In recent years, vitamin D has gained a reputation for being a miracle nutrient of sorts— boasting benefits from improving physical and mental wellbeing. But as more research emerges, just how vast the vitamin’s value is has become the subject of scientific debate.

While some physicians argue most Americans would be wise to consume more of the vitamin, others say further research needs to be conducted before health care professionals urge their patients to soak up too much D.

So, what’s all the fuss about?

What it is

Technically, vitamin D is a type of hormone; it helps the body absorb calcium from food and plays a role in keeping bones dense. It’s hard to get from food sources alone, but most people can synthesize it in their skin when exposed to UVB light, which is present in sunlight. People in areas with frequent overcast days, older adults, and some women may benefit from a supplement.

READ MORE: 5 Reasons to Avoid Vitamin D Deficiency

“To sum it all up, nearly every cell in the body needs vitamin D to function at full capacity,” says Dr. Robert Heaney, professor of medicine at Creighton University. He is also research director at GrassrootsHealth, a nonprofit that promotes the benefits of vitamin D.

“Deficiency in vitamin D inhibits these processes, causing the cellular systems to break down, and leading to chronic disease,” he says.

Research suggests low vitamin D levels could play a role in various health conditions such as Type 1 and Type 2 diabetes, multiple sclerosis, hypertension and glucose intolerance. Those risks have compelled physicians to begin routinely testing patients’ D levels.

READ MORE: These Screenings and Blood Tests are Probably Free to You

The wave of research, increased serum testing, and emergence of organizations devoted to vitamin D has compounded, amounting to a kind of vitamin D movement. But why?

“Nearly 40 percent to 75 percent of the world's population is vitamin D deficient,” says Carole Baggerly, director of GrassrootsHealth. “The deficiency and disease that follow do not need to exist. GrassrootsHealth was founded to eradicate the vitamin D deficiency epidemic.”

Last October, researchers from the University of Alberta submitted a letter and calculations to the journal Nutrients claiming that the current recommended daily intake of vitamin D is too low— about 10 percent of what it should be to maintain adequate blood levels. This March, with financial help from GrassrootsHealth, Heaney submitted calculations confirming the authors’ findings to the same journal.

Hold on a minute

While some physicians and researchers are excitedly promoting the benefits of vitamin D, others are urging caution.

“We are at a crossroads,” says Dr. JoAnn Manson, professor of medicine at Harvard Medical School. “Observational studies suggest low vitamin D levels are associated with increased risks of many chronic diseases. We don’t yet have compelling evidence from randomized clinical trials that supplementation with high doses of vitamin D will reduce risk of these conditions.”

Randomized clinical trials can show causality— meaning, they can prove that raising D levels would reduce risk of disease. So far, all we know is that low vitamin D levels and certain diseases are occurring at the same time. It could be the case that for many people who are already sick, getting enough vitamin D through diet and sunlight is too difficult, leading to low levels.

READ MORE: Play it Sun-Smart this Summer

How much is too much?

“The vitamin D serum level should be checked every year, preferably in March or April when levels are lowest,” Baggerly says. “From there, you can target intake to keep your blood levels in the 40-60 [ng/mL] range.”

That’s the optimal level for warding off disease, GrassrootsHealth says. But not everybody agrees.

The NIH says that levels above 50 ng/mL may be dangerous, and levels above 60 ng/mL are linked to serious side effects, though this is disputed in some scientific literature. And just last month, researchers out of the University of Copenhagen provided evidence that too low or too high of vitamin D levels are linked to an increase in premature death.

READ MORE: Eat Right to Live Longer

Like many other nutrients, you can have too much vitamin D, though toxicity is rare. Vitamin D toxicity can lead to heart arrhythmias, kidney stones, or calcification of blood vessels and organs, causing serious damage.

That’s precisely the problem of promoting it too much, Manson says. “People should avoid taking megadoses of anything until a clear benefit is established,” she says.

People curious about vitamin D may not have to wait much longer.

“There are at least four large, randomized clinical trials in progress on vitamin D, and several smaller ones,” Manson says. “We’ll have the answers to these questions in just a few years.”