Vitamin D Status: UV-exposure, Obesity and Cancer

Abstract

Sun is the main vitamin D source for humans. Sun exposure during the summer may provide large amounts of vitamin D3, which is stored in the fat tissue and released during the winter. Serum 25(OH)D levels > 80 nmol/L at the end of the summer are considered necessary in order to avoid vitamin D deficiency during the winter. However, it is not clear how much sun exposure is needed to achieve this concentration. In this study we simulated a Norwegian summer by using commercially available tanning equipment. We found that moderate exposures given during 5-7 weeks may raise serum 25(OH)D concentrations from typical winter values to typical summer values. A UV dose equal to a whole body sun exposure of 5 - 10 MED at sunny summer midday increased serum 25(OH)D by 15-23 nmol/L. The increase was dependent on the initial vitamin D status: persons with the lowest baseline 25(OH)D concentrations got the largest increase. However, for persons with vitamin D deficiency (25(OH)D < 50 nmol/L) the mentioned UV exposure was not large enough to reach the threshold of vitamin D sufficiency (25(OH)D 75 nmol/L). Moreover, a daily whole body sun exposure of ~0.2 MED seems to be almost equal to an oral vitamin D intake of 2,000 IU.<br> Body composition and BMI are important predictors of vitamin D status. Our earlier investigations suggested that serum 25(OH)D levels decrease proportionally with increasing BMI. The key mechanism behind is possibly increased sequestration of fat-soluble vitamin D in a large volume of fat tissue. However, other factors, such as low sun exposure and inadequate vitamin D intake, may also contribute to the low vitamin D status in overweight and obese persons. In this study we have investigated the impact of excess body weight on serum 1,25(OH)2D concentrations. According to our results high BMI and adiposity in adults were associated with decreased serum 1,25(OH)2D. Serum 25(OH)D was the strongest predictor of 1,25(OH)2D values. A decrease in 25(OH)D by 1 nmol/L was associated with a decrease in 1,25(OH)2D concentrations by 0.4 pmol/L (P<0.001). There was no correlation between serum concentrations of 25(OH)D and 1,25(OH)2D in obese children and adolescents, although the prevalence of vitamin D deficiency and insufficiency in adolescents was high (58 %).<br> Low vitamin D status as a consequence of low sun exposure and/or high BMI may play a role in cancer development and prognosis. Our calculations suggest that a low vitamin D status may explain at least 20% of the cancer risk attributable to high BMI. It also seems that the contribution of low 25(OH)D to the increased cancer risk with increasing BMI may be different for different cancer types being highest for colorectal and breast cancers. A panel of 25 world leaders in vitamin D research recommended that the serum 25(OH)D concentrations should be at least 75 nmol/L (30 ng/ml) in order to provide optimal health outcomes. These values may be achieved by moderate UV exposure or by high vitamin D intake. The dose of vitamin D supplementation and UV exposure should be adjusted according to BMI.