Vitamin D helps our bodies regulate levels of calcium and phosphate－nutrients that keep bones, teeth and muscles healthy. Often, sunlight on our skin can be enough to enable us to produce all the vitamin D we need. But when sunshine is lacking, vitamin D must be ingested, and it can be difficult to meet the recommended levels from food alone. This matters because the health benefits of adequate vitamin D intake may be even greater than previously thought.

Vitamin D deficiencies are widespread, with about 1 billion people, from all age groups and ethnicities, suffering from them, even in countries with year-round sunshine. Indeed, they are particularly common in the Middle East, owing partly to the prevalence of skin-covering clothes and a cultural habit of staying out of the sun. That same habit, together with darker skin, contributes to lower levels of vitamin D among Africans.

Even in industrialized countries, doctors are seeing the resurgence of rickets, a bone-weakening disease that had been largely eradicated through vitamin-fortified milk and other products. And rickets is far from the only disease to which vitamin D deficiency may contribute. Research conducted over the last decade suggests that vitamin D plays a much broader disease-fighting role than once thought.

For example, the Health Professional Follow-Up Study (sponsored by Harvard School of Public Health) found that men with vitamin D deficiencies were twice as likely to have a heart attack as men who had adequate levels. Several other studies have found links between low vitamin D levels and higher risk of heart failure and sudden cardiac death, stroke, and overall cardiovascular disease. Yet another study showed that daily vitamin D supplements may improve cardiac function in people with chronic heart failure.

Dozens of other studies point to a potential link between low levels of vitamin D and increased cancer risk, particularly the risk of colorectal cancer (though, based on current evidence, it remains unclear whether vitamin D supplements actually lower cancer risk). Likewise, vitamin D levels may go some way toward predicting cancer survival (though the evidence remains limited here, as well).

Then there is multiple sclerosis (MS), which occurs at much lower rates in sunnier climates, possibly owing in part to higher vitamin D levels. One study found that, of patients who already have MS, those who took a high dose of vitamin D supplements had lower relapse rates. Another study showed that those with the highest vitamin D blood levels had a 62 percent lower risk of developing MS than those with the lowest vitamin D levels.

Likewise, a 30-year study in Finland found that children who regularly received vitamin D supplements during infancy had a nearly 90 percent lower risk of developing type 1 diabetes than those who received none. Other European case-control studies reinforce the conclusion that vitamin D may help protect against type 1 diabetes.

Even obesity may have connections to vitamin D. A clinical study found that providing supplements to obese and overweight people with vitamin D deficiency aids weight loss and enhances the benefits of a reduced-calorie diet. Another study revealed that women dieters with insufficient calcium who took a calcium and vitamin D supplement had more fat loss than those who did not.

As if that were not enough, vitamin D also helps to fight infection. Scientists have discovered that vitamin D is necessary to activate the immune system's T-cells, which identify and attack bad pathogens circulating throughout the body.

That may explain, at least partly, why flu virus peaks in the winter months (in temperate climates). This seasonality led a British physician to hypothesize that influenza outbreaks are affected by sunlight-related "seasonal stimulus". A randomized controlled trial in Japanese children found that type A influenza rates in children taking vitamin D supplements were about 40 percent lower than in those taking a placebo; there was no significant difference in type B influenza rates. A recent review found that vitamin D supplementation was safe and protective against acute respiratory tract infection overall.

Vitamin D's role in fighting infection is not exactly news. Before antibiotics were discovered, sunlight was part of the standard treatment for tuberculosis. A study showed that TB patients recovered at a faster rate when given vitamin D alongside antibiotics. And several studies, when analyzed together, suggest that people diagnosed with TB have lower vitamin D levels than healthy people of similar age and other characteristics.

More research is required before we can be certain of any of these benefits, but recent results are promising. In any case, there is no doubt that vitamin D is crucial for human health.

So how much vitamin D do we need to reap its disease-fighting rewards? The Institute of Medicine recommends tripling the recommended daily intake of vitamin D for children and adults in the United States and Canada, to 600 IU (15 micrograms) per day. Public Health England's recommended dietary allowance for adults and children over the age of one is 400 IU (10 mcg) per day. Given how few foods contain vitamin D naturally, reaching these levels may require conscious effort.

Some people may need more than the recommended dietary allowance－if they are obese or taking anticonvulsive medications, immuno-suppressants, antifungals such as ketoconazole, or medications for HIV/AIDS－though it is important to consult a physician before initiating such a regime. Ingesting too much vitamin D－above 10,000 IUs per day－may cause kidney and tissue damage and promote hypercalcemia (the buildup of calcium in the blood).

We are still a few years away from clinical trials that explore the possible link between vitamin D supplements, higher vitamin D levels, and reduced risk of disease. But given the public-health implications of such a connection－low vitamin D levels have been linked to a doubled risk of premature death－the research could not be more important. In public health, there aren't many game changers. Vitamin D may yet prove to be one.

The author is a regional medical expert at Sanofi Pasteur.