Circulating 25(OH)D concentrations in overweight and obese adults is explained by sun exposure, skin reflectance, and body composition

Authors: PICCOLO, BRIAN - University Arkansas For Medical Sciences (UAMS); HALL, LAURA - University Of California, Davis; Stephensen, Charles; Gertz, Erik; VAN LOAN, MARTA - University Of California, Davis

Submitted to: Current Developments in Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/22/2019
Publication Date: 5/27/2019
Citation: Piccolo, B.D., Hall, L.M., Stephensen, C.B., Gertz, E.R., Van Loan, M.D. 2019. Circulating 25(OH)D concentrations in overweight and obese adults is explained by sun exposure, skin reflectance, and body composition. Current Developments in Nutrition. doi.org/10.1093/cdn/nzz065.
DOI: https://doi.org/10.1093/cdn/nzz065

Interpretive Summary: Overweight and obese individuals are at greater risk of vitamin D deficiency compared to normal weight individuals. Synthesis of vitamin D in the skin during sun exposure is an important source for this nutrient, in addition to diet, but methods of measuring sun exposure have not been validated in overweight and obese individuals. The goal of this study was to assess, in overweight and obese individuals, the validity of a mathematical model that was developed in lean individuals that uses sun exposure to estimate vitamin D status. The study used data collected during a 15-week controlled feeding study investigating the effects of dairy consumption on body composition. Information regarding sun exposure (duration, time of day, clothing worn) were collected in diaries and skin pigmentation was assessed using a skin-reflectance spectrophotometer. Sun exposure energy (joules) was determined for each study volunteer based on their diaries, skin reflectance, and the intensity of ultraviolet light during period exposure reported from a UV spectrophotometer at a local weather observation station. Vitamin D status was assessed by measuring serum 25-hydroxy vitamin D (25(OH)D) concentrations. The study found that sun exposure was positively related to serum 25(OH)D, inversely related to body fat levels and body mass index. However, the model developed for lean individuals significantly over-estimated the vitamin D status of overweight and obese individuals so a new mathematical model was developed that performed better in this study group. The model included sun exposure, skin reflectance, total fat mass, total lean mass, and intra-abdominal adipose tissue as predictors. In summary, this study showed that additional factors need to be considered in predicting the effect of typical sun exposure on the vitamin D status of overweight and obese individuals, as compared to lean individuals, emphasizing the role of adipose tissue in affecting vitamin D status.

Technical Abstract: Background Obese individuals are known to be at risk for vitamin D deficiency compared to normal weight individuals. Cutaneous synthesis is a major source of vitamin D; however, objective measurements of sun exposure are lacking in this population. Objective To assess the validity of a regression model using sun exposure in lean individuals to estimate serum 25-hydroxyvitamin D (25(OH)D) in overweight and obese individuals, and to develop a prediction equation for serum 25(OH)D in overweight and obese adults. Methods This study was a secondary analysis of a 15-week controlled feeding study investigating the effects of dairy consumption on body composition. Information regarding sun exposure day, hour, time outside, and clothing were self-assessed in sun exposure diaries. Personal sun exposure energy (joules) was assessed by downloading time specific ultraviolet-B energy data from climate stations. Skin reflectance was measured using a Minolta 2500d spectrophotometer. Dietary intake of vitamin D was known. Serum 25(OH)D concentration measured by radioimmunoassay. Body composition was determined from whole-body dual energy x-ray absorptiometry and computed tomography scans. Results Sun exposure was positively related to serum 25(OH)D (r = 0.26; P = 0.05), inversely related to total fat mass, android fat, and BMI (r = - 0.25, - 0.30, and - 0.32, respectively). The modified Hall model significantly over-estimated serum 25(OH)D in overweight and obese adults by 27.33–80.98 nmol/L, depending on the sun exposure calculation. A new regression model was developed for overweight and obese persons that explained 29.1% of the variance in post-intervention 25(OH)D concentrations and included sun exposure, skin reflectance, total fat mass, total lean mass, and intra-abdominal adipose tissue as predictors. Conclusion Major determinants of serum 25(OH)D concentration in healthy overweight and obese individuals include sun exposure, skin reflectance, and adiposity. Addition of adiposity terms to the prior model significantly improved predictive ability in overweight and obese men and women. (clinicaltrials.gov: NCT00858312)