Annie WC Kung, Keith DK Luk, LW Chu, and Peter KY Chiu

Age-related osteoporosis in Chinese: an evaluation of the response of intestinal calcium absorption and calcitropic hormones to dietary calcium deprivation 1 3 Annie WC Kung, Keith DK Luk, LW Chu, and Peter KY Chiu ABSTRACT Background: Age-related osteoporosis may be associated with inefficient intestinal calcium absorption and bone remodeling. Objective: We investigated the pathogenesis of age-related osteoporosis in Chinese women with habitual low calcium intakes. Design: We studied the response of intestinal calcium absorption, calcitropic hormones, and biochemical bone markers to graded dietary calcium deprivation. Results: The osteoporotic subjects (n = 25) had higher urinary calcium excretion (P < 0.05) and lower plasma 1,25-dihydroxyvitamin D concentrations (P < 0.02) than did age-matched control women (n = 25). Parathyroid hormone was not significantly different from that in age-matched control women but was significantly higher than in young women (n = 15, P < 0.05). Fractional 45 Ca absorption was 61% in all 3 groups when the diet was unmodified and increased to 71%, 69%, and 68% in the osteoporotic subjects, age-matched control women, and young women, respectively, when dietary calcium was reduced to 300 mg/d. When the osteoporotic women were calcium deprived, serum 1,25-dihydroxyvitamin D failed to increase but urinary calcium excretion persisted. In contrast, supplementation with 1200 mg Ca resulted in a lowering of parathyroid hormone (P < 0.005 compared with the unmodified diet) and 1,25-dihydroxyvitamin D (P < 0.01) and decreased fractional 45 Ca absorption (P < 0.01), suggesting that the increased calcium intake was associated with a potent compensatory ability of the intestine and calcitropic hormones to adapt. Calcium supplementation lowered osteocalcin (P < 0.05) but not alkaline phosphatase, which remained elevated in the osteoporotic subjects at all stages. Conclusions: Elderly osteoporotic women had reduced 1,25-dihydroxyvitamin D production, excessive urinary calcium loss, and high bone turnover. The Chinese women had exceptionally potent intestinal calcium absorption. Am J Clin Nutr 1998;68:1291 7. KEY WORDS Osteoporosis, calcium absorption, calcitropic hormones, Chinese, women, 1,25-dihydroxyvitamin D, parathyroid hormone INTRODUCTION Osteoporosis is in part a disorder of bone remodeling. Although the cause of abnormal bone remodeling is not well See corresponding editorial on page 1153. understood, a common feature of the osteoporotic process is a negative calcium balance due to more calcium exiting the skeleton than returning. Two pathophysiologic categories of osteoporosis have been defined. In postmenopausal osteoporosis, the primary defect is estrogen deficiency, with a resultant increase in bone resorption. In age-related osteoporosis, the process of bone remodeling becomes inefficient. The inefficiency relates to either excessive bone resorption or defective bone formation. More likely, it is a combination of both abnormalities. Apart from the critical events that constitute the bone remodeling cycle, the gastrointestinal tract looms as another pathophysiologic focus. Aging is believed to be associated with a defect in the ability to adapt to a low-calcium diet (1, 2). Inefficient absorption or malabsorption of calcium from the gastrointestinal tract can be due to a reduction in renal 1,25-dihydroxyvitamin D formation, a decline in intestinal vitamin D receptors, 25-hydroxyvitamin D deficiency (3 5), or inadequate dietary calcium. These mechanisms are based on Western populations in whom habitual calcium intake is higher than in Asian populations. Calcium intake is an important risk factor for the development of osteoporosis. Recent studies have confirmed that supplementation of the diet with calcium is associated with reduced bone loss (6 8). Recent recommendations by the Food and Nutrition Board of the National Academy of Science increased the dietary reference intake for calcium from 800 to 1000 mg/d for adults (9). Whether the same requirement applies to Asian populations, whose body sizes and skeletons are smaller, is unknown. In Hong Kong, the Chinese diet contains an average calcium content of <500 mg/d (10), much lower than that of a typical Western diet. Whether racial factors in the Chinese serve to enhance calcium absorption is unknown. To begin to address this question, we evaluated fractional calcium absorption, calcitropic 1 From the Departments of Medicine and Orthopaedic Surgery, The University of Hong Kong, Queen Mary Hospital, Hong Kong, China. 2 Supported by CRCG grants 337.041.0060 and 337.041.0048 and grant 394.041.1510 from the Osteoporosis and Endocrine Research Fund, The University of Hong Kong. 3 Address reprint requests to AWC Kung, Department of Medicine, The University of Hong Kong, Queen Mary Hospital, 102 Pokfulam Road, Hong Kong, China. E-mail: awckung@hkucc.hku.hk. Received February 11, 1998. Accepted for publication July 2, 1998. Am J Clin Nutr 1998;68:1291 7. Printed in USA. 1998 American Society for Clinical Nutrition 1291

1292 KUNG ET AL hormones, and biochemical bone markers in response to dietary calcium deprivation in a group of elderly osteoporotic subjects and age-matched as well as young healthy control subjects. SUBJECTS AND METHODS Subjects Twenty-five southern Chinese women with postmenopausal osteoporosis were recruited. All osteoporotic subjects had at least one clinically significant vertebral fracture and a total lumbar spine bone mineral density (L1 4 BMD) <2.5 SDs below the mean for a young, control population (t score < 2.5). The women were studied 6 mo after the fracture. Twenty-five agematched, healthy postmenopausal women and 15 healthy, premenopausal women with normal BMDs were also studied. The healthy women were recruited from the community. Women with underlying metabolic or genetic bone disease, with premature menopause (age <40 y), who had had a bilateral ovariectomy, or who used drugs that could affect bone mineral metabolism were excluded. The protocol was approved by the Ethics Committee, the University of Hong Kong. Study design Dietary calcium intake was assessed with a food-frequency questionnaire before the study (11). The study was divided into 4 phases, each of which consisted of a treatment period of 4 wk and a washout period of 2 wk. In the first phase, subjects were given a calcium supplement of 1200 mg/d in 3 divided doses in the form of calcium citrate (Citracal; Mission Pharmacal, San Antonio, TX). During the second phase, subjects received a calcium supplement of 600 mg/d as calcium citrate (200 mg 3 times daily after meals). During the third phase of the study, subjects were advised to return to their unmodified diets. Dietary calcium intake was reassessed by questionnaire during this period. In the final phase, subjects were placed on a low-calcium diet, during which they were asked to avoid foods containing substantial amounts of calcium, such as dairy products, eggs, bean curd and related products, seafood, and calcium-containing vegetables. The low-calcium diet was estimated to contain <300 mg dietary calcium/d. In addition, subjects were given oral phosphate (Phosphate Sandoz, 1 g 3 times daily; Sandoz, Bunchbowl, Australia) with meals to inhibit intestinal calcium absorption. The sequence of high to low calcium intake was used because it is the one under which the adaptation period is defined. Measurements were taken at recruitment and at the end of each study period. Subjects were asked to bring a 24-h urine collection for calcium and creatinine measurement. Fasting blood was obtained for measurement of calcium, phosphorus, albumin, creatinine, total alkaline phosphatase, parathyroid hormone (PTH), 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, and osteocalcin. A 2-h fasting morning urine sample was also collected for n-telopeptide measurement. Calcium absorption study Fractional 45 Ca absorption was estimated at the end of each treatment phase from the appearance of 45 Ca in blood after the ingestion of 100 ml of an aqueous solution containing 3 Ci 45 Ca and 100 mg cold Ca as the chloride salt (12), followed by ingestion of three 50-mL portions of deionized water. Before ingestion, duplicate 60- L standards were removed from each test dose for counting. Exactly 3 h after ingestion of the tracer, 15 ml blood was drawn. Two 2-mL portions of serum and the standards were each added to 18 ml scintillation fluid and -emissions were counted in a scintillation counter (model 1900TR; Packard Instrument Co, Meridian, CT). Counts were corrected for quenching. In the first 8 women, blood was drawn immediately before the second tracer was administered and counted as described above to evaluate residual counts from the first tracer dose. Residual counts were all <1%, suggesting that the subsequent tracer test was not affected by the previous one. The fraction of 45 Ca absorption index, based on evidence compiled by Manery (13) and used by others (14), is the fraction of the 45 Ca count ingested and multiplied by 15% of body weight. 45 Ca was purchased from Amersham Corp (Buckinghamshire, United Kingdom) and spectral analyses were carried out on each batch before its use to ensure purity. Laboratory assays Plasma 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D were measured with kits from Incstar Corporation (Stillwater, MN). Plasma 25-hydroxyvitamin D was extracted with acetonitrile and assayed by an equilibrium radioimmunoassay procedure (Incstar Corporation). The intra- and interassay CVs were 10.6% and 14.9%, respectively. Plasma 1,25-dihydroxyvitamin D was extracted by using C 18 OH and silica cartridges and assayed by using a competitive radioimmunoassay procedure (Incstar). The percentage recovery was monitored by the addition of known amounts of 1,25-dihydroxycholecalciferol (calcitriol) in ethanol in 3 serum samples in each assay. The overall recovery was 105 ± 10%. The intra- and interassay CVs were 7.8% and 8.3%, respectively. Serum intact PTH was measured with chemiluminescence immunometric assay kits from Corning Magic Lite (Chiron Diagnostic Corporation, Medfield, MA) with intra- and interassay CVs of 5.6% and 6.6%, respectively. Serum calcium, phosphorus, albumin, total alkaline phosphatase, and creatinine were measured with a Hitachi 747 random access analyzer (Boehringer Mannheim, Mannheim, Germany). Urinary calcium and creatinine were determined by colorimetric reaction with the Synchron CX5 (Beckman Instruments, Palo Alto, CA). Serum intact osteocalcin was measured by enzyme-linked immunosorbent assay with commercial kits (Novocalcin; Metra Biosystems Inc, Mountain View, CA). The intra- and interassay CVs were 8.8% and 10.1%, respectively. Urinary n-telopeptide was also measured by enzyme-linked immunosorbent assay (Osteomark; Ostex, Seattle). The intra- and interassay CVs were 8.7% and 10.9%, respectively. All samples for individual subjects were measured in a single assay. Densitometry Lumbar spine (L1 4) and femoral neck BMDs were measured by dual-energy X-ray absorptiometry (Hologic QDR 2000 plus; Hologic Inc, Waltham, MA). The in vivo precision of the machine for lumbar spine and femoral neck measurements was 1.2% and 1.5%, respectively. Statistical analysis Baseline values were compared by using one-way analysis of variance (ANOVA) with Tukey s test for repeated comparisons. Results are reported as means ± SDs. For the analysis of longitudinal data, two-factor ANOVA was used to determine the inter-

CALCIUM ABSORPTION AND OSTEOPOROSIS IN CHINESE 1293 actions between calcium intakes and between the various patient groups. Tukey s test was used as a post hoc test to compare variables identified as being significant (P < 0.05) by ANOVA between calcium intakes and between the various patient groups. The statistical package used was SPSS for WINDOWS (SPSS Inc, Chicago). n-telopeptide excretion. Serum total alkaline phosphatase and osteocalcin but not urinary n-telopeptide were significantly higher in osteoporotic subjects than in age-matched control subjects. Bone mineral densities were significantly lower in osteoporotic subjects than in age-matched or young control subjects. RESULTS Baseline data Dietary calcium intakes and plasma 25-hydroxyvitamin D concentrations of osteoporotic subjects were not significantly different from those in age-matched or young healthy control subjects (Table 1). None of the subjects had 25-hydroxyvitamin D concentrations < 37.5 mmol/l (15 mg/l). Serum calcium and phosphate concentrations and creatinine clearance were not significantly different between osteoporotic subjects and agematched control subjects, but urinary calcium excretion was higher in osteoporotic subjects. Although PTH concentrations were not significantly different between osteoporotic subjects and age-matched control subjects, plasma 1,25-dihydroxyvitamin D concentrations were significantly decreased in osteoporotic subjects. Compared with young women, the osteoporotic subjects had higher albumin-adjusted serum calcium concentrations and elevated urinary calcium excretion. Furthermore, the osteoporotic subjects had lower plasma 1,25-dihydroxyvitamin D concentrations but elevated serum PTH. Bone turnover was increased in the osteoporotic subjects compared with the agematched or young control subjects as evidenced by elevated serum total alkaline phosphatase, serum osteocalcin, and urinary Longitudinal data Calcitropic hormones The response of the kidneys and gastrointestinal tract to calcium deprivation was studied. With a gradual reduction in calcium intake, serum calcium and urinary calcium excretion decreased in parallel in both age-matched and young control subjects (Table 2). In osteoporotic subjects, although serum calcium concentrations decreased when dietary calcium intake was gradually lowered, urinary calcium excretion was still higher than that in the control subjects, even during calcium deprivation. Serum 25-hydroxyvitamin D was not significantly different throughout the study in all 3 study groups (data not shown). PTH concentrations increased during calcium deprivation in both osteoporotic subjects and age-matched control subjects; concentrations in these subjects were significantly higher than in young control subjects (Figure 1). When subjects were given 1200 mg Ca, PTH concentrations were suppressed in both osteoporotic subjects and age-matched control subjects to concentrations similar to those in young control subjects (2.27 ± 1.09, 2.10 ± 0.80, and 1.89 ± 0.93 pmol/l, respectively). Plasma 1,25-dihydroxyvitamin D increased appropriately during calcium deprivation in the age-matched and young control subjects, but not in the osteoporotic subjects (Figure 2). Plasma 1,25-dihydroxyvitamin D TABLE 1 Baseline characteristics of the study subjects 1 Osteoporotic Age-matched control Young control subjects subjects subjects (n = 25) (n = 25) (n = 15) ANOVA Age (y) 67 ± 6 a 67 ± 6 a 43 ± 6 b < 0.0001 Height (cm) 150 ± 8 a 151 ± 5 b 156 ± 4 b < 0.05 Weight (kg) 51 ± 11 55 ± 8 53 ± 7 NS Menarche (y) 15 ± 2 15 ± 2 14 ± 2 NS Time since menopause (y) 20 ± 8 20 ± 6 NS Calcium intake (mg/d) 561 ± 183 587 ± 219 554 ± 191 NS Bone mineral density Femoral neck (g/cm 2 ) 0.54 ± 0.12 a 0.64 ± 0.08 b 0.78 ± 0.13 c < 0.0001 Lumbar (L2 4) spine (g/cm 2 ) 0.71 ± 0.15 a 0.89 ± 0.14 b 1.02 ± 0.10 c < 0.0001 Biochemical indexes Albumin-adjusted calcium (mmol/l) 2.42 ± 0.08 a 2.40 ± 0.10 a 2.27 ± 0.18 b < 0.05 Serum calcium (mmol/l) 2.39 ± 0.10 2.40 ± 0.10 2.35 ± 0.09 NS Phosphorus (mmol/l) 1.16 ± 0.25 1.09 ± 0.19 1.05 ± 0.19 NS Albumin (g/l) 44 ± 3 46 ± 2 46 ± 2 NS Creatinine ( mol/l) 73 ± 14 a 78 ± 9 a 62 ± 8 b < 0.05 Creatinine clearance (ml/min) 65 ± 18 a 63 ± 12 a 86 ± 21 b < 0.05 Urinary calcium (mol/mol Cr) 0.63 ± 0.47 a 0.28 ± 0.25 b 0.27 ± 0.16 b < 0.05 Plasma calcitropic hormones 25(OH)D (nmol/l) 82 ± 20 80 ± 23 82 ± 30 NS 1,25(OH) 2 D (pmol/l) 74 ± 26 a 110 ± 30 b 108 ± 33 b < 0.02 PTH (pmol/l) 3.67 ± 1.54 a 3.37 ± 1.41 a 2.67 ± 1.62 b < 0.05 Bone turnover indexes Total alkaline phosphatase (IU/L) 84 ± 21 a 72 ± 18 b 54 ± 16 b < 0.05 Osteocalcin (mg/l) 9.5 ± 4.2 a 6.7 ± 3.9 b 4.5 ± 2.3 b < 0.05 Urinary NTx (mmol BCE/mol Cr) 50 ± 21 36 ± 21 28 ± 15 NS 1 x ± SD. Cr, creatinine; 25(OH)D, 25-hydroxyvitamin D; 1,25(OH) 2 D, 1,25-dihydroxyvitamin D; PTH, parathyroid hormone; NTx, n-telopeptide; BCE, bone collagen equivalent. Means in the same row with different superscript letters are significantly different at the P value shown in the ANOVA column.

1294 KUNG ET AL TABLE 2 Changes in biochemical indexes during calcium deprivation 1 1200-mg Ca 600-mg Ca Unmodified Low-calcium Two-factor supplement supplement diet diet ANOVA 2 Total calcium (mmol/l) Osteoporotic subjects 2.41 ± 0.11 2.41 ± 0.09 2.37 ± 0.10 2.34 ± 0.10 D, G Age-matched control subjects 2.41 ± 0.11 2.40 ± 0.09 2.38 ± 0.09 2.34 ± 0.07 Young control subjects 2.41 ± 0.10 2.39 ± 0.08 2.31 ± 0.07 2.29 ± 0.07 Albumin-adjusted calcium (mmol/l) Osteoporotic subjects 2.41 ± 0.10 2.39 ± 0.08 2.38 ± 0.08 2.34 ± 0.09 D, G Age-matched control subjects 2.41 ± 0.10 2.40 ± 0.07 2.37 ± 0.10 2.33 ± 0.09 Young control subjects 2.41 ± 0.06 2.37 ± 0.05 2.31 ± 0.03 2.28 ± 0.08 Phosphorus (mmol/l) Osteoporotic subjects 1.06 ± 0.16 1.10 ± 0.12 1.15 ± 0.11 1.15 ± 0.13 None Age-matched control subjects 1.07 ± 0.14 1.08 ± 0.12 1.09 ± 0.13 1.11 ± 0.13 Young control subjects 1.11 ± 0.15 1.14 ± 0.11 1.15 ± 0.17 1.17 ± 0.20 Urinary calcium (mol/mol Cr) Osteoporotic subjects 0.60 ± 0.57 0.42 ± 0.24 0.33 ± 0.15 0.36 ± 0.19 D, G Age-matched control subjects 0.43 ± 0.28 0.40 ± 0.22 0.28 ± 0.25 0.25 ± 0.20 Young control subjects 0.40 ± 0.25 0.44 ± 0.25 0.28 ± 0.15 0.27 ± 0.14 1 x ± SD; n = 25 osteoporotic subjects, 25 age-matched control subjects, and 15 young control subjects. Cr, creatinine. 2 Significant main effects of diet (D), group (G), or a significant D G interaction, P < 0.05. concentrations in the osteoporotic subjects were significantly lower than in in the age-matched or young control subjects at all times except when they were given the 1200-mg Ca supplement. In the postmenopausal, age-matched, healthy control subjects, plasma 1,25-dihydroxyvitamin D concentrations were not significantly different from those in the young control subjects at all phases of the study. Biochemical bone markers No significant changes were noted in total alkaline phosphatase in all 3 groups during calcium deprivation. The osteoporotic subjects, however, had significantly higher concentrations than those in the young control subjects during all phases of the study (Table 3). In the osteoporotic subjects, significant elevations of serum osteocalcin occurred during the unmodified diet period and the low-calcium diet period compared with the 1200-mg Ca supplementation period. When subjects were given 1200 mg Ca, osteocalcin concentrations were significantly reduced in osteoporotic subjects as well as in age-matched control subjects. TABLE 3 Changes in biochemical bone markers during calcium deprivation 1 Similarly, osteoporotic subjects had significantly higher osteocalcin concentrations than did young control subjects during all phases of the study. Serum osteocalcin remained unchanged during calcium deprivation in the young control subjects. Urinary n-telopeptide excretion in the osteoporotic subjects showed a decreasing trend with both 600-mg and 1200-mg Ca supplementation, but the results were not significant (Table 3). Similar results were observed in the age-matched control subjects. n-telopeptide excretion increased when the young control subjects were consuming the low-calcium diet, but not significantly so. Fractional 45 Ca absorption Fractional 45 Ca absorption during the unmodified diet was 61% in all 3 groups of subjects (Table 4). During the lowcalcium period, 45 Ca absorption was 71%, 71%, and 68% in the osteoporotic, age-matched control subjects, and young control subjects, respectively. These results were not significant when compared with the unmodified diet period. During calcium supplementation, fractional 45 Ca absorption showed a decreasing 1200-mg Ca 600-mg Ca Unmodified Low-calcium Two-factor supplement supplement diet diet ANOVA 2 Alkaline phosphatase (IU/L) Osteoporotic subjects 76 ± 18 79 ± 20 73 ± 2 76 ± 21 G Age-matched control subjects 65 ± 12 66 ± 17 68 ± 15 70 ± 17 Young control subjects 55 ± 16 56 ± 17 54 ± 16 58 ± 19 Osteocalcin (mg/l) Osteoporotic subjects 8.07 ± 3.47 8.60 ± 3.41 9.50 ± 3.68 10.40 ± 3.33 D, G Age-matched control subjects 6.07 ± 2.84 6.15 ± 3.05 6.28 ± 3.92 7.03 ± 3.54 Young control subjects 5.00 ± 2.55 5.00 ± 2.71 5.02 ± 2.26 5.50 ± 2.48 Urine NTx (mmol BCE/mol Cr) Osteoporotic subjects 44 ± 28 38 ± 17 50 ± 21 55 ± 30 None Age-matched control subjects 35 ± 28 37 ± 25 40 ± 28 50 ± 28 Young control subjects 29 ± 14 23 ± 7 32 ± 15 47 ± 34 1 x ± SD; n = 25 osteoporotic subjects, 25 age-matched control subjects, and 15 young control subjects. NTx, n-telopeptide; BCE, bone collagen equivalent; Cr, creatinine. 2 Significant main effects of diet (D), group (G), or a significant D G interaction, P < 0.05.

CALCIUM ABSORPTION AND OSTEOPOROSIS IN CHINESE 1295 FIGURE 1. Mean (± SD) parathyroid hormone (PTH) concentrations of osteoporotic subjects (, n = 25), age-matched control subjects (, n = 25), and young control subjects (, n = 15) during different dietary calcium intakes. * Significantly different from 1200-mg Ca supplementation, P < 0.005. Significantly different from young control subjects, P < 0.05. Significance of two-factor ANOVA: diet, P < 0.05; group, P < 0.001; diet group interaction, NS. trend in both the age-matched and the young control subjects, but the results were not significant. Significant differences in fractional 45 Ca absorption between the low-calcium diet and the 1200-mg Ca supplementation period were noted in all 3 groups. However, 45 Ca absorption was suppressed to 49% in the osteoporotic subjects when they were supplemented with 1200 mg Ca. We also examined the interactions between age, group, and calcium intake on fractional absorption and calcitropic hormones. With use of two-factor repeated-measures ANOVA, no significant interactions between age, group, and calcium intake were detected for any of the indexes studied. DISCUSSION Our results showed that fractional 45 Ca absorption was significantly altered by changes in dietary calcium intake, which ranged from >1500 to <300 mg/d. However, no significant differences were detected in fractional 45 Ca absorption between postmenopausal osteoporotic subjects; healthy, age-matched control subjects; and healthy, premenopausal control subjects except during supplementation with 1200 mg Ca. The fractional calcium absorption value obtained in our subjects when they were consuming their unmodified diets (with a daily calcium intake of 500 mg) was similar to the result of 58% obtained in another study performed in young children from the same population by using a double stable-isotope method (15). This suggests that intestinal calcium absorption does not change with aging. Intestinal calcium absorption in our Chinese subjects was more than 2- fold higher than that reported in whites (16) or blacks (17) with similar dietary calcium intakes, which is reported to be 20 30%. Whether this represents a racial difference or just an adaptation to chronically low dietary calcium intakes since childhood in our population is unknown. It is a Chinese custom to wean infants from milk at 6 12 mo of age. Also, 70% of Chinese adults have lactose intolerance and do not consume any dairy products. Thus, vegetables are the main source of dietary calcium (11). There is little doubt that humans can adapt to large changes in dietary intake of calcium. The enhanced calcium absorptive effi- FIGURE 2. Mean (± SD) 1,25-dihydroxyvitamin D [1,25(OH) 2 D] concentrations of osteoporotic subjects (, n = 25), age-matched control subjects (, n = 25), and young control subjects (, n = 15) during different dietary calcium intakes. * Significantly different from 1200-mg Ca supplementation, P < 0.01. Significantly different from young control subjects, P < 0.05. Significance of two-factor ANOVA: diet, P < 0.0005; group, P < 0.0005; diet group interaction, NS.

1296 KUNG ET AL TABLE 4 Fractional calcium absorption in Chinese women 1 1200-mg Ca 600-mg Ca Unmodified Low-calcium Two-factor supplement supplement diet diet ANOVA 2 Osteoporotic subjects (n = 25) 49 ± 11 56 ± 16 61 ± 18 71 ± 21 D, G Age-matched control subjects (n = 25) 56 ± 13 58 ± 11 60 ± 12 71 ± 18 Young control subjects (n = 15) 59 ± 19 64 ± 14 61 ± 7 68 ± 11 1 x ± SD. 2 Significant main effects of diet (D), group (G), or a significant D G interaction, P < 0.05. % ciency in this population may represent successful adaptation to generations of low dietary calcium. Furthermore, this efficient intestinal calcium absorption is apparently not impaired by estrogen deficiency due to menopause, so that in healthy, postmenopausal women intestinal calcium absorption was not significantly different from that in premenopausal women at all phases of the study. Also, the efficient intestinal absorption of osteoporotic women apparently overcame the inadequate 1,25-dihydroxyvitamin D production of these subjects because their fractional calcium absorption was not significantly different from that in the young control subjects during the unmodified and the low-calcium diets. When subjects were supplemented with 1200 mg Ca, fractional calcium absorption in the osteoporotic women was promptly suppressed to levels below even those of the healthy control subjects. There is thus no evidence of intestinal malabsorption or defective 1,25-dihydroxyvitamin D action on the gut in these osteoporotic subjects. Although the gastrointestinal tract adapts well to low dietary calcium intakes, the osteoporotic women had significantly higher urinary calcium losses and lower plasma 1,25-dihydroxyvitamin D concentrations during dietary calcium deprivation than did control subjects. The low 1,25-dihydroxyvitamin D concentrations could have been due to either defective renal calcidiol 1-monooxygenase activity or enzyme resistance to the action of PTH (4). We further observed that despite a low 1,25-dihydroxyvitamin D concentration and excessive urinary calcium loss, PTH concentrations in the osteoporotic women were not elevated and there was no evidence of secondary hyperparathyroidism in these subjects. Previous studies conducted in this population also revealed an absence of secondary hyperparathyroidism in patients with hip fracture (18). Our finding of a lack of secondary hyperparathyroidism is in contrast with other reports in white women that age-related bone loss is associated with increased PTH as a result of a negative calcium balance in the body. Thus, to explain the high bone turnover in the presence of low 1,25-dihydroxyvitamin D and normal PTH concentrations in our Chinese osteoporotic women, we postulate that these women had specific differences in bone and calcium homeostasis compared with subjects with normal bone mass, and that they had altered bone turnover responses to changes in calcium intake and abnormal skeletal responses to endogenous 1,25-dihydroxyvitamin D and PTH concentrations. Previous studies of female patients with vertebral fractures also reported evidence of suppression of PTH, consistent with the primary pathophysiologic disorder resulting in the bone (19, 20). Recently, O Brien et al (21) reported that girls and women from osteoporotic families have a significantly altered bone turnover response to acute changes in calcium intake, so that the net balance in bone turnover was significantly lower than in healthy control subjects. This finding also suggests a primary abnormality in the regulation of bone turnover in osteoporotic subjects that is probably genetically determined. In this study, we also gained information about the amount of calcium required to suppress bone turnover in osteoporotic subjects. Supplementation with 1200 mg Ca was associated with suppressed PTH concentrations and lowered serum osteocalcin concentrations, suggesting that this degree of calcium supplementation can retard bone turnover. Failure to detect a significant change in n- telopeptide excretion was probably related to the large biological intraindividual variation in n-telopeptide excretion (22). However, we also observed that with 1200-mg Ca supplementation, 1,25- dihydroxyvitamin D concentrations were further suppressed in these osteoporotic subjects who had preexisting low concentrations. The reduction of plasma 1,25-dihydroxyvitamin D was probably secondary to a reduction in the serum PTH of these patients. 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