Here’s what Dr. Joseph Mercola had to say about vitamin D (right beside his ad hawking a month’s supply of the stuff for $29.97): “There’s very few supplements I recommend for almost everyone but vitamin D is one of them.” And this from Life Extension Magazine: “Why isn’t everyone supplementing with Vitamin D?” Finally, from Dr. Mehmet Oz: “If I could think of one vitamin to push to everybody to get into their lives, it’s vitamin D.” Make no mistake, vitamin D is the “it girl” of the vitamin world—the sleek, sexy cure-all for falls, fractures, muscle weakness, heart disease, stroke, cancer, diabetes, asthma, and multiple sclerosis. A few years ago, on Good Morning America, Diane Sawyer stated that: “One-hundred million Americans have vitamin D deficiency. Think about it—one-hundred million of us, including children.” Americans listened, and now more than one-hundred million Americans are taking some form of vitamin D supplement. But if vitamin D is so great, then why did the USPSTF (US Preventative Services Task Force) recommend against using it at current doses in postmenopausal women to prevent fractures, the very group touted to have the greatest need?
Vitamin D, known as the “sunshine vitamin,” is complicated. Technically, it’s not a vitamin at all, but rather a hormone that has multiple effects throughout the body, although for the sake of this post I’ll primarily be discussing vitamin D’s effects on calcium metabolism and bone mineralization. Despite misconceptions that vitamin D is all about maintaining bone density, its real purpose is to keep a tight rein on the amount of calcium in the bloodstream. Calcium is the star here. Muscles, including the heart, can’t contract without it. The body has a vested interest in maintaining blood levels within a narrow range. Although the most abundant mineral in the body, 98% of our calcium is stored within bone, unavailable for immediate use. And this is where vitamin D comes in. Low blood calcium signals the parathyroid gland to release parathyroid hormone, which stimulates the kidney to churn out the biologically active form of vitamin D3 [1,25-(OH)2-D3, calcitriol], a potent hormone that, in turn, increases intestinal absorption of dietary calcium while simultaneously activating osteoclasts to mobilize a portion of the calcium stored within bone. Both of these effects serve to raise serum calcium levels. And as with every homeostatic process, there are counter hormones—in this case, calcitonin, synthesized in the thyroid gland—that work to do the opposite. This helps explains why multiple trials involving hundreds-of-thousands of patients have failed to show that vitamin D supplementation increases bone density. That’s right—taking vitamin D by itself doesn’t work to “build stronger bones,” because that’s not its purpose in the body. The true function of vitamin D isn’t to maintain bone calcium but rather to maintain blood calcium. Along the way, the skin, kidney, intestine, bone, parathyroid, and thyroid gland all have key roles to play. When there is plenty of vitamin D and calcium available, the result is that blood levels of calcium are normal and the excess is shunted to bone. This, in turn, adds to bone strength and reduces the risk of fracture. Calcitonin—not vitamin D—is the hormone responsible for building bone.
But this doesn’t mean that vitamin D isn’t important to bone health. When the supply is inadequate, either through a lack of sunlight or dietary deficiency, the body will do whatever is necessary to maintain normal blood calcium levels, even if that means weakening bones by leeching calcium from them. In children, the result is rickets, a disease of marked demineralization and subsequent long bone deformities. In the late 1800’s, German physicians noted that the disease could be entirely prevented by administering a daily teaspoon of cod liver oil. Although a serendipitous observation, we now know that cod liver oil contains huge amounts of vitamin D, on the order 1,350 IU per teaspoon (more than twice the recommended daily intake), and far more than other foods like salmon (450 IU/3 ounces), tuna (150 IU/3 ounces), or eggs (40 IU/egg yolk). In the 1930’s, milk suppliers began fortifying milk with vitamin D (115 IU/cup), and the daily teaspoon of cod liver oil recommended by your great grandmother went the way of the Model T.
There are 3 ways to meet your daily need; food, supplements, or sunlight. Of these, sunlight is the most efficient. Between 10- to 60-minutes of sun exposure (without sunscreen) twice weekly between the hours of 10 AM and 3 PM will fulfill the requirement for most, with the duration of sun exposure required dependent upon the latitude and the season. In the skin, pre-vitamin D3 is converted to vitamin D3 through the effects of UVB rays from the sun. Cloud cover blocks out roughly 50% of these rays; sitting in the shade, 60%; SPF sunscreen of 30 or greater, 95%. Glass also filters out UVB light, so driving on a sunny day with the windows up prevents your skin from synthesizing vitamin D.
In areas with a UV Index of 8-10, as little as 5-10 minutes of sun exposure is adequate for vitamin D synthesis; in areas with an index of 6-8, 10-20 minutes; areas of 3-6, 20-40 minutes; areas of 1-3, 30-60 minutes; while areas with an index of less than 1 are unlikely to result in vitamin D synthesis regardless of exposure duration.
Fortunately for those who live in Buffalo (311 cloudy days/year), Seattle (308 cloudy days/year), or Pittsburgh (306 cloudy days/year), you can still get your daily fix from food, but note that vitamin D is not widely prevalent. For most, fortified milk remains the go-to product. Salmon, tuna, and mackerel contain moderate amounts, as does orange juice and yogurt. Margarine, eggs, and beef liver contain smaller amounts. The majority of vitamin D in food comes in the form of vitamin D3 (cholecalciferol) with smaller amounts as D2 (ergocalciferol).
Last, there are supplements, ranging from multivitamins, to stand-alone vitamin D products, to combination products containing both vitamin D and calcium. Note to vegans: If you live in a northern latitude and don’t get outside much, you might want to consider taking a vitamin D3 supplement. Otherwise, not so much. Why not? Let’s follow the evidence.
First of all, where did Diane Sawyer get off saying that more than one-hundred-million Americans are vitamin D deficient? Was she lying, or was this a case of garbage in/garbage out? A lot of “health experts” are now recommending that everyone get their vitamin D level checked, because people won’t know they’re deficient without checking (which begs the question that if you’re healthy and asymptomatic, who cares what your level is?). For a number of reasons, the form of vitamin D doctors measure in the blood is 25-(OH)-D3 (calcidiol). Unfortunately, there is no accepted standard of what this level should be. Should it be the lowest level needed to prevent disease, or the level needed for optimum health? What if that level isn’t the same for everybody, or even the same in a given individual as he/she ages?
The daily RDA (Recommended Dietary Allowance) for vitamin D is currently set at 600 IUs for adults between the ages of 18 to 70, and 800 IUs for those older than 70, an intake generally adequate to achieve 25-(OH)-D3 levels of approximately 50 nmol/L. When tested against this standard, roughly a third of people fall below this number. Extrapolating to a population of 318 million, and you arrive at Sawyer’s “hundred million” number. If the Endocrine Society’s higher cutoff of 75 nmol/L is used to define “insufficiency,” then that number swells to nearly 245 million people. Does anybody really believe that the most overweight, overfed, over-tanned nation on earth really has 245 million citizens deficient in anything? This number has no face validity, utterly failing the “sniff test.” In other words, it stinks.
The reason this number is a fallacy is due to a misunderstanding about what the RDA (Recommended Dietary Allowance) represents. It is not the intake below which people are deficient, but rather a number determined by the IOM (Institute of Medicine) to meet the needs of at least 97.5% of the population, assuming a bell-shaped needs curve. The average 25-(OH)-D3 level in people not taking supplements is about 40 nmol/L, a level corresponding to a daily intake of 400 IUs/day. But since different people have different needs, the RDA was intentionally set well above this intake to ensure that nearly everybody meeting the standard would have adequate vitamin D levels. The RDA is not the intake below which disease occurs, but rather the intake that virtually guarantees its absence. Due to confusion over what the RDA represents, millions have been inadvertently labeled as being vitamin D “insufficient” or “deficient.” Obese people typically have lower blood levels despite having higher total body stores, given that vitamin D is fat soluble. Meanwhile, blacks are 2- to 9-times more likely to have low blood levels, despite having just half the fracture risk relative to whites. When it comes to preventing fractures, it’s clearly not just the amount of measured 25-(OH)-D3 in the blood that counts. It’s more likely an easily measured marker rather than one that is highly meaningful except at the extremes.
Furthermore, the IOM’s recommendations are based on dietary needs in the absence of significant sun exposure. It’s pretty easy for most of us to meet our vitamin D needs most of the time, simply by taking a 15-minute outdoor walk after lunch—you know, the place where animals run around, trees grow, and flowers bloom. Rather than a supplement, the best way to maintain bone health is to perform outdoor weight-bearing exercise, like running, skating, or walking, 2 to 3 times a week (applying sunscreen after several minutes of exposure). You’ll not only get your daily vitamin D, but you’ll also ensure strong bones. To lower your fall risk, add a few balance exercises and some yoga, and viola—adequate vitamin D, strong bones, and balance! One again, exercise trumps supplements.
But some counter that much higher levels of vitamin D are required for optimum health. Okay, so where’s the evidence? In reading the studies, I found results all over the board—one study reported a 20% reduction in hip fractures in women taking vitamin D supplements, another reported no benefit at all. I found similar results as regards to cancer, fall risk, and heart health. Most of the trials were fraught with bias, so I turned instead to the most reliable sources I know—the Cochrane Collaborative and the USPSTF (US Preventative Services Task Force). Neither accepts pharmaceutical monies or advertising. Both deliver high quality systematic reviews.
And here’s what I found: Vitamin D supplementation, at doses between 400 to 800 IUs/day, does not increase bone density, decrease fracture risk, reduce blood pressure to any clinically significant degree, or lower all-cause mortality. As to whether vitamin D can reduce the risk of falling by improving muscle function, the data is mostly negative. Similarly, when it comes to cancer prevention, the majority of trials have failed to show an association. In a recent meta-analysis combining the results of trials involving more than 50,000 patients, vitamin D failed to lower the all-cause cancer risk. So, too, in the largest single trial looking at vitamin D and colon cancer (Women’s Health Initiative), the supplement didn’t work. Nor does it prevent heart attacks and strokes. Proponents argue that these negative results are due to inadequate doses of vitamin D used in the trials, but does it really make sense to think that the optimum level of vitamin D is one that can only be obtained through massive supplementation? Evolution doesn’t work that way. Although patients with very low baseline vitamin D levels are at increased risk for cardiovascular events, supplementing patients with normal levels doesn’t prevent those events. About the only place where vitamin D appears to help is asthma, where supplements have been associated with fewer flare-ups and less severe attacks. The mechanism is unknown.
If calcium is added to the mix at doses between 1,000 and 1,200 mg/day for 2 years, you can expect an increase in bone density on the order of 0.7 to 1.8%. As to whether this reduces fractures is another matter, but not a trivial one given that 70% of postmenopausal American women are taking calcium supplements. Although earlier trials reported a significant reduction in hip fractures, none of the five more recent trials published since 2005 have confirmed the association. The only consistency is that the results have been inconsistent. As to harm, calcium supplements are associated with an increased risk of developing kidney stones. Other trials have reported an increased incidence of stroke and heart attack, although a 2016 meta-analysis failed to confirm the association. So, despite the fact that doctors have recommended calcium supplements for decades, the benefits and harms of this practice remain uncertain.
Vitamin D and calcium might make sense in frail, elderly, postmenopausal, nursing home patients, where the combination has a larger impact on hip fracture prevention, but even here there is no benefit until the dose of vitamin D exceeds the current recommendation of 800 IUs/day. By way of comparison, bisphosphonate drugs, like Fosamax, Actonel, Boniva, and Reclast, are far more effective, cutting the fracture risk in half. Remember Sally Field hyping Boniva? The drug works far better than vitamin D and calcium, but is also far more expensive. Still, if fracture prevention is the goal, then the bisphosphonates are a better place to start.
After reviewing hundreds of studies, the USPSTF concluded that the data was insufficient to recommend either for or against vitamin D and calcium supplementation to prevent fractures in men and premenopausal women. The task force recommended against vitamin D and calcium at doses of 400 IU and 1,000 mg, respectively, to prevent fractures in active postmenopausal women. There remains uncertainty as to whether higher doses of vitamin D might be beneficial. In a separate report, the USPSTF felt the evidence was insufficient to recommend or reject routine screening of vitamin D levels in asymptomatic adults. For a supplement that has been as exhaustively studied as this one, there’s still a good bit of waffling.
So, who do I think should consider vitamin D screening and supplementation?
- Women with osteoporosis who aren’t candidates for bisphosphonate therapy.
- People who, through inclination or climate, receive little to no sun exposure.
- Those whose diets are deficient in vitamin D containing foods.
As for me, I believe I’ll go for a run in the sun and have some salmon for dinner.
To estimate your 10-year risk of sustaining a hip fracture use the FRAX Tool at www.shef.ac.uk/FRAX/.
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