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Researchers from the Oxford Centre for Diabetes, Endocrinology and Metabolism (OCDEM) have found that increased exposure to bright sunlight may be associated with a lower risk of type 2 diabetes and heart disease by lowering blood insulin and triglyceride levels.

The study, published in the Journal of Clinical Endocrinology & Metabolism, is the first to suggest that regular exposure to bright sunlight might improve glucose and fat metabolism.

A team of researchers led by Dr Costas Christodoulides and Professor Fredrik Karpe from the Radcliffe Department of Medicine at the University of Oxford examined data from local weather stations coupled with blood and body fat measurements from over 10,000 healthy middle-aged people from the Oxford Biobank and the Netherlands Epidemiology of Obesity studies – these are large-scale databases of health information from volunteers living in Oxfordshire, UK, and Leiden, Netherlands. The Oxford team worked closely with colleagues at the Leiden University Medical Centre in the Netherlands for this study.

We were initially looking for associations between measures of cardiometabolic health and outdoor temperature, since previous work in both humans and rodents suggested that colder temperatures may activate biological processes that reduce the risk of type 2 diabetes and obesity.
- Dr Costas Christodoulides.

Instead, the research team found that if anything warmer rather than colder temperatures seemed to be linked to more healthy indicators.

So the investigators also used the data recorded at the Radcliffe Meteorological Station at the University of Oxford, and the Royal Dutch Meteorological Institute, to work out how many hours of bright sunshine the study volunteers in Oxfordshire and in Leiden, Netherlands were exposed to in the two weeks before they were tested.

For both locations, the researchers found that the amount of bright sunlight was associated with lower insulin and triglyceride levels. Higher insulin levels are indicative of a reduced body response to the actions of insulin and are one of the first signs of type 2 diabetes whereas higher triglyceride levels can indicate the start of atherosclerosis, a condition where fatty deposits accumulate inside the arteries. Atherosclerosis increases the risk of heart disease and stroke.

The association between hours of bright sunshine and better cardiometabolic health was stronger in the group of people living in Oxfordshire, though it was also present in the Netherlands group. Once the amount of bright sunshine was taken into account, the association between better cardiovascular indicators and warmer weather nearly disappeared, suggesting that it was the amount of bright sunshine that was the more decisive factor.

The association was also independent of how much body fat a person had.

Eating a healthy diet and exercising regularly are still absolutely the best ways to reduce your risk of developing type 2 diabetes and heart disease, but our research suggests that some environmental factors may also play a role. 
- Dr Christodoulides

“These could act indirectly, with better weather encouraging more outdoor exercise and a lighter diet, and/or directly – previous work finds that night-time blood levels of the hormone melatonin increase with longer exposure to bright light, and studies in rodents and humans indicate that melatonin levels might modulate diabetes risk”

Professor Fredrik Karpe, the Director of the Oxford BioBank as well as a consultant physician working with diabetic patients, said “We still don’t know exactly what is happening in the body to explain these associations, or why our results differ from previous studies which suggest that colder rather than warmer temperatures reduce type 2 diabetes rates.

“These differences could potentially be because our study involved healthy people rather than people with diabetes, and these results do demonstrate the power and utility of having large and detailed health datasets available to researchers.”

This study was enabled by funding from the British Heart Foundation, the European Commission, the Dutch Heart Foundation, the European Foundation for the Study of Diabetes, and the Netherlands Cardiovascular Research Initiative.

Read the full paper