Prevalence of Micronutrient Malnutrition Worldwide

Transcription

1 May 2002: (II)S46 S52 Prevalence of Micronutrient Malnutrition Worldwide Usha Ramakrishnan, Ph.D. Recent estimates indicate that globally over two billion people are at risk for vitamin A, iodine, and/or iron de ciency, in spite of recent efforts in the prevention and control of these de ciencies. The prevalence is especially high in Southeast Asia and sub-saharan Africa, and pregnant women and young children are at greatest risk. Other micronutrient de ciencies of public health concern include zinc, folate, and the B vitamins. However, there is limited data on the actual prevalence of these de ciencies. Finally, in many settings, more than one micronutrient de ciency exists, suggesting the need for simple approaches that evaluate and address multiple micronutrient malnutrition International Life Sciences Institute Micronutrient de ciencies are common in many developing countries and are typically due to inadequate food intake, poor dietary quality, poor bioavailability (because of the presence of inhibitors, mode of preparation, and interactions), and/or the presence of infections. Micronutrient status can affect health outcomes such as child survival, growth, and development either directly (e.g., de ciencies of vitamin A, iodine, iron, zinc, or folic acid) or indirectly through interactions with each other (e.g., interactions with vitamin A, zinc, or iron), increasing food intake owing to improved appetite, and reducing morbidity. Most discussions of micronutrient de ciencies have been limited to the Big 3, namely vitamin A de ciency (VAD), iodine de ciency disorders (IDD), and iron de ciency anemia (IDA). While these problems remain signi cant public health concerns, it is important to recognize other micronutrient de ciencies such as zinc, folate, and multiple micronutrient malnutrition (MMM). Even the authors of the recent micronutrient update in the fourth report of the World Nutrition Situation acknowledge that other micronutrient de ciencies have not been addressed adequately. 1 This paper provides a brief review of current information on the global Dr. Ramakrishnan is Assistant Professor, Department of International Health, Rollins School of Public Health at Emory University, 1518 Clifton Rd, N.E., Atlanta, GA 30032, USA. prevalence of VAD, IDD, iron, zinc, and other vitamin and multinutrient de ciencies. Vitamin A De ciency (VAD) Severe vitamin A de ciency typically entails clinical signs of xerophthalmia and very de cient serum levels of vitamin A ( 0.35 mol/dl), whereas subclinical de - ciency (mild and moderate VAD) is de ned as tissue concentrations of vitamin A low enough to have adverse health consequences, even if there is no evidence of clinical xerophthalmia. Serum retinol is the most widely used biochemical indicator of vitamin A status and the levels used to de ne mild, moderate, and severe VAD are g/dl ( mol/l), 0 20 g/dl ( mol/l) and 10 g/dl ( 0.35 mol/l), respectively. Further, VAD is de ned as a public health problem if more than 2% of the population has serum retinol levels 20 g/dl and the degree of severity is classi ed as follows: mild, 2 10%; moderate, 10 20%; and severe, 20%. 2 The most recent estimates of the prevalence of VAD in young children are available from two sources. The WHO-MDIS system indicated that as of 1995, there were 28 million children under the age of 5 with clinical VAD, and 251 million with subclinical VAD. 2 More recently, Mason et al., 3 using a slightly different approach, estimated the numbers to be 3.3 million with clinical VAD and million with subclinical VAD. Some of the reasons for the variations in estimates relate to the lack of reliable representative data. Nevertheless, it is a major public health problem in more than 60 countries and a large proportion of these children live in Southeast Asia, followed by Africa and the Americas (see Table 1). Considerable efforts have been directed in the last decade towards the elimination of VAD, but it is dif cult to assess progress, as estimates of VAD are still in the process of being updated. Mason et al. 3 concluded that there was evidence of improvement by comparing data on clinical VAD that were available from repeated national surveys for 8 countries and regional estimates for 1985 and Comparison of trends could not be done for subclinical VAD because data is scarce. One of the hurdles in the assessment of subclinical VAD has been the absence of eld-based technologies that would per- S46 Nutrition Reviews, Vol. 60, No. 5

2 Table 1. Global Prevalence of Micronutrient Malnutrition Region Vitamin A Iodine Iron Zinc (%) a (%) b (%) c (%) d Southeast Asia West Paci c Africa East Mediterranean Americas Europe a Clinical and subclinical de ciency in young children. 2 b Living in a region where total goiter rates in school age children 5%. 1,4 c Based on the prevalence of iron de ciency anemia (IDA) and assuming that iron de ciency is 2.5 times that of IDA. 5,6 d Risk of low intake based on national food balance data corrected for phytate: Zn ratio; the estimates for Southeast Asia, Europe, and the Americas do not include South Asia (95.4%), Eastern Europe (12.8%), and North America (0.9%), respectively. 7 mit easy and rapid assessment using, say, just a drop of blood. Although VAD has been reported among women of reproductive age, there is a dearth of reliable prevalence data. Surveys based on recall have reported high rates of night blindness during pregnancy in Nepal, 8 India, and Laos. 2 Similarly, studies in pregnant women from Nepal, Philippines, Zambia, Ghana, and Mali have reported prevalence values for subclinical VAD (serum retinol 20 g/dl) from 19 38%. 2,9 Low levels of breast milk retinol are also common in many developing countries. 10 The use of this indicator in eld-based surveys merits further attention as it may serve as an appropriate, less invasive, and culturally acceptable indicator of maternal and infant vitamin A status. Iodine De ciency Disorders (IDD) Endemic goiter and cretinism are the well-known clinical signs of iodine de ciency and it is estimated that globally 740 million are affected by goiter. 1 The prevalence of IDD based on total goiter rates (visible and palpable) is the highest in the Eastern Mediterranean region (32%), followed by Africa (20%), Europe (15%) and Southeast Asia (12%). However, it is important to recognize that these clinical signs are seen only in severe cases, and subclinical de ciency, which is also associated with a range of intellectual and behavioral de cits, affects many more individuals. Nearly one-third of the world s population (2.23 billion) is considered to be at risk for iodine de ciency, of which a large proportion live in Southeast Asia. 1 The global prevalence of the percentage at risk of iodine de ciency is shown in Table 1. The commonly used indicators of subclinical de - ciency include blood concentrations of thyroid stimulating hormone (TSH), and the thyroid hormones, thyroxin (T 4 ) and 3, 5, 3 triiodothyronine (T 3 ). 11 Urinary iodine has also been used widely in recent times, especially to evaluate progress of IDD control efforts, as it is cheaper, less invasive, and relatively easy to measure in population-based surveys. An individual is usually classi ed as having severe, moderate, mild, or no IDD if the median urinary iodine values are 20, 20 49, and 100, respectively. At the population level, the currently accepted indicator of IDD elimination is if 50% of the samples have a median value 100 g/dl and not more than 20% have a median value 50 g/dl. 11 The IDD control program has been very successful in the past decade with the widespread adoption and coverage of iodized salt worldwide. Nearly 90% of all salt is adequately iodized in Latin America and the Caribbean. Asia, sub-saharan Africa, and Near East and North Africa have coverage rates of 65 75%, 50 74%, and around 50%, respectively. However, comparison of prevalence gures between 1990 and 1998 indicate little change, which may be explained in part by increased efforts to identify the problem, suitability of the indicator, and issues with quality control. 3,4,11 Nevertheless, the stewardship of the salt industry in this effort has to be commended and there is increasing evidence of regional success stories in parts of Asia and Africa. For example the national goiter rate in China has been almost halved from 20% in 1995 to approximately 10%. 4 Iron De ciency Iron de ciency is by far the most widespread nutrient de ciency in the world, affecting more than 2 billion persons. 1 Severe iron de ciency results in anemia, which is the most widely used indicator of this de ciency. Although iron de ciency is the main cause of anemia in most settings, anemia may be the result of other nutrient de ciencies such as vitamin B 12 and folate as well as non-nutritional causes such as malaria, genetic abnormalities (e.g., thalassemia), and chronic disease. 5 However, iron de ciency, like other micronutrient de ciencies, develops over time. Three stages of iron de ciency have been described: (1) iron depletion during which iron stores are reduced as indicated by lowered values of serum ferritin (2) iron de cient erythropoeisis when iron stores are exhausted and there is decreased transferrin saturation and increased erythropoietin (FEP), and (3) iron de ciency anemia, which is characterized by low hemoglobin and is usually microcytic, hypochromic anemia. 12 The recommended cut-off values for hemoglobin levels used to de ne anemia, however, vary by age, race, and physiologic status (Table 2), and adjustments for smoking and altitude have also been recommended. 13 Although these values are useful to de ne and compare Nutrition Reviews, Vol. 60, No. 5 S47

3 Table 2. Recommended Cut-off Values for Hemoglobin Levels Used to De ne Anemia 1 1 Age/Sex Groups Hemoglobin Level (g/l) Children 6 59 months 110 Children 5 11 years 115 Children years 120 Nonpregnant women 120 Pregnant women 110 Adult males 130 the magnitude of the problem, they often provide a limited view. In many settings, comparison of the entire hemoglobin distribution of the affected population with that of well-nourished populations clearly indicates that the entire population is affected. The populations most at risk for iron de ciency and IDA are young children and women of reproductive age, especially during pregnancy (Figure 1). This is due to the increased physiologic requirements, combined with increased losses and poor dietary intakes. Examination of the global prevalence of anemia among children and women of reproductive age in different regions of the world indicates that more than half the pregnant women and young children are anemic in Southeast Asia, West Paci c and Africa (Table 3). 6 Although the prevalence of anemia is slightly lower among school-age children and nonpregnant women, it is still high enough to deserve intervention strategies. These high rates of anemia, especially in Southeast Asia, clearly indicate that it is likely that almost the entire population has some form of iron de ciency. For example, the prevalence of anemia among men and the elderly is quite high in developing countries when compared with industrialized countries (Figure 1). However, efforts in the prevention and control of iron de ciency have been slow and more work is needed to understand the epidemiology of anemia, and, more importantly, to identify and implement successful strategies. Even in industrialized nations and the Americas, pregnant women and young children are at risk of iron de ciency, and widespread forti cation of common foods such as cereals and wheat our has been identi ed as the reasons for successful control of iron de ciency. In recent times, there has been some discussion regarding the cut-off values used to de ne anemia. Stoltzfus 14 has shown that, even though the overall prevalence of anemia may be high and similar in different settings, most of it is mild and differences are seen in the prevalence of moderate-severe anemia (Hb 90 g/dl). For example, the prevalence of moderate-severe anemia among pregnant women was 4 5 times greater in Nepal (21%) compared with Java (6%) and Peru (4%). These differences have implications for programs and have yet to be discerned. Zinc De ciency Although zinc de ciency is increasingly being recognized as a widespread problem, there is very limited nationally representative data on the magnitude and severity of this de ciency. Some of this is due in part to the lack of reliable biomarkers of zinc status. 15 Zinc is present throughout the body and is found in over 200 metalloenzymes. Plasma zinc is the most commonly used indicator, but it is in uenced by infections and diurnal variations. Despite these limitations, there is increasing evidence of widespread subclinical de ciency based on plasma zinc values and the prevalence of inadequate zinc intakes, especially in young children. Data from several trials, including those included in the recent meta-analysis on the effect of zinc supplementation on young child morbidity, indicate widespread zinc de ciency. 16 Mean plasma zinc values in the control groups of children ranged from 65 g/dl in India to 95.8 g/dl. In many sites, for example, Peru, Pakistan, and Papua, New Guinea, the mean values were close to 70 g/dl, the recommended cut-off value for de ciency. Even less data are available for women of reproductive age. Studies from Peru 17 and Indonesia 18 have shown that the prevalence of zinc de ciency among pregnant and postpartum women were 60% and 24%, respectively. Zachar et al. 19 recently reported in a small study (n 18) that nearly one-third of adult women in a semirural Guatemalan community had serum levels below 70 g/dl, which is considered indicative of de - ciency. In terms of dietary intakes, a major concern has been the bioavailability of zinc from different sources. Phytate, a potent inhibitor of zinc absorption, is present in high levels in the cereal-based diets of many developing countries. Data from the Collaborative Research Study Project (CRSP) studies showed that the prevalence of inadequate zinc intakes among young children, assuming either basal or normal requirements, were highest in Kenya (57% and 90%, respectively), followed by Mexico (25% and 68%, respectively), and Egypt (10% and 36%, respectively). 20 Studies conducted in Brazil, Guatemala, Mexico, India, Nepal, Nigeria, Malawi, Egypt, and Kenya have also reported that mean zinc intakes are less than two-thirds of the RDA among women of reproductive age. 21 More recently, Wuehler et al., 7 using data from national food balance sheets compiled by the Food and Agricultural Organization, estimated that nearly half the world s population is at risk of low zinc intakes. The global prevalence of low intakes by region (Table 1) indicates that almost the entire population in South Asia (95.4%) is at risk, followed by Southeast Asia (71.2%), S48 Nutrition Reviews, Vol. 60, No. 5

4 Figure 1. Prevalence of anemia among different age-sex groups by level of development. 6 Africa ( %), and Latin America (45.8%). These estimates accounted for the bioavailability of zinc by correcting for phytate:zn molar ratios, and also assumed a 25% coef cient of variation. Although these values may be biased and may overestimate the true magnitude of zinc de ciency, they support the evidence based on biochemical indicators and clearly indicate that this de- ciency is a major public health problem that needs to be addressed, especially in light of the evidence indicating adverse consequences in young children. Other Vitamin De ciencies The global prevalence of other vitamin de ciencies is unknown due to limited nationally representative data. 11 There is some evidence, however, of subclinical de - ciencies of vitamin B 6, vitamin B 12, folate, and ribo avin in different age-sex groups. 5 Surveys conducted in several countries in the 1970s concluded that vitamin B 12 de ciency was not a major problem, with the exception of India where animal intakes are extremely low. 22 However, there are several concerns regarding the methods used and more recent studies using improved techniques have shown that it is more prevalent than was earlier presumed. 5 The CRSP studies have reported the prevalence of vitamin B 12 de ciency was 18% and 41% among preschool-age children in Kenya and Mexico, respectively. 23 These studies have also shown that, among young children and women of reproductive age, the average dietary intakes of vitamins B 6 and B 12 were also below two-thirds of the Recommended Dietary Allowances, suggesting an increased risk of de ciency. 23,24 Recent studies from Mexico and Guatemala have also found some degree of vitamin B 12 de ciency ranging from 20 50% in many segments of the population. 15 In developed countries such as the United States, the elderly have been identi ed as being at risk for vitamin B 12 de ciency. 5 The prevalence estimates from the most recent nationally representative Third Nutrition and Health Examination Survey (NHANES III) show that 7% of elderly have subclinical vitamin B 12 de ciency. 25 There is surprisingly little data on the current situation of folate de ciency, especially in developing countries. Most of the surveys that have shown that folate de ciency is common during pregnancy in Africa and Asia were conducted more than 20 years ago. 26,27 In contrast, the United States has current data on folate status. The overall prevalence of low serum folate ( 3 ng/ml) was approximately 15% during with higher rates among non-hispanic blacks and Mexican- Americans. However, there is preliminary evidence that folate status has improved following the forti cation of cereals that began in Recent studies indicate that folate de ciency may not be a concern in parts of Latin America where anemia is still a problem, 5 but it is a concern among women of South Asian origin. 28,29 Ribo avin de ciency is quite common in many parts of the developing world and the clinical signs of angular stomatitis have been reported in several surveys. However, it has received much less attention, and there is sparse data on the extent of this de ciency, especially in Nutrition Reviews, Vol. 60, No. 5 S49

5 Table 3. Global Prevalence of Anemia Among Children and Women of Reproductive Age 6 Region Children (%) Adult Women (%) Preschool (0 4 years) School age (5 14 years) Pregnant Non-pregnant Southeast Asia West Paci c Africa East Mediterranean Americas Europe the subclinical form. Ribo avin status can be easily measured based on either urinary or serum levels, but is affected by day-to-day variation. Studies in India, 29 China, 30 Guatemala, 31 and the Gambia 32 have found that 50% of pregnant and/or lactating women had subclinical ribo avin de ciency. Multinutrient De ciencies Although there has been much interest in multiple micronutrient de ciencies recently, little is known about the magnitude and signi cance of this problem. 33 The evidence on interactions between micronutrients, however, clearly indicates a need for more work in this area. For example, studies have shown that vitamin A and ribo a- vin play a role in iron metabolism; the provision of these nutrients along with iron signi cantly improved iron status and reduced anemia in young children and pregnant women when compared with those who received only one of the nutrients. 5 A recent study from Zaire also demonstrated how iron status can affect the effectiveness of an iodine intervention to reduce goiter. 34 At the country level, it is obvious that several countries have more than one micronutrient de ciency. For example, nearly three-fourths of the 49 countries in which the prevalence of undernutrition (wt/age 2 SD of NCHS median) is greater than 20%, have at least two micronutrient de ciencies, and almost half have three namely vitamin A, iron, and iodine de ciency. However, few have examined whether clustering of micronutrient de ciencies occurs at the individual level. Dietary intakes from CRSP data provide evidence of inadequate intakes of several nutrients in the three study sites, Kenya (zinc, calcium, ribo avin, and vitamins A, E, and B 12 ), Mexico (zinc, iron, thiamin, ribo avin, and vitamins A, E, C, and B 12 ), and Egypt (iron, calcium, ribo avin, and vitamins A and E). 23,24 In a recent analysis of nationally representative data from Honduras, Albalak et al. 35 found no evidence of an increased risk of having another de ciency if one is present, beyond what might be predicted by chance. Nevertheless, nearly three-fourths of all preschool-age children had at least one de ciency, usually VAD, anemia, or stunting, and one- fth had both VAD and anemia. Studies from Mexico, Thailand, and India also suggest that multinutrient de ciencies are common, and the prevalence of two or more micronutrient de ciencies has been estimated at 10 25% among preschool-age children. 3,21,33 Conclusions Most of our information on the global prevalence of micronutrient de ciencies to date is limited to one population (young children) and three nutrients (vitamin A, iron, and iodine). Without doubt, there is a need for nationally representative data on the prevalence of subclinical de ciencies of other nutrients such as the B vitamins and zinc, and multiple micronutrient malnutrition; and among other age-sex groups, such as adolescents and women of reproductive age who are also at risk of adverse consequences. Interventions for the prevention and control of micronutrient malnutrition typically have focused on pregnant women and young children who are most vulnerable to these de ciencies and their adverse effects, and are a combination of one or more strategies supplementation, forti cation, other food-based strategies, and public health interventions. However, there is increasing evidence for adopting a lifecycle approach as many of these de ciencies do not develop overnight, with the exception of famines and emergencies. 1 In a recent report on the progress of controlling micronutrient de - ciencies, Mason et al. 3 concluded that, while considerable efforts have been made in the past decade, especially in the prevention and control of VAD and IDD, there remains a need for reliable, nationally representative data that is systematically collected at more than one point in time. Speci cally, the development and eld testing of innovative, simple, low-cost techniques will permit us to assess micronutrient status easily, and thereby help identify subgroups at risk, appropriately target interventions, and monitor and evaluate progress in our efforts to prevent and control these de ciencies. For example, the recent inclusion of hemoglobin estimation using the simple portable hemoglobinometer, i.e., Hemocue TM in the Demographic Health Surveys in many countries has already begun to generate valuable nationally representative data on the prevalence of ane- S50 Nutrition Reviews, Vol. 60, No. 5

6 mia in young children and women of reproductive age. Although several micromethods have been developed, they often require adequate laboratory facilities and the collection of at least L of capillary blood by nger prick for each nutrient of interest. This is not feasible in many programmatic settings because of technical reasons as well as cultural barriers to blood collection, especially in young children. Without doubt, methods that minimize the amount of blood to be collected and simplify the handling and analyses of specimens would be of great value. For example, the use of dried blood spots to estimate various micronutrients, namely folate, retinol, and ferritin, would be of great value. In summary, investing in improving the technical expertise to assess micronutrient status and implement and evaluate effective programs will help achieve the goals of reducing this preventable condition of micronutrient malnutrition worldwide. 1. ACC/ SCN. 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