Spotlight on temperature and light

Recent studies looked at light therapy for depression and diabetes, seasonal variation in glycemic control, and associations between outdoor sunlight and temperature and metabolism.

Several recent studies investigated connections between light, temperature, and metabolic health.

One trial, published by Diabetes Care on Feb. 14, tested light therapy for patients with depression and type 2 diabetes. It included 83 Dutch patients who were randomized to either four weeks of light therapy (10,000 lux) or placebo light therapy daily at home. Overall, light therapy did not reduce depressive symptoms or improve insulin sensitivity compared to placebo. However, an analysis of the 51 patients who adhered to the study protocol suggested some positive effects of light therapy on depression, while another prespecified subgroup analysis showed a positive effect among patients with lower insulin sensitivity (P=0.017) and a trend toward effectiveness in those using insulin (P=0.094). The study authors noted that although their results did not show a significant difference between light therapy and placebo, the effect size was actually similar to that in previous studies of light therapy on depression. “The findings of this trial warrant further study of the antidepressant and glucometabolic effects of light therapy in people with highly insulin-resistant type 2 diabetes,” they said.

Another study, also published by Diabetes Care on Feb. 14, investigated seasonal variations in glucose, blood pressure, and cholesterol control among 4,678 type 2 diabetes patients in Japan. The studied patients had their HbA1c (target <7%), blood pressure (target <130/80 mm Hg), and LDL cholesterol (target <100 mg/dL [<2.6 mmol/L]) measured at least 12 times over two years. Fewer patients achieved all three targets during the winter (9.6%) than the summer (15.6%), with the most significant difference in systolic blood pressure (40.9% in winter vs. 56.6% in summer). The findings might relate to seasonal variation in sympathetic nerve activity, the authors said, although they noted that physical activity, food intake, and cultural factors could also play a role. They called for future research to find interventions to reduce seasonal variations in these markers and thereby reduce cardiovascular events.

Finally, a study of patients without diabetes, published by the Journal of Clinical Endocrinology and Metabolism on Feb. 12, searched for associations of bright sunlight and outdoor temperature with measures of glucose and lipid metabolism. It included 4,327 middle-aged adults from the Oxford Biobank and 5,899 from the Netherlands Epidemiology of Obesity Study. In the former cohort, increased bright sunlight exposure was associated with lower fasting insulin levels, insulin resistance, beta-cell function, and triglyceride levels. The other cohort showed generally the same trends, but with weaker associations. The study found no association between outdoor temperature and glucose or lipid metabolism after adjustment for sunlight exposure. The study authors speculated that, in addition to light-related changes in diet and lifestyle, the findings might be explained by melatonin signaling from light exposure.