Vitamin D Ancillary Study in 5 Populations of African Origin

Through its key role in bone mineral metabolism vitamin D deficiency was one of the first nutritional deficits linked to a clinical disease. A range of population and clinical research has now implicated less severe vitamin D deficiency as a potential risk factor for several chronic diseases [Melamed, et al. J Am Soc Nephrol 2009;20:2631-9; Michos et al. Future Cardiol, 2009;5:15-8; Reis et al. Diabetes Metab, 2005;31:318-25] and all-cause mortality [Melamed et al. Arch Intern Med, 2008;168:1629-37; Durazo-Arvizu et al. International Workshop on Statistical Modeling(IWSM);2010] is significantly increased at the low end of the 25(OH)D distribution among blacks and whites in the follow-up data from NHANES III. One out of 2 adults in the US has a 25(OH)D serum level < 30 ng/ml [Ginde et al. Arch Intern Med, 2009;169:626-32], currently considered the threshold for health-promoting levels [Howe et al. N Engl J Med, 2007;357:1981-2]. Light skin facilitates formation of 25(OH)D and consequently only 3% of US blacks have "optimal" levels, with very low levels (<10 ng/mL) found in 29% of blacks, compared to 5% of whites and10% of Mexican Americans [Ginde et al. Arch Intern Med, 2009;169:626-32]. Concern exists therefore that low serum levels of 25(OH)D may contribute to the disproportionately high risk for some chronic diseases in US blacks (eg, obesity, hypertension and CVD) [Pilz et al. Nat Rev Cardiol, 2009;6:621-30; Wang et al. Circulation, 2008;117:503-11]. At the present time the most convincing body of data on 25(OH)D and risk of chronic disease exists for obesity; the study proposed here will be conducted as an extension of a major international project on determinants of weight gain in Afro-origin populations.

25(OH)D has regulatory functions in several physiologic systems. Paradoxically, while US blacks have relatively lower serum 25(OH)D they have higher bone mineral density (BMD) and lower fracture rates, suggesting important variation in metabolic set points across populations [Aloia, Am J Clin Nutr, 2008;88:545S-550S]. However, it is not known if US blacks are at increased risk from the non-skeletal effects of low 25(OH)D. The overall health consequences of this shift in 25(OH)D levels and the threshold of "normal" for US blacks are thus unknown. The primary aim of this study is therefore to define the optimal level of 25(OH)D and the appropriate threshold for supplementation in the multi-ethnic US population. These data could have an important impact on public health recommendations regarding health-promoting levels of 25(OH)D.

Vitamin D levels in West Africans are similar to US whites, and much higher than in US blacks (see preliminary data). These cross-population contrasts represent one of the most significant known examples of a gene-environment interaction. By sampling Afro-origin populations across a range of latitudes, Ca++ intake, diet patterns, and lifestyles the investigators will model the determinants of 25(OH)D and related health effects. This research will incorporate 3 novel aspects. First, data on 25(OH)D will be standardized and a set of key determinants of 25(OH)D levels from 5 geographically separated populations of African descent will be available. Second, the study will generate precise covariate measures for obesity and other CV traits to test prospectively the association with 25(OH)D. Third, the investigators will have DXA measurements to examine 25(OH)D - PTH - bone density relationships and address a fundamental question about regulation of mineral metabolism and bone health. A major strength of this study will be state-of-the-art measurements of covariates for obesity, including doubly labeled water, activity monitoring, body composition and dietary intake. The role of 25(OH)D in obesity risk can therefore be evaluated with control for all of the key determinants of energy balance.

This study will utilize existing data and fasting samples from an ongoing NIH-funded study [Modeling the Epidemiologic Transition Study (METS; DK080763)]. METS began recruiting 2500 adults of African ancestry ages 25-45 from 5 countries (500 each from the US, Jamaica, Seychelles, South Africa, and Ghana) in October 2009. This study extends the follow-up period currently funded by METS to 6.2 years and uses a combined cohort-ecologic design to examine within-person and between-group associations.

Specific Aims:

1. Measure serum 25(OH)D and intact parathyroid hormone (iPTH) levels in 500 adults from the 5 METS sites (total N=2,500) at baseline and determine associations with latitude, diet, physical activity, adiposity, blood pressure (BP), and biochemical risk factors for CVD (eg, insulin, glucose, adiponectin, leptin, cholesterol).
2. Examine the association between baseline serum 25(OH)D and iPTH levels and changes in body composition and blood pressure over 6.2 years of follow-up.
3. Repeat measurement of serum 25(OH)D and iPTH at third follow-up examination (approximately 3.5 years after baseline) and measure BMD using DXA in the available cohort.