An Exploration of How Solar Radiation Affects the Seasonal Variation of Human Mortality Rates and the Seasonal Variation in Some Other Common Disorders

Abstract

Many diseases have large seasonal variations in which winter overall mortality rates are about 25% higher than in summer in mid-latitude countries, with cardiovascular diseases and respiratory infections and conditions accounting for most of the variation. Cancers, by contrast, do not usually have pronounced seasonal variations in incidence or mortality rates. This narrative review examines the epidemiological evidence for seasonal variations in blood pressure, cardiovascular disease rates and respiratory viral infections in relation to atmospheric temperature and humidity, and solar UV exposure through vitamin D production and increased blood concentrations of nitric oxide. However, additional mechanisms most likely exist by which solar radiation reduces the risk of seasonally varying diseases. Some studies have been reported with respect to temperature without considering solar UV doses, although studies regarding solar UV doses, such as for respiratory infections, often consider whether temperature can affect the findings. More research is indicated to evaluate the relative effects of temperature and sun exposure on the seasonality of mortality rates for several diseases. Since solar ultraviolet-B (UVB) doses decrease to vanishingly small values at higher latitudes in winter, the use of safe UVB lamps for indoor use in winter may warrant consideration.

Guidelines for Preventing and Treating Vitamin D Deficiency: A 2023 Update in Poland

“…The incidence and mortality rates for cardiovascular and respiratory diseases are much higher in winter than in summer. As a result, all-cause mortality rates in winter are 25% higher than in summer in the U.S. [ 22 , 23 ]. Since the risk for cardiovascular disease and infectious diseases is inversely correlated with serum 25(OH)D concentrations, it would be very useful to understand why the concentrations change with season and how to maintain summertime concentrations in winter.…”

“…A vitamin D supplementation study reported that for 400, 4000, or 10,000 IU/day vitamin D 3 for 6 months, 162, 320, and 1289 genes were up- or down-regulated in their white blood cells, respectively [ 32 ]. This finding suggests that the increased risk of cardiovascular disease and mortality rates in winter is largely due to lower 25(OH)D concentrations in winter [ 23 ].…”

Evidence That Increasing Serum 25(OH)D Concentrations to 30 ng/mL in the Kingdom of Saudi Arabia and the United Arab Emirates Could Greatly Improve Health Outcomes

“…The authors had no explanation for their findings. However, a report in 2022 showed that seasonal changes in serum 25(OH)D concentrations had the highest impact on seasonal variations in mortality rates, with seasonal changes in temperature having a similar but lesser effect [ 75 ].…”

Comment on: Association between vitamin D and cardiovascular health: Myth or fact? A narrative review of the evidence

“…Additional support for the protective role of vitamin D supplementation comes from the fact that CVD mortality rates are about 25% higher in winter than those in summer as discussed in a recent review of the factors affecting the seasonality of CVD incidence and CVD mortality rates 10 where seasonal changes in temperature and serum 25(OH)D concentrations and elevated parathyroid hormone concentrations were discussed in detail. While the evidence regarding temperature is strong, so is that for vitamin D, and which is more important in particular situations may depend on how much exposure there is to cold temperatures or to extreme heat.…”