does not equate to deficient serum levels in the majority of individuals, as discussed below. National Estimates of Dietary Intake of Vitamin D and Serum Levels Dietary intake levels of calcium and vitamin D are available from the survey report series What We Eat In America. In addition, the National Cancer Institute (NCI) incorporated estimates of intake from supplements, thereby providing an estimate of total calcium and vitamin D intake. Information about dietary intake is taken from the National Health and Nutrition Examination Survey (NHANES; cdc.gov/nchs/nhanes.htm), which was initiated in the 1960s by the Centers for Disease Control and Prevention. NHANES is unique in that it collects and tracks both dietary intake and health measures, including serum 25(OH)D concentrations, to monitor health in a national sample of Americans. The dietary calcium and vitamin D intake data (24) and supplement intake (NCI) is reported in the 2011 IOM report (15) (Figure 1-1). The diet survey relies on the gold standard for dietary measures using two or more 24-hour dietary recalls for each person (15). The dietary intake estimates are limited by survey respondents abilities to accurately report foods consumed and the timeliness of the food composition databases linked to foods in the survey, which may lead to underreporting of intake (IOM, 2000). Median vitamin D intake from foods for all life-stage groups is below the EAR of 400 IU/day (Figure 1-1), yet these data should be considered in light of the corresponding serum 25(OH)D concentrations because intake is often underreported. The mean serum 25(OH)D concentrations in adults in the United States range from 23 to 25 ng/ml (Table 1-3) and from 25 to 31 ng/ml in Canada, which may reflect differences in the assay methods used or in lifestyle (15). Many persons obtain some vitamin D from sun exposure, and as a result, they are at lower risk for inadequacy even when their intakes are below the reference value of IU/day. This would explain why most individuals in the population who are well below the recommended intake of vitamin D (Figure 1-1) do not have low 25(OH)D levels (Table 1-3). The estimated 25(OH)D level of 20 ng/ml in response to dietary intake of IU/day is based on randomized controlled trials conducted at 22 Translational Endocrinology & Metabolism: Vitamin D Update

11 IU/day years years years >70 years Men Diet Men Diet + Supplement Women Women Diet + Supplement RDA EAR FIG 1-1. Usual vitamin D intake from diet and diet plus supplements in men and women by age group. Solid line represents recommended dietary allowance (RDA) and dashed line represents estimated average requirement (EAR) for a given life stage. TABLE 1 3. Mean Serum 25OHD Concentrations for the U.S. by Life Stage Groups a Males ng/ml Females ng/ml 1 3 y 28.4 ± y 28.4 ± y 28.2 ± y 23.6 ± y 26.4 ± y 23.0 ± y 24.0 ± y 25.1 ± y 23.2 ± y 23.0 ± y 23.4 ± y 22.9 ± y 22.9 ± y 22.6 ± y 23.6 ± 0.5 a Data are mean ± SE. Units are in ng/ml (multiply by 2.5 for nmol/l); Source: NHANES, Optimal Vitamin D Levels in Health and Disease 23

12 northern latitudes in Europe or Antarctica during the winter months, when sun exposure for endogenous vitamin D synthesis is minimal. Given that vitamin D production from sunlight varies considerably and the potential health risks of sun exposure, the IOM committee made the dietary recommendations assuming an absence of sun exposure. Sources of Vitamin D Briefly, the dietary source of vitamin D 2 is primarily from plants (through irradiation of ergosterol), analogous to human production of D 3 from sunlight. Vitamin D in food is primarily in the form of D 3 and its metabolite 25(OH)D 3, available from fish sources, such as salmon, mackerel, and herring. Beef liver, cheese, egg yolks, and mushrooms provide smaller amounts. Certain foods in the United States are fortified with vitamin D, typically D 3, such as milk, yogurt, cheeses, various breads, and some juice products. Although ultraviolet radiation can assist in increasing serum 25(OH)D levels, skin cancer and photoaging are increasing epidemics and sun exposure is therefore not recommended. In the case study presented, the patient s low serum 25(OH)D level could be increased by giving her a large weekly replacement dose that is currently only available as D 2. A replacement dose of 1,000 IU/day could be expected to increase levels by 10 ng/ml over the course of 6 12 weeks. Vitamin D and Disease Outcomes Observational studies show that both low and high serum 25(OH)D levels have different risk outcomes for mortality (25-27), fracture and frailty (28-30), and cancer (31, 32) (Figure 1-2), as well as other diseases (33). The complexity arising from endogenous and dietary sources of vitamin D, and the potential for confounders due to race/skin pigmentation, obesity, physical activity, and nutritional status, including supplementation practices, contribute to risk variability. Further, despite the usefulness of 25(OH)D as marker of exposure, it has limitations as a biomarker of effect, since it is a surrogate for the active form, 1,25(OH) 2 D, in various tissues and organs. Skeletal Health Vitamin D deficiency reduces skeletal mineralization, resulting in rickets in children and osteomalacia in adults. Osteomalacia leads to newly deposited bone matrix with osteoid seams of unmineralized matrix. The risk for osteomalacia increases when serum levels of 25(OH)D are below 20 ng/ml (50 nmol/l) according to a study of postmortem bone biopsies in a large 24 Translational Endocrinology & Metabolism: Vitamin D Update

13 sample of adults ( years of age). These data, combined with older literature (15) showing osteomalacia and not osteoporosis as a primary cause of hip fracture, suggest one reason why vitamin D supplementation has been shown to prevent hip fracture in the institutionalized population. Observational studies consistently show a relationship between low serum 25(OH) A summary of the randomized D levels and risk of bone loss, fractures, controlled trials shows that frailty, and osteomalacia (28, 29, 34 40). vitamin D 3 combined with In 1132 women in the Women s Health calcium supplements significantly Initiative study (28), an 8.6 year follow-up reduce non-vertebral and hip shows that ethnicity may influence the relationship (Figure 1-2). A prospective cohort predominantly in older subjects fractures, but the benefit was study in 6307 older women (29) shows living in institutionalized settings. that after 4.5 years of follow-up, there was higher risk of frailty associated with both lower and higher serum 25(OH)D levels (Figure 1-2). The MrOS study used a case-cohort design in 1608 men and after 5 years of follow-up found an increased risk of non-spine fracture when serum levels of 25(OH)D were < 20 ng/ml (30). Bone loss at the hip has been shown to increase at low circulating concentrations of 25(OH)D when the range is from 12 to 32 ng/ml (30 80 nmol/l). There is evidence of a relationship between serum 25(OH)D and BMD at the femoral neck, but randomized controlled trials using vitamin D supplements of 400 1,000 IU/day have not shown significant differences in BMD compared to calcium supplementation alone (15, 41, 42). For fracture outcomes, there is evidence from a randomized controlled trial conducted in female Navy recruits that, during military training, women receiving supplementation of vitamin D (800 IU/day) with calcium (2,000 mg/day) had a reduced risk of stress fractures (as compared to recruits given placebo) (43), but there was no vitamin D arm, so the risk-reduction effect had to be attributed to the combination of nutrients. Most of the fracture studies examining vitamin D have been done in the elderly. A summary of the randomized controlled trials(15, 44) shows that vitamin D 3 ( IU/day) combined with calcium supplements (500 1,200 mg/day) significantly reduced non-vertebral and hip fractures, but the benefit was predominantly in older subjects living in institutionalized settings (hip fractures: odds ratio [OR] = 0.69; 95% CI ). Due to the strong interrelationship between vitamin D and calcium, it is not always clear which nutrient or combination of nutrients contributed to the outcome in many studies. There are also few studies examining a dose-response relationship for vitamin D so more of these studies are needed. Optimal Vitamin D Levels in Health and Disease 25

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15 Numerous ecological and cross-sectional studies show significant associations between low levels of 25(OH)D (below 20 ng/ml) and increased mortality and risk of disease (Figure 1-2). An example is colorectal cancer (31, 46). Serum concentrations of 25(OH)D lower than 30 ng/ml are associated with an increased risk of colorectal cancer in the European Prospective Investigation into Cancer and Nutrition (EPIC) study, which used 1248 cancer cases and an equal number of controls (31) (Figure 1-2). The EPIC findings, however, are not consistent with findings showing that male smokers had significantly elevated colon cancer risks in the highest 25(OH)D quartile (>30 ng/ ml) versus the two lowest quartiles <15 ng/ml (47). Breast cancer and other types of cancer show both positive or null findings (46, 48, 49), but for pancreatic cancer higher levels of 25(OH)D conferred increased risk of disease in 952 cases and 1333 controls, aged 62 years and 6.5 years of follow-up (32, 50) (Figure 1-2). In multiple sclerosis, a recent prospective cohort study demonstrated that for each 10 nmol/l increase in serum 25(OH)D, there is up to a 12% reduction in the hazard of symptom relapse (51). Both diabetes and atherosclerosis are often connected with low levels of 25(OH)D. Evidence supports a potential role for vitamin D supplementation in preventing type 2 diabetes in patients with levels of 25(OH)D below 30 ng/ml, especially in those with insulin resistance (52, 53). A few studies show that cardiovascular diseases (CVD) mortality is increased with low 25(OH)D (26, 27, 54) (Figure 1-2), yet the relationship between 25(OH)D and CVD outcomes in African Americans may differ (55). For those with increased risk of CVD, two recent systematic reviews show limited evidence to suggest that vitamin D supplements at moderate to high doses may reduce CVD risk (56, 57). Data from NHANES III in 7970 individuals (ages years) (33) shows that a low serum 25(OH)D (<20 ng/ml) is associated with higher risk of current depression compared to values between 20 and 30 or >30 ng/ml (33). These data (33), however, do not show that serum 25(OH)D levels are associated with a history of depression. Over all, while low levels of 25(OH)D (<20 ng/ml) are consistently associated with greater disease risk or mortality, there are very few randomized and controlled prospective intervention trials with vitamin D targeting non-skeletal diseases, a lack that leaves major gaps in our understanding of causal relationships. In addition, the shape of the dose response relationship curve over a wide range of vitamin D intake levels remains speculative and was of some concern at higher serum levels (>50 ng/ml). All these reasons support the IOM recommendation that serum 25(OH)D levels should be between 20 and Optimal Vitamin D Levels in Health and Disease 27

16 50 ng/ml. Due to an absence of well-designed randomized controlled trials for non-skeletal health outcomes, the IOM committee used data based on skeletal health to determine the vitamin D requirements (15). Potential Adverse Outcomes of Excess Intake/ Tolerable Upper Level Adverse Outcomes Some of the numerous studies addressing the relationship between serum 25(OH)D and various disease outcomes are shown in Figure 1-2, as described above. The limitations of the studies are many, because diseases associated with low 25(OH)D levels may be a marker of chronic nonspecific illness rather than evidence of a direct pathogenic contribution of low vitamin D to a specific disease. Conversely, higher serum 25(OH)D levels associated with diseases could reflect dehydration in individuals who are less healthy or the patients attempt to counteract symptoms by taking vitamin supplements. The observational studies are limited in their interpretation, yet they offered some guidance for the IOM committee in determining a safe upper level of vitamin D intake. Tolerable Upper Level Setting an upper limit is done to avoid any potential unknown long-term effects of chronically high intakes of a nutrient. In the case of vitamin D, and the absence of studies of the effects of high intakes over long periods of time, the observational studies of serum levels of 25(OH) D over a wide range and disease risk were important in the consideration of the TUL. Emerging evidence that there may be adverse effects in individuals with high serum 25(OH)D levels was considered (Figure 1-2). Hypercalcemia of vitamin D toxicity has been reported for intake of 10,000 IU/day, resulting in serum levels above 150 ng/ml (58), so this intake level provided the starting point for setting the UL for adults. This level is the no observed adverse effect level (called the NOAEL), which is then adjusted by a factor of 2 (0.5) to reach a value of 5,000 IU/ day. This adjustment reflects data concerning adverse effects related to all-cause mortality, falls, fractures, frailty, and CVD risk as well as possible forms of cancer, which taken as a total body of evidence provide reason for caution. Furthermore, preliminary data concerning the possible differences in response for blacks as compared to whites (28, 55) warranted another adjustment of 20% (0.80), yielding a UL of 4,000 IU/ day. The UL value is double that set in 1997 by the previous IOM com- 28 Translational Endocrinology & Metabolism: Vitamin D Update

17 mittee, which followed a very conservative approach based on a single study (59). The UL of 4,000 IU/day is a conservative value that was set to provide public health protection, especially when existing data do not support benefit above such intakes. Intake values in the range of 4,000 IU/day should not cause serum 25(OH)D levels to exceed 50 ng/ml. For example, in a study with 5,000 IU/day supplementation in fortified bread (and 800 mg Ca), serum 25(OH)D increased to 50 ng/ml, with an upper quartile of 64 ng/ml, in nursing home residents. This concentration is at the high end of the serum levels associated with outcomes like all-cause mortality, and it is a concentration above which benefits have not been demonstrated. Populations With Reduced Vitamin D Synthesis from Sun Exposure Upper Latitudes, Sunscreen, and Urban Living People living at latitudes closer to the Equator will have greater yearround UVB exposure and increased synthesis of vitamin D in the body, raising 25(OH)D levels accordingly. It has been shown that only a few degrees difference in latitude can influence serum 25(OH)D. For example, in the United Kingdom, serum summer 25(OH)D levels are 25 ng/ ml at 51 North, compared to 17 ng/ml at 57 North in Scotland (60). This seems to support the concept that individuals at higher latitudes will have lower serum 25(OH)D levels, yet because the average Canadian serum 25(OH)D levels are higher than those in the United States, it is clear that other factors contribute to the levels, making the relationship more complicated. Over all, it is more accurate to compare 25(OH) D levels using data in a single controlled trial (60), rather than survey data from different countries. For example, the higher Canadian values can be, at least partially, explained by the higher values in the assay methods used in Canada (chemiluminescence) compared to the assay method used in the U.S. (radioimmunoassay), and they may also be due to differences in other factors, such as the season in which data were collected and/or the numbers of dark-skinned or obese persons in each country s survey. Sunscreen absorbs UV light and will decrease vitamin D synthesis by 95% to 98% at a sun protection factor (SPF) of It has been suggested that about 20% of the body surface needs to be exposed to UVB to increase serum 25(OH)D (61, 62). Holick has suggested that sun exposure for only minutes per day on the face, hands, and arms for a short Optimal Vitamin D Levels in Health and Disease 29

18 time prevents damage by the sun yet will sufficiently increase vitamin D production (63). However, this is contradicted by mathematical models of observational data of sun exposure showing that the amount of exposure required to achieve a rise in serum 25(OH)D levels would compromise skin health and contribute to the risk of cancer and aging (64). There are a few reasons why urban living might alter vitamin D exposure, but the direction of change is not clear. Urban living is associated with more time spent indoors, which would reduce sun exposure. It is also possible that urban air pollution lowers 25(OH)D levels (65); however, the higher ozone levels in polluted cities would be expected to raise serum 25(OH)D levels (66). One study hypothesized that the urban environment, with tall buildings in close proximity ( urban canyons ), might reduce sunlight exposure and vitamin D synthesis (67). Over all, it is possible that serum 25(OH)D levels would be lower due to urban living, but the relationship remains unclear due to multiple contributing factors. Dark Skin Darker skin pigmentation due to melanin in the epidermal layer can reduce the amount of vitamin D synthesized in the skin. This is consistent with the lower levels of serum 25(OH)D found in African Americans compared to whites, whereas Hispanic populations showed intermediate serum levels of 25(OH)D (68). Lower levels of 25(OH)D have also been found in South Asians (e.g., Indians, Pakistanis, Philippinos, Sri Lankans) residing in the northern United States and Canada (69). The health consequences of lower 25(OH) D levels in dark-skinned individuals are not clear. It is well known that, despite lower serum 25(OH)D levels in African Americans, this population has a lower risk for osteoporosis and fracture than whites with higher serum 25(OH)D. There is also some evidence that black women have an increase in serum PTH at a lower 25(OH)D level than white women (70). Others have shown a relationship between 25(OH)D and aortic and carotid artery calcified athrerosclerotic plaque in African-Americans (55). Data show that while higher 25(OH)D is associated with a lower risk of fracture in white women, higher 25(OH)D is associated with a higher risk of fracture in black women (p trend = 0.024)(28) (Figure 1-2). Other large population studies that compare other dark-skinned populations to whites are needed for interpretation of serum 25(OH)D levels and health outcomes. 30 Translational Endocrinology & Metabolism: Vitamin D Update

19 Indoor Environments and Institutionalized Older Persons With technological advances and many jobs in North Americans that require indoor activity, workers experience reduced sun exposure and and increased risk of vitamin D deficiency. Furthermore, some people wear a greater amount of clothing for religious reasons, and these garments greatly reduce their sunlight exposure. In addition, data for the institutionalized frail elderly suggest that this group has a higher risk for low serum 25(OH)D due to many factors, including: low dietary intake; aging skin, which is less effective in synthesizing vitamin D, partially because of decreased levels of skin provitamin D (7-dehydro-cholesterol); and reduced renal conversion of 25()H)D to active 1,25(OH) 2 D. The low serum 25(OH)D levels in institutionalized older individuals are linked to greater risk of fracture (15, 44) and possibly other disease states. In the case presented here, the 78-year-old woman was not institutionalized, but her risk for low serum 25(OH)D levels was increased because she had limited mobility and food intake, estimated by her relatively low BMI of 21 kg/m2, and because she did not take dietary supplements. As a result, she was at high risk for low levels of 25(OH)D and at potential risk for adverse skeletal outcomes. Decreased skin synthesis of vitamin D, whether it is due to living at high latitude, use of sunscreen, urban living, dark pigmentation, excessive indoor activity, or institutionalization, was factored in the dietary recommendations, which assume minimal sun exposure, and therefore vitamin D intake need not be increased above the recommended level. However, solid evidence indicates that institutionalized older persons are a higher-risk group and may require higher intakes. We still do not know whether certain dark-skinned populations and those exposed to high pollution levels or other living conditions that lower serum 25(OH)D levels require higher vitamin D intake. In addition, there is some evidence (28) that lower serum 25(OH)D levels may be necessary to maintain a similar level of health. Clinical and Pathological Conditions Where Vitamin D Supplementation Guidelines May Not Be Adequate Gastrointestinal Disease Serum 25(OH)D levels reflect total exposure from sun and diet. However, vitamin D is a hormone and is converted to 25(OH)D in the liver, and 1,25(OH) 2 D in the kidney. Numerous disorders and medications can interfere with the metabolism of vitamin D and lead to lower levels. Optimal Vitamin D Levels in Health and Disease 31

20 Among disorders that are associated with low 25(OH)D, gastrointestinal and hepatic diseases are by far the most common. The mechanism of decreased 25(OH)D in these patients is usually either malabsorption of this fat-soluble vitamin or enhanced catabolism of hepatic 25(OH)D. Regardless of the etiology, it is often necessary to provide high-dose vitamin D supplementation to these patients in order to reach and then maintain adequate blood levels (71). A reasonable regimen is 50,000 units of vitamin D 2 once per week for 12 weeks, followed by a repeat serum 25(OH)D level. Some individuals with short bowel or bacterial overgrowth syndromes require higher doses on a daily basis to maintain their levels between 20 and 30 ng/ml. Failure of high-dose supplementation to increase serum 25(OH)D levels necessitates a workup for gastrointestinal disorders. Intramuscular vitamin D is not recommended due to significant pain at the site of injection. Some individuals with short gut syndromes can maintain normal serum levels of 25(OH)D by using a desk solar lamp that provides UVB to the hands in the nm range for minutes daily (72). In the patient noted above, 1,000 IU of vitamin D (either D 2 or D 3 ) should lead to an increase of between 5 and 10 ng/ml in serum 25(OH)D. Monitoring can be performed at 3-month intervals, and, in her case, PTH and alkaline phosphatase can be expected to return to normal within 6 months after treatment commences. Medications A number of medications can lower the serum 25(OH)D level and a careful history should allow a provider to identify the drug that is responsible for this change. The most common cause of drug-induced low vitamin D levels is glucocorticoid therapy, although the individual response can be quite variable and may not be dose or duration dependent (15). The mechanism for lowered 25(OH)D in individuals on longterm steroids is not well delineated and may relate to metabolic changes in the metabolites of vitamin D. Other agents that commonly lower serum 25(OH)D by accelerating metabolism in the liver are anticonvulsants, such as phenobarbital, dilantin, and tegretol; antituberculosis drugs, such as isoniazid; H2 blockers, such as cimetidine; rifabutin, an anti-hiv drug; and some hypolipidemic agents(73). Treatment of medication-induced low serum 25(OH)D usually requires discontinuation of the drug or high-dose supplementation with 50,000 units of vitamin D weekly. In the case presented above, the patient was taking hydrochlorothiazide, a medication that has sometimes been associated with an increase in 25(OH)D (15). 32 Translational Endocrinology & Metabolism: Vitamin D Update

21 Obesity Excess adiposity or obesity (BMI 30 mg/m 2 ) is associated with lower serum 25(OH)D concentrations and higher PTH levels than found in non-obese individuals. This is likely due to vitamin D deposited in excess adipose tissue (7, 74, 75) but could also partially be due to greater clothing cover-up or shade-seeking behavior in the obese population (76). There is also evidence that obese subjects are highly sensitive to serum calcium, causing a left shifted relationship with PTH that is common in other types of patients with vitamin D deficiency (77). It has Obese individuals may require been shown that an increase of 1 kg/m 2 larger than usual doses of in BMI is associated with a decrease of vitamin D to achieve a serum nmol/l in 25(OH)D and a (OH)D level comparable pmol/l decrease in 1,25(OH) 2 D concentrations (78, 79). Consistent with this, counterparts; however, it is not to that of their normal-weight Blum et al. (80) found that, in elderly clear if this is associated with a subjects supplemented with 700 IU of beneficial outcome. vitamin D per day, for every additional 15 kg of weight above normal, the 25(OH)D level is approximately 10 nmol/l lower after 1 year of supplementation. Obese individuals may require larger than usual doses of vitamin D supplements to achieve a serum 25(OH)D level comparable to that of their normal-weight counterparts. In support of evidence that vitamin D is stored in adipose tissue, weight reduction studies show that serum 25(OH)D levels rise when obese individuals lose body fat (81 85). An important concern is whether the lower serum 25(OH)D levels associated with obesity contribute to the diseases associated with obesity, as well as affecting bone health. Evidence for effects of obesity on bone density is mixed. The combined influence of increased weight-bearing activity and endogenous synthesis of estrogen associated with increased adiposity have long been associated with higher bone density (86 88). However, some studies have suggested that increased fat mass itself may be a factor in the development of, rather than prevention of, osteoporosis, particularly in the elderly (89). In addition, others have shown that the quality of bone is altered in obesity (90, 91), and for a given BMD, fracture risk is increased in obese compared to normal-weight older individuals (92). Different studies suggest that either high serum PTH (93) or low 25(OH)D (94) predicts metabolic syndrome in obese adults. Hence, it is possible that the lower serum 25(OH)D due to excess adiposity may adversely Optimal Vitamin D Levels in Health and Disease 33

22 affect a variety of health outcomes associated with metabolic syndrome, including CVD (95), hypertension (96 98), impaired glucose tolerance, type 2 diabetes (52), and inflammation (99 101). However, at this time, there is no evidence that increases in vitamin D intake in the obese can ameliorate these comorbidities, and higher vitamin D requirements than those specified for non-obese persons are not specifically recommended. Summary Overall, low levels of 25(OH)D (<20 ng/ml) remain a concern for many individuals in the North American population (15, 18). Also, there is evidence showing a decline in serum 25(OH)D levels over time (68, 102), even after adjustment for assay shift that occurred over the years. Evidence was found to support a higher recommended intake of vitamin D than in 1997 based on randomized controlled trials in the field of bone health. This level is 600 IU/day in all age groups except for those over age 70, who should consume 800 IU/day. There was not adequate evidence that additional vitamin D had a significant effect on other non-skeletal health conditions, although the biological evidence is strong. Recent evidence shows that high serum levels of 25(OH)D are not associated with continued beneficial effects and, for some outcomes, disease risk is increased when serum levels rise above 50 ng/ml. Observational studies of serum 25(OH)D suggest that the nadir for disease risk and mortality is between 20 and 30 ng/ml. New randomized, placebo-controlled trials are in progress and will help to determine whether non-skeletal health outcomes can be considered in the future. Nevertheless, these new studies need to address potential adverse outcomes of higher serum 25(OH)D and to balance this with the measurable improved clinical benefits. In the meantime, it remains controversial whether all patients should be screened for serum 25(OH) D levels. Certainly, individuals with low BMD and/or fractures should have at least one serum 25(OH)D level obtained during their workup, as was done for the woman in the case study presented. The rationale for this is two-fold: 1) low levels can lead to falls and additional fractures; 2) treatment with the bisphosphonates should be withheld until serum 25(OH)D levels are in the normal range. Other high-risk groups that should be screened include individuals with chronic liver and/or intestinal disease and all patients with a diagnosis of celiac disease, as well as candidates for bariatric surgery. We do not advocate routine screening of every patient, nor do we recommend screening for indi- 34 Translational Endocrinology & Metabolism: Vitamin D Update