Nighttime artificial outdoor lighting was associated with impaired glucose control and increased diabetes risk, a cross-sectional study in China showed.

In a nationally representative sample of 98,658 adults, the prevalence of diabetes was 28% higher among people exposed to the greatest levels of artificial outdoor light compared with those who had the least exposure (adjusted prevalence ratio [PR] 1.28, 95% CI 1.03-1.60), reported Yu Xu, PhD, of the Shanghai Jiaotong University School of Medicine, and colleagues in Diabetologia.

Each quintile increase of artificial light exposure was associated with a 7% increase in diabetes prevalence (adjusted PR 1.07, 95% CI 1.02-1.12). Artificial light exposure also was linked with higher HbA1c and fasting glucose levels as well as reductions in beta cell function, Xu’s group said.

“The current study is the largest epidemiological study to test the hypothesis that increased exposure to outdoor LAN [light at night] is associated with a higher possibility of developing hyperglycemia, insulin resistance, and diabetes,” the researchers wrote.

“Our findings contribute to the growing literature suggesting that LAN is detrimental to health and demonstrate that LAN may be a potential novel risk factor for diabetes,” Xu and co-authors added.

The findings also may have global implications, they suggested: more than 80% of the world’s population and 99% of Americans and Europeans live under light-polluted skies, according to a recent study.

Exposure to artificial light at night may increase metabolic disease risk by disrupting circadian rhythms, the researchers noted. This disruption can lead to changes in physiological and metabolic factors including activity levels, body temperature, food intake, lipid profile, insulin sensitivity, glucose metabolism, and levels of plasma melatonin, glucocorticoids, and fatty acids, they said.

Previous studies have tied artificial outdoor light to health problems such as obesity, psychiatric disorders, and cancer. A prospective study of older adults in Hong Kong, for example, linked artificial light with an increased risk of hospitalization or death from coronary heart disease. Few studies, however, have examined a link with diabetes, Xu’s group observed.

Xu and colleagues used satellite data from the U.S. Defense Meteorological Satellite Program that contained annual composite images of outdoor light at night excluding sun and moon luminance, clouds, atmospheric lighting, and ephemeral events such as fires. “Although these images capture only a fraction of the light originating from the Earth’s surface, they represent the relative intensity of night-time illumination at ground level,” the researchers said.

Study participants came from the China Noncommunicable Disease Surveillance Study. Conducted in 2010, this survey included individuals in 162 study sites in all 31 provinces, autonomous regions, and municipalities of mainland China. Participants were 18 or older and had lived in their current residence for at least 6 months.

Participants were assigned a category of light exposure based on geographic region. Diabetes was defined by American Diabetes Association criteria. Insulin resistance was assessed by HOMA-IR and beta cell function by HOMA-B.

Mean age of participants was about 43 and 49.2% were women. Participants in higher quintiles of outdoor light exposure were more likely to be older, have a higher body-mass index (BMI) and household income, and live in an urban area. Those in lower quintiles of outdoor lighting reported higher levels of physical activity and fewer years of education.

Findings were adjusted for age, sex, education, smoking status, alcohol use, physical activity, family history of diabetes, household income, urban vs rural location, antihypertensive medications, lipid-lowering medications, and BMI.

The study’s cross-sectional design might not accurately reflect long-term exposure to artificial outdoor lights, the researchers acknowledged. In addition, satellite data represented a rough estimate of an individual’s actual exposure, they said.

“Our findings contribute to the growing literature suggesting that LAN is detrimental to health and demonstrate that LAN may be a potential novel risk factor for diabetes,” Xu and colleagues noted. “However, we advise caution against causal interpretation of the findings and call for further studies involving direct measurement of individual exposure to LAN.”

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