In 2005, a forty-five-year-old woman, feeling fatigued, visited her doctor, who ordered a series of tests. She was told that the tests were all within the normal range, including her vitamin D level. Ten years later, the woman had another check of her vitamin D, again because she was feeling fatigued. The vitamin D level was unchanged. This time, however, the doctor told her that her level was inadequate, and recommended supplemental vitamin D. Same doctor, same testing laboratory, same vitamin D levels, same symptoms. What happened?
We’re in the midst of a vitamin D craze. Everyone, it seems, is taking it.1 Vitamin D now outsells every other supplement on the market, including fish oil, probiotics, and multivitamins. Indeed, one in four older adults now takes vitamin D supplements.2
The reason that so many people are taking vitamin D is to prevent thinning or weakening of their bones. But that’s not all. They also take it to treat muscle weakness and fatigue or to boost their immune systems or to slow the aging process or to prevent cancer or heart disease or diabetes. Dr. Clifford Rosen, a bone specialist at Maine Medical Center, calls our compulsion to take vitamin D “a religion.”3
What most people don’t realize is that they’ve been duped.
Vitamins are essential for normal growth and nutrition. Unfortunately, our bodies can’t make vitamins. We can get them only from food. Unlike every other vitamin, however, vitamin D can be made in the body. So, technically, it’s not a vitamin; it’s a hormone.
How do we make vitamin D? Sunlight converts a form of cholesterol in our skin to pre-vitamin D3, which is then modified in the liver. When doctors request a blood test to measure vitamin D, it’s this liver-modified form they’re measuring, even though it’s not the active form of the vitamin. The active form is made in the kidneys.
Vitamin D facilitates the absorption of calcium from the intestine. Without vitamin D, only 10 to 15 percent of dietary calcium is absorbed; with it, 30 to 40 percent is absorbed. Calcium is necessary to make bones strong; indeed, about 99 percent of the body’s calcium is stored in bones. People who don’t make enough vitamin D don’t absorb enough calcium, which leads to bone thinning. This thinning or softening of the bones causes an outward bowing of the knees (rickets) in children and a dramatic increase in the incidence of fractures in older adults. About 50 percent of women and 25 percent of men over fifty will have a fracture due to bone thinning sometime during their life.4 So, it’s not an insignificant problem.
Because vitamin D can be made in the body using ultraviolet light from the sun (hence its nickname the “sunshine vitamin”), we don’t necessarily need to get vitamin D from food. This is fortunate because, unlike most other vitamins, it’s hard to find a lot of natural vitamin D in what we eat. Vitamin D can be found in fatty fish such as salmon, mackerel, sardines, and tuna. It can also be found in beef liver, cheese, egg yolks, white beans, mushrooms, broccoli, collard greens, kale, and soy beverages. Because vitamin D isn’t prevalent in nature, and because some people don’t get adequate amounts of sunlight, years ago federal agencies required food producers to add vitamin D to milk, orange juice, yogurt, and breakfast cereals. Once foods were supplemented, the problem of severe vitamin D deficiency virtually disappeared in the United States. Rickets, which was common among children in the 1920s, has essentially vanished.5 Thinning of the bones in older adults, however, has not.
To understand how we came to believe that so many people are now vitamin D deficient, we need to go back to the beginning.
Humankind’s debut on earth occurred on a prehistoric landmass at or near the equator, where sunshine was plentiful. It was here that our ability to synthesize vitamin D in our skin was born. It wasn’t long, however, before we migrated to areas where sunshine was less plentiful. Today, anyone who lives at or above 37 degrees north of the equator will have limited sunlight during the winter months. In the West, this translates to living north of a line extending from San Francisco to Philadelphia; in the East, from Athens to Beijing. To deal with this lack of sunlight, we developed a mechanism to store vitamin D in fat, which could then be liberated during the winter. This worked well for winters that were shorter than ten weeks. For longer winters, however, vitamin D stores were often depleted. At this point, vitamin D could be obtained only from food.6
Given that most people get enough vitamin D from sunlight and that those who experience long winters get enough vitamin D from fortified foods, why does anyone need to take vitamin D supplements? The answer can be found in the influence of two different types of studies, neither of which, as it turns out, was correct.
First, researchers compared the relative health of people who lived at or near the equator with those who lived farther away. Studies that compare health outcomes in people who live in different parts of the world are called ecological studies. Researchers found that those who lived farther from the equator were more likely to suffer from asthma, diabetes, multiple sclerosis, obesity, schizophrenia, bipolar disorder, and seasonal affective disorder (SAD); they were also more likely to develop colon, breast, and prostate cancer.7 Most impressively, a twenty-year study of thirty thousand people found that those with limited sun exposure didn’t live as long.8 Vitamin D, it appeared, was far more important to human health than simply making strong bones.
Then researchers took the next step. To prove that these dramatic differences in health were caused by vitamin D, they compared the health of people who took vitamin D supplements with those who did not. This time, however, it didn’t matter where people lived or how much they went out in the sun. Studies that compare the health of one group with another where those being studied (not the investigator) decide whether to take a particular medicine or dietary supplement are called observational studies. In this case, the investigator stands back and just observes what is happening.
Again, the results were clear. Observational studies showed that those who took vitamin D supplements were less likely to develop breast, ovarian, skin, prostate, lung, and colon cancer. They were also less likely to have high blood pressure, more likely to have high levels of high-density lipoprotein cholesterol (HDL, the good cholesterol), and less likely to suffer heart attacks. Further, they were less likely to be stricken by infectious diseases such as tuberculosis and influenza or to be diagnosed with mental disorders such as depression, Alzheimer’s disease, and Parkinson’s disease. They were also less likely to suffer from diseases such as diabetes, multiple sclerosis, rheumatoid arthritis, and thyroid disease as well as a variety of other problems such as obesity, chronic kidney diseases, age-related macular degeneration of the eye, chronic pain syndromes, and fractures from falls. In other words, people who took supplemental vitamin D were less likely to be burdened by almost every disease known to humankind.9
How was this possible? If vitamin D affects only the amount of calcium in bones, why would it impact so many other systems in the body? A closer look at the biology of vitamin D appeared to provide the answer. Scientists found that most cells in the body have their own receptors for vitamin D and that when vitamin D binds to cells, it can impact more than two hundred different genes that regulate cellular activity. Further, they found that the enzyme that converted vitamin D to its active form wasn’t found only in the kidneys; it was found also in the colon, prostate, and breast, which might explain why vitamin D lowered the incidence of cancer at those sites. Finally, researchers found that in addition to its effect on bone metabolism, vitamin D impacted the immune system and other hormonal systems as well as how cells grew and differentiated.10
Everything added up. The only thing left was to prove that vitamin D was doing exactly what the ecological and observational studies had shown that it was doing. Researchers then turned to a third group of studies called randomized, controlled clinical trials. Unlike the ecological studies, which observed health differences based on where people lived, or observational studies, which allowed subjects to choose whether they took vitamin D or not, researchers performing randomized, controlled trials determined who would receive supplemental vitamin D. Investigators could now make sure that those who did or did not take vitamin D supplements were identical in their medical backgrounds, socioeconomic status, health care–seeking behavior, and areas of residence.
Again, as was true for the ecological and the observational studies, the results from the randomized, controlled clinical trials were clear. This time, however, they were exactly the opposite of what researchers had expected.
In 2019, researchers from Tokyo divided four hundred patients with cancers of the esophagus, stomach, colon, or rectum into two groups. One group received 2,000 international units (IU) of vitamin D every day; the other, a placebo. Researchers found no differences in the survival rates or relapse rates for any cancer.11
In 2014, researchers from Denmark reviewed the results of eighteen studies involving 50,000 middle-aged women. The cancer rate was 7.6 percent in those who took vitamin D and 7.7 percent in those who didn’t. The only difference between these two groups was that those who took vitamin D had an increased risk of kidney stones.12
In 2017, researchers from Creighton University in Nebraska reported the results of a four-year study of vitamin D involving 2,300 middle-aged women who did or didn’t receive vitamin D plus calcium. The incidence of cancer in the two groups was indistinguishable.13
In 2018, researchers from the United Kingdom reviewed the results of thirty studies of vitamin D involving 19,000 men and women followed for up to six years. The incidence of cancer and cancer-related deaths was the same in both groups.14
In 2019, researchers from Harvard Medical School reported the results of a five-year study in which they divided 26,000 older adults into two groups. One group received 2,000 IU of vitamin D every day; the other group didn’t. At the end of the study, the incidence of cancer was the same in both groups.15
In 2014, researchers from New Zealand reviewed the results of eight studies involving 46,000 participants, finding that vitamin D with or without calcium was of no value in preventing stroke or heart disease. They also reviewed the results of nine studies with a total of 49,000 participants and found no benefit in the prevention of heart attacks.16
In 2012, researchers from New Zealand studied 320 healthy adults who either did or did not receive vitamin D monthly for eighteen months. The incidence of respiratory infection was the same in both groups. No differences were seen in the number of infections, the severity of symptoms, the duration of symptoms, or work days missed.17
In 2017, French researchers reviewed eighty-three studies, finding no differences in the incidence of obesity, diabetes, mood disorders, muscular function, tuberculosis, or cancer in those who did or didn’t take vitamin D with or without calcium.18
In 2010, Australian researchers studied 2,300 women over age seventy who either were or were not given vitamin D every autumn for three to five years. Shockingly, those who received vitamin D were more likely to fall and more likely to suffer fractures than those who didn’t.19
In 2014, French researchers reviewed the results of twenty-two studies involving a total of 76,000 participants, finding no differences in the incidence of fractures in those who did or didn’t receive vitamin D with or without calcium. They also reviewed the results from twelve trials involving 28,000 participants who did or didn’t receive vitamin D, again finding no differences in the incidence of hip fracture.20
In 2017, researchers in China reviewed the results of thirty-three studies involving 51,000 participants over age fifty who either did or didn’t receive vitamin D plus calcium. They found no differences in the incidence of fractures regardless of the gender, history of previous fractures, dietary calcium intake, baseline vitamin D levels, or dose of vitamin D and calcium given.21
In 2018, researchers from New Zealand reviewed eighty-one studies involving 54,000 people who did or did not receive vitamin D supplements. They found that vitamin D had no effect on the number of total fractures, hip fractures, or falls, or on bone mineral density.22
Worse, researchers performing randomized, controlled studies found not only that vitamin D didn’t work but that high blood levels of vitamin D could cause weakness (which might explain why the 2010 Australian study found an increased risk of falls resulting in fractures), nausea, vomiting, frequent urination, and kidney stones.23
What happened?
In observational studies, individuals decide whether to take vitamin D. In randomized, controlled studies, researchers make that decision. This is an important difference. As it turned out, people who chose to take vitamin D were more likely to be wealthier, to have health insurance, to visit a doctor when sick, and to exercise; they were also less likely to smoke.24
When one factor (e.g., vitamin D levels) is associated with a particular disease but not the cause of that disease, it’s called an epiphenomenon.25 One notable example of an epiphenomenon is something called the anemia of chronic disease. Many people with inflammatory diseases (e.g., infections, rheumatologic diseases, or cancer) are anemic, meaning that they have low numbers of circulating red blood cells. In these cases, because the anemia isn’t causing the disease, giving blood transfusions won’t treat it. Similarly, if low levels of vitamin D are just a marker for chronic illness, it shouldn’t be surprising that giving vitamin D won’t treat or prevent that illness.
In the United States, no group has been diagnosed more frequently or more inaccurately as vitamin D deficient than African Americans. Because vitamin D levels are consistently lower in black people than in white people, black people are often told that they need to take supplemental vitamin D. At the same time, however, their bone mineral density is significantly higher than that in white subjects. How is this possible? In a paper published in the prestigious New England Journal of Medicine, Harvard researchers found the answer. Although black people clearly have lower levels of vitamin D, they also have much lower levels of vitamin D–binding protein, something that is used by the body to store vitamin D in the winter. Because less vitamin D is bound by this protein, more is available to absorb calcium from the intestine and build strong bones. By choosing to measure only vitamin D levels in black patients, doctors have consistently misdiagnosed those patients.26
The failure of physicians to measure vitamin D–binding protein is only part of the problem. In addition to its importance in building strong bones, calcium is also critical to the normal function of electrical impulses in the heart. Too much or too little calcium in the bloodstream could cause the heart to stop beating. Because specific levels of calcium in the bloodstream are essential to life, it’s a highly regulated process. And neither vitamin D nor vitamin D–binding protein is doing the regulating. Rather, both are regulated by a gland located behind the thyroid, in the neck. This gland, called the parathyroid gland, releases a substance called parathormone (PTH), which regulates the conversion of vitamin D to its active form in the kidney. If calcium levels in the bloodstream are too low, PTH liberates calcium from bones, even if that means that the bones become dangerously thin. The body would rather suffer weakened bones than a heart that doesn’t beat properly.
The interplay among the synthesis of vitamin D by the sun, liver, and kidneys; the storage of vitamin D in fat by vitamin D–binding protein; and the regulation by PTH of both vitamin D and calcium is complex. So, when doctors get a vitamin D level, they are looking at only one part of the process, which can be misleading. It would be far more accurate to measure calcium, PTH, vitamin D–binding protein, and vitamin D at the same time.27 It is the rare doctor who does this.
Studies showing that people who lived at or near the equator were healthier than those who lived farther away were misleading for another reason. In addition to vitamin D, sunlight also enhances the production of a variety of other substances, such as beta-endorphins, which modify pain; calcitonin, which works in opposition to PTH; substance P, which affects blood flow, inflammation, pain, mood, anxiety, and cell growth; adrenocorticotropic hormone (ACTH), which controls the immune system and inflammation; and melanocyte-stimulating hormone, which reduces appetite, increases libido, and is responsible for skin pigmentation.28 All these substances affect health, all are enhanced by exposure to the sun, and none was measured in studies that determined the effect of sun exposure on human health.
With all this information in hand, two federal advisory bodies, the Institute of Medicine (IOM) and the U.S. Preventive Services Task Force (USPSTF), convened a panel of experts to determine who should be tested for vitamin D deficiency and what levels of vitamin D in the bloodstream were adequate. Both groups reached the same conclusions: (1) Exposure to sunlight and fortified foods alone provide enough vitamin D for 97.5 percent of the U.S. population; (2) blood levels above 20 nanograms (a nanogram is one billionth of a gram) of vitamin D are adequate; and (3) the only people who should be tested for vitamin D deficiency are those with thinning bones; those with conditions that affect the absorption of fat from the intestine, such as weight-loss surgery or celiac disease; and those taking medicines that affect the absorption or processing of vitamin D such as steroids and antiseizure medications. For people unable to absorb vitamin D from food, the IOM recommended the daily intake of 600 IU up to seventy years of age and 800 IU for those older. Everyone else should stop getting tested for vitamin D and stop taking vitamin D supplements.29
So, if, according to the IOM and the USPSTF, most people have adequate levels of vitamin D and get enough vitamin D from the sun and from fortified foods, why are so many people now told that they are vitamin D deficient? The reason is that although the IOM and the USPSTF agreed that levels of vitamin D greater than 20 nanograms were adequate, most laboratories (e.g., Quest Diagnostics and LabCorp) list 20 to 30 nanograms as insufficient. Why?
On April 10, 2017, Gina Kolata pulled back the curtain. In an article written for the New York Times titled “Why Are So Many People Popping Vitamin D?” Kolata noted that Dr. Michael Holick, a professor of medicine, physiology, and biophysics at Boston University School of Medicine, was a “leading proponent of the idea that just about everyone needs a vitamin D supplement.” Contrary to the IOM and the USPSTF, Holick believed that people needed vitamin D levels to be at least 30 nanograms, not 20. This level cannot easily be achieved by diet and would require almost constant exposure to the sun. The only way that these higher levels could be obtained would be by taking vitamin D supplements. It’s not unusual to find the occasional doctor who disagrees with recommended federal guidelines. But Michael Holick wasn’t just any doctor. In 2011, Holick headed a committee of the Endocrine Society that recommended that vitamin D levels be at least 30 nanograms, which meant that about half the U.S. population was suddenly vitamin D deficient. As a consequence of this recommendation, commercial labs ignored federal advisory groups and began listing levels of 20 to 30 nanograms of vitamin D as insufficient. Most continue to do so today.30
Holick’s advocacy has been embraced by Gwyneth Paltrow’s lifestyle and wellness company, Goop, and by Mehmet Oz, who describes vitamin D as “The No. 1 thing you need more of.” Oz tells his audience that vitamin D will help avoid heart disease, depression, weight gain, memory loss, and cancer, despite abundant evidence to the contrary.31
Ravinder Singh, who runs a testing lab at the Mayo Clinic, was shocked by the sudden change in recommended levels. “Demand for vitamin D testing went through the sky,” he said. “It was almost as though there was nothing else in clinical practice.”32
On August 18, 2018, Liz Szabo, in an article for the New York Times titled “Vitamin D, the Sunshine Supplement, Has Shadowy Money Behind It,” took a closer look at exactly who was funding Dr. Michael Holick. Szabo noted that Holick’s guidelines had helped push vitamin D sales up to $936 million a year, a ninefold increase over the previous decade. Holick acknowledged in an interview that he was receiving $1,000 a month from Quest Diagnostics, a commercial lab that was clearly benefiting from his recommendations. Holick also had extensive ties to the pharmaceutical industry. Between 2013 and 2017, he received $163,000 for consulting services, according to Medicare’s Open Payments Database, which tracks payments from drug manufacturers. The companies that paid him include Sanofi-Aventis, which markets vitamin D supplements; Shire, which makes drugs for hormonal disorders that are treated with vitamin D; Amgen, which makes a drug for the treatment of bone thinning; and Roche Diagnostics and Quidel Corporation, both of which make vitamin D tests.33
Dr. Holick’s ties to the tanning bed industry have also come under fire. In 2008, David Armstrong, in an article for the Wall Street Journal titled “Researcher Received Industry Funds,” noted that Holick had published an article recommending the moderate use of tanning beds to increase vitamin D levels—no doubt to the dismay of dermatologists, who know that tanning beds increase the risk of skin cancer. According to the Wall Street Journal article, Boston University had received a series of grants totaling $162,014 from the UV Foundation, an organization funded by the now-defunct Indoor Tanning Association, whose board of directors was composed entirely of tanning bed industry executives, in order to fund vitamin D research by Holick.34 For this reason, Holick was asked to step down from his position in Boston University’s dermatology division, which he did. But he remained a professor in the medical school’s Department of Endocrinology, Diabetes, Nutrition, and Weight Management.35 Although Holick argued that his industry funding “doesn’t influence me in terms of talking about the health benefits of vitamin D,” his financial association with a testing laboratory that has directly benefited from his advocacy is, at best, unseemly.36
The clash between the levels of vitamin D listed by testing laboratories and those recommended by the two federal advisory panels isn’t the only force working against patients. Advocacy organizations such as the U.S. National Osteoporosis Foundation (NOF) and the Europe-based International Osteoporosis Foundation (IOF)—osteoporosis is the medical term for thinning bones—also ignore federal guidelines and constantly promote the notion that only vitamin D levels above 30 nanograms are adequate and that otherwise healthy people should be routinely screened. Twelve of the twenty-two NOF corporate sponsors and fourteen of the twenty-five IOF corporate sponsors sell nutritional products that promote vitamin D.37
Consumers today have a choice to make. Either they can believe that both the IOM and the USPSTF are correct and that 97.5 percent of the U.S. population are getting adequate amounts of vitamin D from sunlight and fortified foods; or they can believe that diagnostic labs are correct and that roughly half the U.S. population (about 165 million people) is vitamin D deficient. Dr. JoAnn E. Manson, a preventive medicine researcher at Brigham and Women’s Hospital in Boston, summed it up best: “A lot of clinicians are acting like there is a pandemic,” she said. “This gives them justification to screen everyone.”38 In the end, patients should ask themselves who is really benefiting from all this testing, all these prescriptions, and all these office visits.