Relationship of vitamin D and parathyroid hormone with obesity and body composition in African Americans

Abstract

Background: Obesity disproportionately affects African Americans (AA) (especially women), and is linked to depressed 25-hydroxyvitamin D (25-OH D) and elevated parathyroid hormone (PTH). The relationship of 25-OH D and PTH with body composition and size in AA is not well known.

Objective: To determine the relationship of 25-OH D and PTH levels with body composition and anthropometric measures.

Design: A cross-sectional study was conducted in 98 healthy, overweight, adult AA enrolled in an NIH/NIEHS-sponsored weight loss/salt-sensitivity trial.

Measurements: Multivariable linear regression analyses were used to explore the relationship of 25-OH D and PTH with body composition, determined by dual-energy X-ray absorptiometry, and anthropometric measures. Body composition and size were contrasted across vitamin D/PTH groups using general linear models: (i) normal (25-OH D >50 nmol/l, PTH <or=65 pg/ml), (ii) low 25-OH D and normal PTH and (iii) low 25-OH D and high PTH.

Results: Age, gender and season-adjusted regression analyses showed that PTH was directly correlated with total (P = 0.02), truncal (P = 0.03) and extremity (P = 0.03) fat mass, while 25-OH D was inversely related to truncal fat mass (P = 0.02). Total fat mass in groups 1-3, respectively, was 30.0, 34.0 and 37.4 kg (P = 0.008); truncal fat mass was 13.4, 15.9 and 17.6 kg (P = 0.006) and extremity fat mass was 15.8, 16.9 and 19.7 kg (P = 0.02). Lean mass did not differ across the three groups.

Conclusions: Our findings show that lower 25-OH D and raised PTH are both correlated, though in opposite directions, with fat mass, fat distribution and anthropometric measures in adult AA.

Figure 1. Fat Distribution across the Vitamin D/PTH Groups

Least square means with corresponding standard errors of fat mass across the vitamin D/PTH groups using general linear models adjusted for age, sex and season. Total, truncal and extremity fat mass were incrementally higher from the normal to the most abnormal vitamin D/PTH groups to a significant degree. Bonferroni’s correction detected significant differences between (*) the normal and SHPT groups and between (+) the normal and LN groups. a: p = 0.01 versus LN + SHPT; b: p = 0.003 versus LN + SHPT; c: p = 0.08 versus LN + SHPT. Normal = normal 25-OH D and normal PTH (N = 24); LN = low 25-OH D and normal PTH (N = 63); SHPT = secondary hyperparathyroidism group or low 25-OH D and high PTH (N = 11). Normal 25-OH D > 50 nmol/l; Normal PTH ≤ 65 pg/ml.