When asked about how people can stay healthy in the winter months, Dr. John Young has an unexpected answer: vitamin D supplements.

Young, medical director at Young Foundational Health Center in Largo, Florida, regularly encourages his patients to take around 10,000 International Units of the hormone each day and 40,000 IU when they're feeling ill.

"(Patients) will tell me, 'The next morning I had to remind myself that I was sick,'" he said, calling the extra large dose a "trick of the trade."

Although most national health organizations set the recommended daily dose of vitamin D between 400 and 1,000 IU, Young is not alone in crediting higher doses with healing power. Over the last few years, vitamin D has emerged as a darling of the health industry, tied to reduced risk of osteoporosis, heart disease, some cancers, multiple sclerosis and even the seasonal flu, according to the Harvard School of Public Health.

However, supplement stardom has brought with it increased scrutiny. In January 2014, a group of researchers published a review of recent vitamin D research, concluding that the nutrient is overhyped and urging members of the health industry to adopt a more measured approach to researching and promoting it.

As consumers weigh advice from friends, medical professionals and health magazines when deciding whether or not to add vitamin D supplements to their shopping lists, the question remains: What exactly do they do for the body?

The basics

Contrary to its name, vitamin D is actually a hormone, produced by the skin as a result of sun exposure, Young said. It's more like testosterone or estrogen than other vitamins of the alphabet, which are predominately taken in through food consumption.

Confusion is to be expected, Young noted, because for many months of the year vitamin D is only available through food or supplements due to the position of the sun. The skin's production of the nutrient is made possible by UVB rays, which don't reach most Americans throughout the winter.

Young's advice about taking supplements is inspired not only by reduced exposure to this type of sunlight, but also by how difficult it is for the average person to maintain a vitamin D-rich diet. The nutrient is found in oily fish like salmon and fortified dairy and grain products, a far dietary cry from the red meats and side dishes associated with the holiday season.

Beyond claims about its ability to strengthen the bones and reduce the risk of various diseases, vitamin D has a basic role to play in the body: modulating the immune system. It keeps the body healthy, aiding in the absorption and regulation of other nutrients.

"Every cell in the body has a vitamin D receptor," said Michael Holick, a prominent vitamin D researcher based at Boston University. "Every cell in the body needs it for maximum cellular health."

Controversial performance

Holick's resume is filled with the sunshine vitamin. After reluctantly joining a vitamin D research project while in graduate school, he's made the nutrient his life's work, publishing numerous studies and books like "The Vitamin D Solution: A 3-Step Strategy to Cure Our Most Common Health Problems."

Holick believes that vitamin D might be "the nutrient of the century," citing the thousands of success stories he's witnessed while serving at Boston University's General Clinical Research Center.

It's this hands-on experience that he uses to counter naysayers such as Mark Bolland, who co-authored last year's review of the benefits of vitamin D. The research determined that vitamin D supplements did not reduce the risk of heart attacks, strokes, cancer or bone fractures by more than 15 percent, in spite of previous claims to the contrary.

Bolland, who is part of a research group focused on osteoporosis, said it was never his intention to launch a broad takedown of vitamin D. Instead, he and his colleagues sought to stem an increasingly powerful tide of support for the nutrient in the health community by returning some skepticism to the research process.

"A number of very large and expensive trials are now being undertaken," wrote Bolland, an associate professor at the University of Aukland, in an email. "Collectively, these trials will cost in the hundreds of millions of dollars."

Vitamin D is now so popular, particularly as a treatment for osteoporosis, that Bolland said doctors are failing to pay attention to studies that show the nutrient is not living up to researchers' promises.

Concerns about misleading consumers are largely ignored, and the vitamin D supplement industry takes in millions of dollars each year, he said, noting that annual sales of vitamin D in the U.S. increased from $184 million to $748 million between 2007 and 2013.

"(The supplements') benefits are small, if any, for the overwhelming majority of people who take them," Bolland said. "This has led to a lot of wasted money that could be better spent elsewhere in the health system."

Holick's answer to the criticism is that researchers like Bolland "play with the numbers only," instead of watching real people flourish as a result of taking vitamin D supplements.

To D or not to D

Vitamin D is not the first nutrient to receive the star treatment. Previously, there were vitamins A and E, both of which were hailed as key supplements before their fame flamed out, whether from lack of results or interest, Holick said.

One key factor supporting the nutrient's recent popularity is the sheer number of reports expressing concern over worldwide vitamin D deficiency. Very few studies have highlighted the risk of overprescription.

"Worldwide, an estimated one billion people have inadequate levels of vitamin D in their blood, and deficiencies can be found in all ethnicities and age groups," the Harvard School of Public Health reported.

Young said that the only way vitamin D can do harm in the body is if calcium levels are too high. He monitors calcium for each of his patients taking vitamin D supplements.

Although he's aware no nutrient is a miracle cure, Holick said vitamin D is supported by more than a decade of research. He has taken 4,000 IU per day for over a decade.

"Even if it only ends up being effective in preventing one disease, you're ahead of the game," he said.

Email: kdallas@deseretnews.com Twitter: @kelsey_dallas