The amount of sunlight received could be linked to changes in blood sugar, particularly during the least sunny months.
Researchers studied data from the UK Biobank, a large British medical database. Their question was simple: do people exposed to more light before a blood test have different blood sugar levels? To estimate this, the team used satellite measurements of ultraviolet A, or UVA, based on the place of residence and the date of the examination.
In winter, variations in UVA can be measured in blood sugar levels.
Illustration image Pixabay
The researchers examined average exposure during the two and seven days preceding the blood sugar measurement. In winter, greater UVA exposure was associated with slightly lower blood sugar. The association also appeared in spring for exposure calculated over seven days, but it was weaker.
In winter, a 1 W/m² increase in average UVA exposure corresponded to a decrease of 0.015 mmol/L over two days. Over seven days, the difference reached 0.027 mmol/L. Most importantly, the decrease was more pronounced when available light was low, then tended to level off.
Why look at UVA? This part of solar radiation passes through the atmosphere and reaches our skin. Several biological mechanisms could link light to the way the body functions.
The team also used another piece of information available in the UK Biobank: visits to tanning salons. During the follow-up period, occasional users had fewer diagnoses of type 2 diabetes than non-users. The difference was even more pronounced among frequent users.
This result calls for great caution. People who use tanning beds may differ from others in age, lifestyle, or other characteristics. The analyses adjusted for several known factors, but an observational study can never eliminate all these differences. It therefore does not demonstrate that UV tanning beds protect against diabetes.
Nor is this an invitation to increase ultraviolet exposure. UV radiation is a recognized risk factor for skin cancer. The importance of these results lies elsewhere: understanding whether certain biological effects of light could contribute to glucose control without reproducing its harmful effects.
The researchers must now clarify the mechanisms involved and verify these associations in other populations and using other methods. One question remains open in particular: how much of the phenomenon comes directly from light, and how much depends on seasonal habits and the time spent outdoors?