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Vitamin D Response to Summer Sun: A 2-Year Study

10/29/2019 4:59:02 PM
sunRisk factors for vitamin D deficiency include obesity, type 2 diabetes, and living at high latitude. Vitamin D3 (cholecalciferol) is primarily synthesized with exposure to ultraviolet B sunlight. However, it is not known whether summer sun exposure is sufficient to achieve adequate serum 25-hydroxyvitamin D (25[OH]D) in at-risk populations.
Over the course of 2 consecutive summers in Stockholm, Sweden (59°N latitude), researchers investigated the effects of summer sun exposure on vitamin D levels in middle-aged, overweight men and women with either normal or impaired glucose metabolism.

Vitamin D levels examined in obese adults

A total of 158 overweight or obese adults (aged 45-69 years) participated in the study. They were classified into 2 groups according to glucose tolerance: normal glucose tolerance and impaired glucose tolerance. Serum 25(OH)D levels were tested before and after summer for 2 consecutive years.
The mean concentration of 25(OH)D for the whole study population before summer was in the low-to-deficient range (55.1 nmol/L). After summer, the mean concentration had increased to 66.3 nmol/L but was still below the recommended range of ≥ 75 nmol/L. This finding was independent of sex or glucose tolerance status. The result was confirmed during the second year with no significant difference between years.

Vitamin D levels rise slightly in summer, but not enough

There was a weak inverse correlation between 25(OH)D and body composition variables in the whole study population, but the correlation only held true for percent fat mass in women. Only 21 participants (13%) reported vitamin D supplementation. The mean serum 25(OH)D concentration among supplement users was higher before summer (64.2 nmol/L) and after summer (71.1 nmol/L) than nonusers, but the seasonal change was similar for both groups.
The key finding from this study was that low vitamin D levels before summer did increase from summer sun exposure—but not enough to achieve adequate recommended levels. The authors conclude that vitamin D supplementation should be considered during both summer and winter in people at risk of vitamin D deficiency and living at higher latitudes. 
Lundström P, Caidahl K, Eriksson MJ et al. Changes in Vitamin D Status in Overweight Middle-Aged Adults with or without Impaired Glucose Metabolism in Two Consecutive Nordic Summers. J Nutr Metab. 2019; 2019: 1840374.