Dr. Michael Holick's enthusiasm for vitamin D can be fairly described as extreme. The Boston University endocrinologist, who perhaps more than anyone else is responsible for creating a billion-dollar vitamin D sales and testing juggernaut, elevates his own levels of the stuff with supplements and fortified milk. When he bikes outdoors, he won't put sunscreen on his limbs. He has written book-length odes to vitamin D, and has warned in multiple scholarly articles about a "vitamin D deficiency pandemic" that explains disease and suboptimal health across the world.

His fixation is so intense that it extends to the dinosaurs. What if the real problem with that asteroid 65 million years ago wasn't a lack of food, but the weak bones that follow a lack of sunlight? "I sometimes wonder," Dr. Holick has written, "did the dinosaurs die of rickets and osteomalacia?"

Dr. Holick's role in drafting national vitamin D guidelines, and the embrace of his message by mainstream doctors and wellness gurus alike, have helped push supplement sales to $936 million in 2017. That's a ninefold increase over the previous decade. Lab tests for vitamin D deficiency have spiked, too: Doctors ordered more than 10 million for Medicare patients in 2016, up 547 percent since 2007, at a cost of $365 million.

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But few of the Americans swept up in the vitamin D craze are likely aware that the industry has sent a lot of money Dr. Holick's way. A Kaiser Health News investigation for The New York Times found that he has used his prominent position in the medical community to promote practices that financially benefit corporations that have given him hundreds of thousands of dollars — including drug makers, the indoor tanning industry and one of the country's largest commercial labs.

In an interview, Dr. Holick acknowledged he has worked as a consultant to Quest Diagnostics, which performs vitamin D tests, since 1979. Dr. Holick, 72, said that industry funding "doesn't influence me in terms of talking about the health benefits of vitamin D."

There is no question that the hormone is important. Without enough of it, bones can become thin, brittle and misshapen, causing a condition called rickets in children and osteomalacia in adults. The issue is how much vitamin D is healthy, and what level constitutes deficiency.

Dr. Holick's crucial role in shaping that debate occurred in 2011. Late the previous year, the prestigious National Academy of Medicine (then known as the Institute of Medicine), a group of independent scientific experts, issued a comprehensive, 1,132-page report on vitamin D deficiency. It concluded that the vast majority of Americans get plenty of the hormone naturally, and advised doctors to test only patients at high risk of certain disorders, such as osteoporosis.

A few months later, in June 2011, Dr. Holick oversaw the publication of a report that took a starkly different view. The paper, in the peer-reviewed Journal of Clinical Endocrinology & Metabolism, was on behalf of the Endocrine Society, the field's foremost professional group, whose guidelines are widely used by hospitals, physicians and commercial labs nationwide, including Quest. The society adopted Dr. Holick's position that "vitamin D deficiency is very common in all age groups" and advocated a huge expansion of vitamin D testing, targeting more than half the United States population, including those who are black, Hispanic or obese — groups that tend to have lower vitamin D levels than others.

The recommendations were a financial windfall for the vitamin D industry. By advocating such widespread testing, the Endocrine Society directed more business to Quest and other commercial labs. Vitamin D tests are now the fifth-most-common lab test covered by Medicare.

The guidelines benefited the vitamin D industry in another important way. Unlike the National Academy, which concluded that patients have sufficient vitamin D when their blood levels are at or above 20 nanograms per milliliter, the Endocrine Society said vitamin D levels need to be much higher — at least 30 nanograms per milliliter. Many commercial labs, including Quest and LabCorp, adopted the higher standard.

Yet there's no evidence that people with the higher level are any healthier than those with the lower level, said Dr. Clifford Rosen, a senior scientist at the Maine Medical Center Research Institute and co-author of the National Academy report. Using the Endocrine Society's higher standard creates the appearance of an epidemic, he said, because it labels 80 percent of Americans as having inadequate vitamin D. "We see people being tested all the time and being treated based on a lot of wishful thinking, that you can take a supplement to be healthier," Dr. Rosen said.

Patients with low vitamin D levels are often prescribed supplements and instructed to get checked again in a few months, said Dr. Alex Krist, a family physician and vice chairman of the United States Preventive Services Task Force, an expert panel that issues health advice. Many physicians then repeat the test once a year. For labs, "it's in their financial interest" to label patients with low vitamin D levels, Dr. Krist said.

In a 2010 book, "The Vitamin D Solution," Dr. Holick gave readers tips to encourage them to get their blood tested. For readers worried about potential out-of-pocket costs for vitamin D tests — they range from $40 to $225 — he listed the precise reimbursement codes that doctors should use when requesting insurance coverage. "If they use the wrong coding when submitting the claim to the insurance company, they won't get reimbursed and you will wind up having to pay for the test," Dr. Holick wrote.

Dr. Holick acknowledged financial ties with Quest and other companies in the financial disclosure statement published with the Endocrine Society guidelines. In an interview, he said that working for Quest for four decades — he is currently paid $1,000 a month — hasn't affected his medical advice. "I don't get any additional money if they sell one test or one billion," he said.

A Quest spokeswoman, Wendy Bost, said the company seeks the advice of a number of expert consultants. "We feel strongly that being able to work with the top experts in the field, whether it's vitamin D or another area, translates to better quality and better information, both for our patients and physicians," Ms. Bost said.

Since 2011, Dr. Holick's advocacy has been embraced by the wellness-industrial complex. Gwyneth Paltrow's website, Goop, cites his writing. Dr. Mehmet Oz has described vitamin D as "the No. 1 thing you need more of," telling his audience that it can help them avoid heart disease, depression, weight gain, memory loss and cancer. And Oprah Winfrey'swebsite tells readers that, "knowing your vitamin D levels might save your life." Mainstream doctors have also urged Americans to get more of the hormone, including Dr. Walter Willett, a widely respected professor at Harvard Medical School.

Today, seven years after the dueling academic findings, the leaders of the National Academy report are struggling to be heard above the clamor for more sunshine pills. "There isn't a 'pandemic,'" said A. Catharine Ross, a nutritional sciences professor at Penn State and chairwoman of the committee that wrote the report, in an interview. "There isn't a widespread problem."