Abstract
Background In the 1980s, analysis of 30 countries suggested that people living farther from the Equator had a higher incidence of cardiovascular disease. Some have hypothesized that sunlight plays a role. Methods We analyzed 180 or more countries’ age-standardized average cholesterol, age-standardized mean systolic blood pressure (BP), and age-standardized prevalence of raised BP, by geographic latitude, over decades. In addition to latitude, we performed analysis by ultraviolet B light (UVB) exposure averaged over several years. Results Mean cholesterol increases with the distance of a country from the Equator. This relationship has changed very little since 1980. Similarly, in 1975, mean systolic BP and prevalence of raised BP were higher in countries farther from the Equator. However, the relationship between latitude and BP has changed dramatically; by 2015, the opposite pattern was observed in women. Countries’ average UVB exposure has a stable relationship with cholesterol over recent decades, but has a changing relationship with BP. Conclusions Since sunlight exposure in a country is relatively fixed and its relationship with BP has changed dramatically in recent decades, countries’ average sunlight exposure is an unlikely explanation for contemporary country-level variation in BP. However, our findings are consistent with a putative effect of sunlight on countries’ average cholesterol, as well as a no longer detectable effect on BP decades ago. A parsimonious potential explanation for the relationship between light and cholesterol is that 7-dehydrocholesterol can be converted to cholesterol, or in the presence of ultraviolet light, it can instead be converted to vitamin D.
Significance Statement This study provides a comprehensive analysis of geographic latitude and cardiovascular risk factors. The results show a stable latitude-cholesterol relationship and a changing latitude-blood pressure relationship over recent decades. The findings shed light on whether contemporary differences between countries’ average blood pressure are attributable to a previously hypothesized effect of sunlight—it is unlikely. However, the results are consistent with an effect of sunlight on blood pressure several decades ago, and on cholesterol—including in recent years.
Taken together with prior ecological studies, animal studies, and clinical trials, our analysis suggests a role for sunlight in determining countries’ average cholesterol. These findings shed new light on the influence of geography on population health and suggest new avenues of investigation.