The ability to make vitamin D from the sun varies according to geography: The sun’s ultraviolet B (UVB) rays trigger the skin to produce vitamin D. Solar UVB radiation is weaker at higher latitudes and, in turn, people who live in higher latitudes tend to have lower vitamin D levels than people who live closer to the equator. For decades, scientists have been intrigued by north-south geographical differences in rates of type 1 diabetes, colon cancer, multiple sclerosis, and other diseases—a child in Finland, for example, is 400 times more likely to have type 1 diabetes than a child in Venezuela—and they wondered whether vitamin D or sunlight exposure might have a role. (, )
Many of the scientific hypotheses about vitamin D and disease stem from studies that have compared solar radiation and disease rates in different countries or regions. These types of descriptive studies, called “ecological” studies or “geographic” studies, can be a good starting point for other scientific research. But they do not provide the most definitive information. Why? Ecological studies look at large groups of people, but they don’t collect individual information on diet or lifestyle. So they can’t account for all of the individual differences that might also vary by geography—and might increase someone’s risk for disease. That’s why researchers need to conduct other kinds of studies that can tease out whether other factors might account for the relationship.
The story ofprovides a good example of how scientific views can change as more definitive evidence trickles in: Early ecological studies suggested that high fiber intakes would be protective against colon cancer. But later studies, which followed large groups of people for long periods of time and gathered detailed data on individual diet and lifestyle factors, failed to find any protective benefit for fiber.
So far, the more-definitive types of studies on vitamin D and disease have tended to support the early ecological findings. More research is still needed, especially randomized controlled trials and prospective studies that follow large groups of people over a long period of time.
To learn more about how to interpret studies on diet and health, read.
1. Garland CF, Garland FC. Do sunlight and vitamin D reduce the likelihood of colon cancer? Int J Epidemiol. 1980; 9:227-31.
2. Gillespie KM. Type 1 diabetes: pathogenesis and prevention. CMAJ. 2006; 175:165-70.
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