For a while vitamin D was looking like a bona fide health elixir. It was recognized a century ago as the cure for rickets, a childhood disease that causes weak and deformed bones. Then, in the early 2000s, researchers began amassing a pile of studies suggesting that low vitamin D levels could be a factor in cancer, cardiovascular disease, dementia, depression, diabetes, autoimmune diseases, fractures, respiratory illnesses and Parkinson’s disease. It seemed reasonable to think that raising our levels of this simple vitamin—one that our bodies make when lit up by sunshine and that we can get more of from supplements—could cure practically whatever ailed us.
At least two books called The Vitamin D Cure were published, along with other books and news reports whose titles include words like “revolution” and “miracle.” There was also a growing concern that we weren’t getting enough of the vitamin. Good Morning America aired a segment that began with reporter Diane Sawyer declaring 100 million Americans were deficient. Her guest was Dr. Oz, who told viewers they could determine their vitamin D level with a simple blood test. Sunshine is the best way to get this vitamin, he said. But if that wasn’t enough, he advised cod liver oil or supplements.
Numerous celebrities and vitamin companies raised hopes that vitamin D could be a panacea, says JoAnn Manson, an endocrinologist and epidemiologist at Harvard Medical School and a lead investigator on some of the biggest vitamin D studies to date. Sales of supplements containing the vitamin soared, as did rates of vitamin D testing.
Then the bottom fell out. Although thousands of studies had linked low levels of vitamin D to an assortment of medical conditions, when scientists tried administering it as a means to prevent or treat those problems, the wonder supplement failed miserably. The notion that our lives would be better if we all just raised our vitamin D levels began to look like a fantasy. The idea that vitamin D deficiency was widespread also crumbled. It turned out that notions of what constitutes a deficiency were based on a dubious understanding to begin with. National population sampling showed that most people were already getting enough of the vitamin.
The study findings were a shock. Vitamin D did not make a dent in cancer or heart disease, and it did not prevent falls, improve cognitive function or reduce bone fracture risk.
There’s no question that vitamin D plays an important role in health. It helps your body absorb and retain calcium and phosphorus; both are critical for building bone. But except for a few subsets of the population (such as breastfed infants and people with particular medical conditions), most people probably don’t need supplements.
The story of how vitamin D was discovered, rocketed to miracle status and then returned to Earth illustrates the sometimes jagged path of scientific discovery. It’s also a cautionary tale about the need to interpret scientific results with humility. Ultimately it’s about the self-correcting nature of science and how knowledge becomes honed over time.
For much of human history, people got their vitamin D mostly from the sun. It turns out humans are a little bit like plants—we can turn ultraviolet light into something our bodies need in a process akin to photosynthesis.
When the high-energy rays of UV light—UVB—hit your skin, they start a chain reaction that converts a compound in your skin called a sterol into a vitamin D precursor. This molecule, after a few more steps, becomes a form of the vitamin that promotes calcium absorption from the gut and increases bone mineralization. Vitamin D also seems to bolster the immune system and tamp down inflammation. It does these things in part by influencing the production of inflammatory compounds and suppressing the buildup of proinflammatory cells. Researchers have studied whether vitamin D can prevent dangerous inflammatory reactions in people with COVID.
Producing vitamin D became increasingly difficult for human bodies during the Industrial Revolution, when smoke and soot darkened the skies and children spent more time in the shade of crowded cities, leading to an increase in rickets. By the late 1800s researchers had documented geographic differences in the prevalence of rickets that pointed to a possible link to sunlight.
Despite the disappointing trials on vitamin D, it’s not time to dismiss the vitamin completely, Manson says. There’s still plenty more to understand. For instance, the VITAL trial showed that among slender or normal-weight people, defined as having body mass indexes of 25 or less, vitamin D supplements appeared to lower the incidence of cancer, cancer deaths and autoimmune disease. This protective effect did not show up among heavier people with higher body masses. Manson cautions that these numbers need to be verified by further work because they are from a smaller subanalysis of the main study. But it’s possible that excess body fat may somehow hamper the effectiveness of vitamin D. Obesity itself is a risk factor for both cancer and autoimmune disease, so it’s likely that any connection is complex.
Read the full article by CHRISTIE ASCHWANDEN / Scientific American
