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MedGenMed. 2006; 8(4): 59.
Published online Dec 19, 2006.
PMCID: PMC1868363

Associations Between Body Mass Index and the Prevalence of Low Micronutrient Levels Among US Adults

Joel E. Kimmons, PhD, Division of Nutrition and Physical Activity, Heidi Michels Blanck, MS, PhD, Division of Nutrition and Physical Activity, Beth Carlton Tohill, MPH, PhD, Division of Nutrition and Physical Activity, Jian Zhang, DrPH, Division of Nutrition and Physical Activity, and Laura Kettel Khan, PhD, Division of Nutrition and Physical Activity

Abstract

Background

Low micronutrient levels observed with increasing adiposity may result from inadequate nutrient intake and/or alterations in nutrient metabolism.

Objective

To examine the association between body mass index (BMI) and micronutrient levels among a nationally representative sample of US adults aged ≥ 19 years.

Design

Using nationally representative cross-sectional data from the National Health and Nutrition Examination Survey III (NHANES III), we examined odds ratios of low micronutrient levels using logistic regression adjusting for covariates.

Measurements

Nutritional biomarker levels (as indicated by serum levels of total carotenoids, alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein/zeaxanthin, lycopene, vitamin E, vitamin C, selenium, vitamin A, vitamin D, folate, vitamin B12, and red blood cell folate) among men and nonpregnant women, by BMI category.

Results

Overweight and obese adults had higher odds of low levels for a number of nutrients compared with normal-weight adults. Odds of being low in multiple micronutrients was most common among overweight and obese premenopausal women.

Conclusion

These findings underscore the need for further assessment of specific micronutrient inadequacies among persons who are overweight or obese. Specifically, research is needed to determine whether these inadequacies are due to insufficient dietary intake, altered metabolic processes, or both.

Introduction

More than 60% of Americans are overweight or obese, conditions that are associated with increased risk for hypertension, dyslipidemia, type 2 diabetes, coronary heart disease, stroke, and certain cancers.[1,2] Because overweight or obese individuals consume adequate total energy, low micronutrient levels may result from inadequate nutrient intake (diet, supplements) and/or alterations in nutrient absorption or metabolism.[35] Aspects of being overweight or obese have been shown to alter the absorption, distribution, metabolism, and/or excretion of micronutrients.[69] For example, among persons who are obese, vitamin D has been shown to have decreased bioavailability from cutaneous and dietary sources and possibly to be sequestered by adipose tissue.[4,9] Also, thiamin metabolism of obese persons appears to be altered, leading to a decrease in cellular absorption and an increase in intracellular conservation.[6]

Concentrations of biochemical markers of nutritional status have also been associated with increases in body mass index (BMI) and other measures of adiposity.[919] However, these associations have been inconsistent in direction and magnitude, and the studies have often been limited by small sample sizes, the examination of relatively few subpopulations, and/or the examination of only a few micronutrients.

Our objective in this study was to describe the prevalence and estimate the odds of low biochemical nutritional levels among normal, overweight, and obese adults using nationally representative data from the Third National Health and Nutrition Examination Survey (NHANES III: 1988–1994).

Methods

Sample

Our sample included adults ≥ 19 years with BMI ≥ 18.5 from NHANES III, a cross-sectional nationally representative survey of the US civilian, noninstitutionalized population conducted between 1988 and 1994. The survey employed a complex, stratified, multistage probability cluster sampling design.[20] All nutritional biomarkers were measured in serum samples with the exception of red blood cell (RBC) folate. The clinical and laboratory methods used to evaluate NHANES III samples have been described previously.[21]

In addition to excluding underweight NHANES III participants, those with a BMI <18.5 (n = 378), we also excluded those whose sex-specific values were extreme for weight, height, or BMI (upper or lower 0.1 percent, n = 52), and women who were pregnant (n = 243) or lactating (n = 91). The sample sizes for each biochemical measure varied slightly because participants missing values for a particular micronutrient were not excluded from the overall analysis. After these exclusions, our analytic sample consisted of 16,191 participants.

Variables

The sample was stratified by sex, BMI category, and either age group (men) or menopausal status (women). Men were categorized as 19.0–64.9 years of age (n = 5875) or ≥ 65 years of age (n = 1933). Women were categorized as premenopausal (n = 4327) or postmenopausal (n = 3810) to account for the potential influence of the hormonal changes. Premenopausal women were defined as women who answered yes to the question “Have you had a period in the last 12 months?,” and no to the questions “Were both ovaries removed?” and “Has your uterus/womb been removed?” All women not defined as premenopausal were considered postmenopausal. The rationale for the age categories for men was based on physical, hormonal, and lifestyle changes associated with aging and the life event of retirement. Study subjects' BMI was calculated by dividing their measured weight in kilograms by the square of their measured height in meters, and they were categorized as normal (BMI 18.5–24.9), overweight (BMI 25.0–29.9), and obese (BMI ≥ 30) in accordance with the National Heart, Lung, and Blood Institute (NHLBI) guidelines.[2] Their race/ethnicity was categorized as non-Hispanic white, non-Hispanic black, Mexican American, or other.

We defined low biochemical nutrient status using cutoffs approximating marginal deficiency, with the exception of vitamin E and carotenoids. Cutoff values were (≤ 30 ug/dL for vitamin A, < 0.4 mg/dL for vitamin C, ≤ 299 pg/mL for vitamin B12, < 100 ug/L for selenium, ≤ 15 ng/mL for vitamin D, < 3.0 ng/mL for serum folate, and < 109 ng/mL for RBC folate.[2224] Marginal RBC folate status was estimated by using the National Center for Health Statistics (NCHS) correction factors applied to the upper end of the deficient range.[24] Because there is no generally accepted value for low levels of vitamin E and the carotenoids (alpha carotene, beta-carotene, beta-cryptoxanthin, lutein /zeaxanthin, and lycopene), we used the 20th percentile by age and sex among NHANES III participants as our cutoff. Finally, because the concentration of carotenoids and vitamin E varies with lipid levels, we also adjusted measured levels of vitamin E and the carotenoids for total cholesterol.[25]

Data Analysis and Statistical Methods

We used SUDAAN software (SAS-callable version, 8.0.2, Research Triangle Park, NC) with appropriate weighting and nesting variables. We examined categorical outcomes and calculated odds ratios (OR) and their corresponding 95% confidence intervals (CI) for low vs sufficient levels of each biomarker, using logistic regression to adjust for age (continuous within categories), race/ethnicity, hormone therapy/oral contraceptive use (women), and serum cotinine concentration (continuous). We adjusted for cotinine levels as a marker of tobacco exposure because of its inverse association with levels of numerous nutritional biomarkers, including vitamin C, vitamin E, selenium, and the carotenoids.[26] The referent BMI category was normal weight (BMI 18.5–24.9). Linear trend tests were calculated for the adjusted prevalence estimates, and statistical tests were determined to be statistically significant at a P value of .05.

Results

The prevalence of overweight and obesity by sex and age is shown in Table 1 . The prevalence and odds of low micronutrient levels by BMI category are presented by sex and age group in Table 2 for women and Table 3 for men.

Table 1
Characteristics of NHANES III Participants Examined for Micronutrient Adequacy, by BMI Category
Table 2
Prevalence and Odds of Low Micronutrient Levels Among US Women, by BMI Category and Menopausal Status, NHANES III (1988–1994)
Table 3
Prevalence and Odds of Low Micronutrient Levels Among US Men, by BMI Category and Age, NHANES III (1988–1994)

In the normal weight groups, the prevalence of low micronutrient levels was ≥ 20% for several micronutrients, including vitamin C among premenopausal women (Table 2); vitamin C, vitamin E, and folate among men aged 19.0–64.9 years; and vitamin C, vitamin E, beta-cryptoxanthin, lycopene, and vitamin B12 among men aged 65 years or older (Table 3).

Among women, increasing BMI category was associated with low biochemical micronutrient levels as evidenced by significant linear trend tests for vitamin E, alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein/zeaxanthin, lycopene, total carotenoids, vitamin C, selenium (premenopausal), vitamin D, and folate (Table 2).

Among men aged 19.0–64.9 years, increasing BMI category was associated with low biochemical micronutrient levels as evidenced by significant linear trend tests for alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein/zeaxanthin, total carotenoids, vitamin C, selenium, and folate (Table 3). Among men aged 65 years and older, increasing BMI category was associated with low biochemical nutritional status as evidenced by significant linear trend tests for beta-carotene, lutein/zeaxanthin, and total carotenoids (Table 3).

Discussion

In this cross-sectional analysis of a nationally representative sample of American adults, we found that those who were overweight or obese, particularly premenopausal women, were more likely to have low levels of various micronutrients than were normal-weight adults in the same sex/age category. Our findings are similar to those of other studies that have assessed the relationship between adiposity and levels of a more limited number of nutrients.[919]

We found a higher prevalence of low carotenoid levels among overweight and obese persons of both sexes and a higher prevalence of low vitamin E levels among women. Mechanistically, beta-carotene and vitamin E are important in protecting LDL against oxidation, which is recognized as an initial step in the development of atherosclerosis.[2737] Low carotenoid and alpha-tocopherol serum levels have also been associated with increases in oxidative stress, insulin resistance, impaired glucose metabolism, some cancers, and age-related macular degeneration.[5,3846] We speculate that the observed low carotenoid and vitamin E levels among overweight and obese persons may result from the increased systemic and adipose tissue-specific oxidative stress found in overweight persons, which may lead to increased oxidative catabolism of these lipid-soluble nutrients.[5,7,47] Mechanisms that may lead to an increase in oxidative stress in the adipose tissue of the obese include an upregulation of the renin-angiotensin system and a reduction in the erythrocyte glutathione and glutathione peroxidase antioxidant defense mechanisms.[4851] Increased adipocyte oxidative stress may lead to systemic increases in reactive oxygen species, which in turn may lead to obesity-related metabolic syndrome and other conditions related to oxidative stress, such as age-related macular degeneration.[7,48]

The one exception we found to the generally positive correlations between BMI category and the prevalence of low micronutrient levels was that overweight men aged 19.0–64.9 had a lower odds of low vitamin E status than normal-weight men. Although there may be a biological explanation for this finding, we suspect that it was due to chance, given that in the other 3 age/sex groups we found a positive correlation between BMI category and the prevalence of low vitamin E levels.

This study consistently found higher odds of low vitamin C levels in obese compared with normal-weight persons. The results of population studies suggest that higher intakes and serum levels of vitamin C are associated with a lower risk for some chronic diseases.[5,5256] Although dose responses to vitamin C or differences in dietary requirements at different BMIs have not been demonstrated, higher intakes of vitamin C are required to achieve the same vitamin C status in persons of increased body weight.[57] Again, but this time in a water soluble-nutrient, increased oxidative stress associated with obesity may lead to vitamin C destruction contributing to the observed lower levels among obese persons.[47,48]

The lower selenium levels observed in this study among the overweight and obese may also reflect an increase in oxidative stress in obese and, possibly, overweight persons. However, the overall prevalence of low selenium levels in our study population was low. Low selenium levels have been associated with increased rates of a variety of cancers and with high homocysteine levels, an independent risk factor for stroke and vascular disease.[5862] The results of a recent pooled analysis showed an inverse association between blood selenium concentrations and rates of colorectal adenoma.[63] Furthermore, numerous studies have found low selenium levels to be associated with increased rates of prostate cancer.[6366] The cutoff value signifying low selenium of < 100 ug/L, as used in this study, is at the lower end of estimated values optimal to prevent cancers; however, slightly lower values may be sufficient for preventing vascular disease and optimizing immune function.[58] Thus, glutathione peroxidases, selenium-dependent antioxidant enzymes, are maximized at serum levels above 100 ug/L.[6769] As mentioned earlier, low levels of glutathione peroxidases have been associated with obesity and indicate an elevated oxidant stress.

The odds of low vitamin D levels increased with increasing BMI among women in our study. These results agree with previous studies, including those which used other measures of adiposity.[9,19,70] Suggested mechanistic explanations for lower vitamin D levels among obese persons include adipose sequestration and a decrease in the bioavailability of vitamin D from both cutaneous and dietary sources.[4,9,71] Although the sequestration of vitamin D into adipose tissue would eventually lead to saturation, it is possible that the metabolic catabolism of vitamin D may increase as it builds up in adipose tissue.

The increased odds of low folate levels among obese premenopausal women may help explain why they are at a higher risk than normal-weight women for cancer, cardiovascular disease, and birth-defect-affected pregnancies. Mechanistically, low folate levels may lead to inefficiencies in purine and pyrimidine synthesis, methionine regeneration, and amino acid metabolism.[7276] The lack of an association between RBC folate and BMI suggests a difference in the metabolic handling of folate by obese persons. The observed low serum folate but adequate RBC levels may suggest adequate storage but inability to mobilize and circulate folate stores.

Although there is limited research to support a particular theory of why micronutrient levels would be associated with BMI, it is probable that different mechanisms explain nutrient-specific low levels among the overweight and obese, particularly for water- vs lipid-soluble nutrients. One or more of the following mechanisms may contribute to the observed differences: absorption, excretion, storage/distribution (fat sequestering, tissue dispersion), metabolism (catabolic loses, possibly oxidative), increased physiologic requirements, and lower absolute total dietary intake.[47,9,48]

Limitations of this study include the cross-sectional nature of NHANES, which prevents directional conclusions or causality about the relationship between BMI and nutrient levels. Also, it is possible that a more precise or specific measure of adiposity could alter these results. For example, Snijder and colleagues[9] found stronger associations between percent adiposity and vitamin D status with total body fat (as measured by DEXA) than BMI. Strengths of this study include the use of data from a large nationally representative survey of adults that included measured weight and height and a wide range of biochemical micronutrient assessments.

In conclusion, this study demonstrates a pattern of low micronutrient levels among overweight and obese adults compared with normal-weight adults. Odds of being low in multiple nutrients were the highest among overweight and obese premenopausal women. Whether the relatively high prevalence of low micronutrient levels that we found among persons who are overweight or obese is due to insufficient micronutrient intake, altered metabolic processes, or both should be further investigated in order to better understand the mechanisms underlying the association between adiposity and various chronic diseases.

Acknowledgments

The authors would like to thank Christine Pfeiffer, PhD and Rosemary Schleicher, PhD for their expertise and assistance in laboratory method interpretation.

Contributor Information

Joel E. Kimmons, Centers for Disease Control and Prevention, Atlanta, Georgia.

Heidi Michels Blanck, Centers for Disease Control and Prevention, Atlanta, Georgia.

Beth Carlton Tohill, Centers for Disease Control and Prevention, Atlanta, Georgia.

Jian Zhang, Centers for Disease Control and Prevention, Atlanta, Georgia.

Laura Kettel Khan, Centers for Disease Control and Prevention, Atlanta, Georgia.

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