Coronary heart diseaseExposure to sunlight

There is limited evidence that exposure to sunlight reduces the risk of CHD. That evidence is primarily of two types:

  • variations of incidence and death from CHD as a function of season and
  • variation with residence altitude.

It is well-known that CHD rates are higher in winter than in summer1 2. It is tempting to ascribe this seasonality to the annual variation in solar ultraviolet-B (UVB) doses and vitamin D production. However, there is another factor related to season that may also contribute to CHD: temperature.

One effect of lower temperature is higher blood pressure2. Hypertension is a risk factor for CHD3. Serum lipid levels have been found to be elevated in winter in Iran4 and Japan5. The annual variation may be related to seasonal variations in diet, with more energy-dense foods consumed in winter to help keep the body warm. 

Ecological (geographical) studies found links between risk-modifying factors and disease by treating people living in geographically defined regions and using region-averaged disease outcomes (incidence, prevalence, or mortality rates) with region-averaged risk-modifying factors. This approach has worked very well for solar UVB and cancer6 7.

There is only one multicountry study that purported to do so for CHD8. However, the vitamin D data in that paper did not support the hypothesis, and other work showed that serum 25(OH)D levels actually increase with latitude in Europe, likely due to supplementation at higher latitudes9. It was pointed out that the increase in CHD mortality rate with increasing latitude in Europe was much more likely due to the increasing prevalence of apolipoprotein E epsilon4 [ApoE4] allele with increasing latitude10. ApoE is a gene that controls lipid levels in the blood, and ApoE4 increases survival for hunter-gatherer peoples by increasing cholesterol production in the liver in order to save as much food as fat between feast and famine. Also, dietary gradients in Europe could also contribute to the latitudinal gradient11

As with other ecological studies, the results of multi-country studies may disagree with results from single countries. The reason is that some risk-modifying factors might vary considerably from country to country but very little within a country. Some of the important factors that could change include genetics, diet, smoking, and alcohol consumption.

However, a couple of ecological studies found that living at higher altitude is associated with reduced incidence of or death from CHD. Solar UVB doses increase by about 10% per kilometer of altitude.

In a multivariate analysis comparing those living in mountainous regions to those living in lowlands in Greece, coronary mortality were lower for residents of the mountainous village in comparison with residents of the lowland villages [hazard ratios (95% CI) for men and women, respectively: total mortality, 0.57 (0.38–0.84) and 0.69 (0.47–1.02); coronary mortality, 0.39 (0.16–0.98) and 0.46 (0.20–1.05)12.

A study in Switzerland found an adjusted risk ratio of 0.78 (95% CI, 0.73–0.82) for CHD mortality rate per 1000-m increase in altitude over a range from 1500 m13. However, living between 1500 m and 3500 m correlates with higher prevalence of CHD in Sudan14.

Page last edited: 09 May 2011


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