During the Industrial Revolution, as more children started growing up in cities, physicians noticed a new disease, which weakened children's bones so they were no stronger than cartilage. Researchers eventually linked a vitamin D deficiency to the disease and recommended sunbathing to cure the condition, called rickets.
Vitamin D remains somewhat of a mystery. Half of Americans and Western Europeans suffer from deficiencies of vitamin D, which aids the body's ability to absorb calcium. Most people don't realize that they experience a deficiency. Vitamin D is hard to find—few foods are rich in vitamin D and the best way to absorb it is through sun exposure. A recent study sheds new light on vitamin D's importance; people with vitamin D deficiencies experience significant cognitive decline.
David J. Llewellyn of the University of Exeter in England published a paper in the Archives of Internal Medicine examining the relationship between vitamin D levels and brain function. Starting in 1998, he sampled the blood of 858 adults, aged 65 or older, to test their vitamin D levels. The subjects also took a trio of tests, measuring brainpower—one test evaluated general cognition, one examined attention, and one measured executive functions, which includes skills such as cognitive flexibility, abstracting thinking, planning, and organizing. Llewellyn took blood samples and asked the subjects to complete tasks again three and six years later. He found that people with dangerously low levels of vitamin D were 60 percent more likely to show a significant decline in overall cognition. Those with lower vitamin D levels were also 31 percent more likely to show insufficient executive functioning.