Demographic, dietary, and biochemical determinants of vitamin D status in inner-city children

Abstract

Background: Reports of clinical rickets are particularly evident in minority infants and children, but only limited analyses of vitamin D are available in this demographic group.

Objective: We sought to characterize circulating 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D [1,25(OH)(2)D], and their determinants, including circulating parathyroid hormone (PTH), total alkaline phosphatase activity (ALP), calcium, and phosphorus, in minority infants and children.

Design: We obtained demographic information and blood samples for measurement of PTH, ALP, 25(OH)D, and 1,25(OH)(2)D in >750 6-mo- to 3-y-old children. Dietary intake data were obtained and analyzed.

Results: The mean (±SD) 25(OH)D concentration was 66 ± 22 nmol/L (26.3 ± 8.7 ng/dL). A total of 15% of children had 25(OH)D concentrations less than the recommended target threshold of 50 nmol/L. Combined elevations of PTH and ALP occurred in only 2.5% of children. Determinants of 25(OH)D included vitamin D intake, age (decreasing with age), skin type (greater concentrations in lighter-skinned children than in darker-skinned children), formula use (higher intakes), season (greater concentrations in the summer and fall than in the winter and spring), and, inversely, PTH. The mean 1,25(OH)(2)D concentration was 158 ± 58 pmol/L (60.6 ± 22.5 pg/mL), which was consistent with a reference range of 41-274 pmol/L or 15.7-105.5 pg/mL. Determinants for 1,25(OH)(2)D were age (decreasing with age), sex (greater concentrations in girls than in boys), skin type (greater concentrations in lighter-skinned children than in darker-skinned children), and, inversely, serum calcium and phosphorus.

Conclusions: Although 15% of subjects were vitamin D insufficient, only 2.5% of subjects had elevations of both PTH and ALP. The greater 25(OH)D concentrations observed with formula use confirm that dietary vitamin D fortification is effective in this demographic group. Circulating 1,25(OH)(2)D is higher in infants than in older children and adults and, in contrast to 25(OH)D, is not directly correlated with nutrient intakes.

FIGURE 1. Biochemical variables.

Values for biochemical variables are shown by using box plots of 25(OH)D (A), calcium (Ca) (B), phosphate (P) (C), ALP (D), PTH (E), and 1,25(OH)₂D (F). The 25th, 50th, and 75th percentiles for each variable are represented by the bottom, middle line, and top of the box, respectively. The lower and upper whiskers extend to the 10th and 90th percentiles, respectively. The mean value for the population is represented by an asterisk. Circles represent values less than the 10th or greater than the 90th percentile; because the population was large, one circle may represent multiple subjects for a given extreme value. 25(OH)D concentrations were <50 nmol/L (20 ng/mL) in 15% of children, in 10% of children 0–12 mo of age, and in 16% of children >12 mo of age. ALP, total serum alkaline phosphatase activity; PTH, parathyroid hormone; 1,25(OH)₂D, 1,25-dihydroxyvitamin D; 25(OH)D, 25-hydroxyvitamin D.

FIGURE 2.

Daily nutritional intake variables. Daily dietary intakes of protein (A), calcium (Ca) (B), magnesium (Mg) (C), and vitamin D (D) are represented by box plots. Protein and mineral nutrient intakes were normalized to BW (mg or g of nutrient/kg BW of the subject), and vitamin D intake is shown as the total ingested amount per day in food plus supplements. The 25th, 50th, and 75th percentiles for each variable are represented by the bottom, middle line, and top of the box, respectively. The lower and upper whiskers extend to the 10th and 90th percentiles, respectively. The mean value for the population is represented by an asterisk. Circles represent values less than the 10th or greater than the 90th percentile; because the population was large, one circle may represent multiple subjects for a given extreme value. BW, body weight; Pro, protein; Vit D, vitamin D.

FIGURE 3.

Scatterplot of 25(OH)D (top) or 1,25(OH)₂D (bottom) related to age in months. The regression equation for the relation of 25(OH)D to age was as follows: 25(OH)D (nmol/L) = 74.9–0.44 (age in mo) (R = 0.18, P < 0.001). The regression equation for the relation of 1,25(OH)₂D to age was as follows: 1,25(OH)₂D (pmol/L) = 188.7–1.5 (age in mo) (R = 0.23, P < 0.001). Regression lines (solid lines) and their 95% CIs (dashed lines) are shown. 1,25(OH)₂D, 1,25-dihydroxyvitamin D; 25(OH)D, 25-hydroxyvitamin D.

FIGURE 4.

Circulating vitamin D metabolites by self-identified ancestry, sex, and season of sampling. A: Concentrations of 25(OH)D in white subjects were greater than in subjects of African ancestry (P = 0.049), values in Hispanic subjects were greater than in subjects of African ancestry (P < 0.002), and values were greater in samples obtained in the summer and fall than in the winter and spring (P = 0.001). There was no sex difference. B: 1,25(OH)₂D concentrations in subjects of African ancestry were greater than in Hispanics or whites (P < 0.001), and girls had greater values than did boys (P = 0.038). There was no effect of season. The 25th, 50th, and 75th percentiles for each variable are represented by the bottom, middle line, and top of the box, respectively. Lower and upper whiskers extend to the 10th and 90th percentiles, respectively. The mean value for the population is represented by an asterisk. Circles represent values less than the 10th or greater than the 90th percentile; because the population was large, one circle may represent multiple subjects for a given extreme value. 1,25(OH)₂D, 1,25-dihydroxyvitamin D; 25(OH)D, 25-hydroxyvitamin D.