My extensive studies have helped redefine what it means to be vitamin D deficient. Before one of my publications in the Lancet in 1998, vitamin D deficiency was defined as a having 25-vitamin D levels below 10 nanograms per
milliliter. I demonstrated, however, that a blood level of twice that—20 nanograms per milliliter—is needed to prevent an unhealthy elevation in parathyroid hormone level. I and my colleagues, including Dr. Robert Heaney at Creighton University, established that for every 100 IU of vitamin D ingested, the blood’s level of 25-vitamin D goes up by 1 nanogram per milliliter. Don’t worry about what that really means; the important point is that based on these observations, it is now recognized the international community now defines vitamin D deficiency as a having a 25-vitamin D level below 20 nanograms per milliliter. Vitamin D insufficiency is between 21 and 29 nanograms per milliliter.
Ideally, you should aim to have a 25-vitamin D level of at least 30 nanograms per milliliter, and 100 nanograms per milliliter is safe.
Clearly, you’re not going to see a vitamin D supplement that says “take this pill once a day and it will boost your 25-vitamin D levels to 40 nanograms per milliliter.” And the sun won’t stop shining once you’ve obtained enough to boost your vitamin D supplies. So how do you know how much you’re getting and how that all translates to sufficient levels in your body?
That’s where my three-step strategy comes in, which I’ll take you through step by step in part II. But first, it’s imperative to clear up a few common misconceptions about vitamin D that probably still have you scratching your head.
Your 25-vitamin D level is the barometer for your vitamin D status and is a summation of your dietary, supplemental, and sunlight sources of vitamin D. See chapter 7 for more about testing.
How Much Is Too Much?
Between 1987 and 1991, as many as forty-four thousand households in the Boston area were at risk of vitamin D “intoxication,” the scientific word for overdosing. The culprit was home-delivered milk that had been overly fortified by one careless employee at a dairy company (rather than 400 IU of vitamin D per quart, there were up to 250,000 IU per quart). The index cases included a healthy seventy-six-year-old woman who died due to vitamin D intoxication, which can increase blood calcium and phosphate and calcify blood vessels and kidneys, and a young child who developed kidney failure, cases we reported in the New England Journal of Medicine. I helped uncover the source of the outbreak and put a stop to distribution of the tainted milk. The experience inspired me to conduct a study, published in the New England Journal of Medicine at the same time, that examined milk and infant-formula preparations.
Milk rarely contains the amount of vitamin D stated on the label, but in most cases it’s not overfortified—it’s underfortified. Which is why my conclusion called for better monitoring of the fortification process. (We did, however, find that infant formulas contained at least the amount of vitamin D stated on the label.)
I’ll give you one more example of an unusual case of vitamin D OD. At 7:00 one morning in the mid-1990s, I picked up the phone to hear an irate Florida lawyer accuse me of ruining his health by suggesting he take vitamin D supplements.
“Are you Dr. Holick?” he said.
“No.” (I wasn’t interested in talking to a lawyer at that hour. Who is?)
“Well, I know you’re Dr. Holick because you’re the only one there that early in the morning. And I’m gonna sue you.”
“Why”? (Now I was interested.)
“Because I’ve been following your advice and I went on the Internet, bought some vitamin D, and now I’ve landed in the emergency room with vitamin D intoxication!”
No wonder I was at the top of his list of people to call and threaten with a lawsuit. Rather than hang up on the poor fellow and call my own lawyer, I kindly asked him to send his vitamin D to me so I could test it. I found it hard to believe he could be vitamin D intoxicated from the amount he believed he was taking.
Sure enough, the company had forgotten to dilute it. The two teaspoons a day he was taking in powder form weren’t delivering a healthy 2,000 IU a day. He was taking 1,000,000 IU a day—plenty to cause vitamin D intoxication. He didn’t sue me in the end, but rather he asked for my advice on how to recover from his vitamin D intoxication. He then took his complaint up with the supplement company, which quickly went out of business.
Sunlight destroys any excess vitamin D that your body makes, so you could never become vitamin D intoxicated from sun exposure.
You would need to ingest more than 10,000 IU of vitamin D a day for at least half a year to even begin to worry about potential toxicity from supplements. Symptoms of toxicity include nausea, vomiting, loss of appetite, constipation, increased frequency of urination, increased thirst, disorientation, and weight loss.
Despite these two cases, vitamin D intoxication is extremely rare. And what I continually tell people is that there is absolutely no way to overdose on vitamin D through exposure to sunlight. The body, not surprisingly, has a way of regulating the amount required so that there’s never an excess amount of vitamin D circulating in the bloodstream. If you expose yourself to the sun in a bathing suit for long enough to see a light pinkness twenty-four hours afterward, that’s the equivalent of taking between 15,000 and 20,000 IU of vitamin D. For a white adult, that would be the equivalent of being exposed to sunlight in June at noon for about ten to fifteen minutes on a Cape Cod beach. Your body has a huge
capacity to make vitamin D at any age. While aging does decrease levels of the provitamin D molecule responsible for making vitamin D in the skin, your body still has enough ingredients to make adequate vitamin D with adequate amounts of sunshine, even when you are ninety years old.