Interpreting Advice on Vitamins
Most of us are familiar with constantly changing nutritional advice as new evidence comes in.
For example, some years ago, eggs were considered healthy and people were advised to eat at least one egg a day. Then when cholesterol was considered evil, eggs went from healthy to almost poisonous, and one got the impression that eating just one egg yolk would send one to the emergency room with a heart attack. Today the tide is turning again, and eggs are supposed to be healthy in moderation.
The same is true of vitamins and supplements. One example is vitamin D.
In preindustrialized, equatorial parts of the world, where lots of clothing is not necessary and where humans evolved, most people get plenty of vitamin D by going outside. Rays from the sun reaching the skin convert cholesterol into vitamin D, and nothing more is needed. You can’t get too much vitamin D from sunshine, because if levels in the skin get too high, the body starts breaking the vitamin down.
But when we live in northern climes where a lot of clothing is necessary in the winter, we don’t produce as much vitamin D naturally, especially when we wear sunscreen when we do go outside. In this case, supplementation may be necessary, especially in people with dark skin, and in older people, who don’t produce vitamin D as easily.
Today, a few foods like milk and orange juice are supplemented with some vitamin D, but not a lot. Fish like wild salmon also contain vitamin D, but most salmon on the market is farmed and contains a lot less. Canned salmon is between these two. People like the Inuit, who live where the sunlight is never intense enough for vitamin D synthesis in the skin, have traditionally gotten their vitamin D through their diet, which in times past included a lot of wild fatty fish.
But most Americans do not get enough vitamin D that way, even when drinking milk and taking multivitamins. This is especially true of people who are unable to spend much time outdoors.
How do you know if you’re getting enough? What are the optimal levels in the blood?
There are several different forms of vitamin D, and formal recommendations usually use the 25-hydroxy version (sometimes written as 25(OH)D) as the standard. The National Institutes of Health Office of Dietary Supplements currently considers less than 12 nanograms per milliliter (ng/mL) deficient. Such low levels can lead to rickets in children and in adults, osteomalacia, which results in weak bones. They consider 12–20 ng/mL insufficient for good health and anything greater than 20 ng/mL adequate. The 20 ng/mL was decreased from 30 ng/mL fairly recently.
Vitamin D levels can also be reported in units of nanomols per liter (nmol/L); 1 nmol/L = 0.4 ng/mL. So in this case the cutoffs are less than 30, 30–50, and greater than 50. This can be confusing, as we often remember the numbers but not the units.
But not everyone agrees with the NIH cutoffs. Grassroots Health, a consortium dedicated to promoting optimal vitamin D, says levels should be 40–60 ng/mL. Dr Neil Binkley, an expert in vitamin D assays, suggests 40–80 ng/mL. Michael Holick, a vitamin D researcher, suggests 30 n/mL as the lower limit of adequate vitamin D. Other groups recommend 50–80, 50–65, 50–70, and so forth. Different labs may have different normal ranges. Here is a chart comparing a few different recommendations. (Click on table and scroll left and right if you can't see it all.) Clearly, there’s no consensus on the cutoff levels.
Vitamin D Council*
*Expressed in ng/mL.
**Expressed in nmol/L
In the recent past, as more and more medical conditions were found to be associated with low levels of vitamin D, people began suggesting higher and higher intakes producing higher and higher serum levels of the vitamin. Most studies showed no toxicity with the higher levels, so it seemed prudent to try. Some were urging us to get our levels above 50 ng/mL.
Just recently another study showed that blood vessels in people with diabetes are less likely to get clogged when they have adequate vitamin D, where “adequate” was defined as greater than 30 ng/mL.
But then, as often happens, some contradictory evidence emerged. A surprising study reported in the Journal of Biological Chemistry showed that low vitamin D levels were associated with longevity!
And a study in the American Journal of Cardiology said that increased levels of vitamin D were associated with lower levels of C-reactive protein, a marker of inflammation, as long as the vitamin D levels were below 21 ng/mL but higher levels of CRP when the vitamin D levels were higher than 21 ng/mL. However, the statistics were criticized by the Vitamin D Council, which supports higher levels.
What should we think of all this? How can we know what to do if even the experts can’t make up their minds?
Well, the first thing to remember is that correlation doesn’t equal causation. Some Supplement A may be correlated with a Condition B, but that doesn’t mean that Supplement A causes Condition B. Owning 5 houses is correlated with being rich. But going out and buying 5 houses won’t cause you to be rich, in fact, more likely the opposite.
The researchers in the JBC article on longevity postulated that the long-lived people had less of a gene called klotho, which is an “aging suppressor.” But the gene also causes lower vitamin D levels. So the lower vitamin D levels might have been a side effect, not a cause, of the longevity.
If people with adequate vitamin D have more or less of some medical condition or live shorter or longer lives, this doesn’t necessarily mean that taking the vitamin D will have the same result. It’s possible that some underlying condition that causes low vitamin D levels is also causing various medical conditions. In order to prove that vitamin D would be helpful in such cases, you’d have to give people vitamin D supplements for a long time and show that they had less of the medical condition.
This was tried some time ago with vitamin A. Studies had shown that higher levels of vitamin A were correlated with lower rates of a cancer. But when they studied supplementation in people at high risk, they found that the people given the supplements actually had higher heart attack rates.
Similarly, low levels of vitamin D have been associated with cardiovascular disease. But a meta-analysis looking at pooled results from more than 30,000 people found no statistically significant effects of supplementing with vitamin D on cardiac or all-cause mortality. It’s possible that the low levels of vitamin D were a result rather than a cause of the cardiac disease.
So because they’re not apt to come up with firm answers for vitamin D in the near future, what should we do about it now?
I think one approach is common sense. Have your vitamin D levels checked, and definitely make sure they’re not below the lowest cutoff point anyone suggests (20 ng/mL); consider getting them even higher than that, maybe 30 to 40 ng/mL. There’s no guarantee this will protect you from myriad diseases, but it’s not likely to hurt, and vitamin D is cheap.
But don’t go overboard and try to get your levels above 100. More is not always better, and sometimes it can cause harm. The fact that our bodies regulate vitamin D levels by destroying the excess in skin when sunshine makes the levels reach a certain point suggests that our bodies don’t want superhigh levels.
Be aware the opinions about this differ, and keep your eyes open for any new evidence that would be convincing one way or another.
Don’t supplement with huge amounts of any supplement on the basis of a popular press article. They tend to overplay the benefits of supplements. Try to find the original article (science news services like Eurekalert and Science Daily usually have links to the articles) to see if the results really seem good. Or better, wait until the results have been confirmed by other researchers to start taking something new. If it’s a food, you don’t need to be so cautious.
Talk to your doctor. Have any of the doctor’s patients had side effects? Did the supplement help them? If your doctor doesn’t have time or doesn’t know much about it, talk with your pharmacist or other people who have tried the supplement. Have they had side effects? One nice thing about vitamin D is that there seems to be a large window between too little and too much, so there’s less worry about toxicity unless your intake is astronomical.
If the supplement is new or the recommended dosage is high, wait a bit to see if there’s contradictory evidence next month. This is especially important with supplements that could be toxic.
Remember that all drugs interact, even common vitamins and minerals. For example, quite often two different minerals will use the same method to get taken up in the gut. If you take a huge amount of one mineral, you may become deficient in another mineral, because the first one is hogging all the transporters. This is a real problem, because most of us aren’t experts in balancing levels of the numerous minerals we need, and the people who claim they are often have financial interests in promoting some supplement, even a simple one like calcium or magnesium. Again, use common sense. Take extra minerals if you think you need them, but don’t go overboard.
Sometimes supplements and foods can interact with your prescription medications. If you’re considering a new one, ask your pharmacist and look on the Internet for possible interactions.
Check the units used in the articles you read. Vitamin D is a good example of a common vitamin whose concentration can be expressed two different ways. You might read that your vitamin D should be 100. But the author meant 100 nmol/L, which is 40 ng/mL, and you might be trying to get your levels to 100 ng/mL, which may be too high.
I think getting vitamin D naturally, the way we are meant to get it, by going out in the sun without sunscreen for at least a short time, makes more sense than taking supplements. According to Holick, five to 30 minutes (depending on the season, latitude, skin color, and time of day) between 10 a.m. and 3 p.m. twice a week is usually sufficient. However, these recommendations are for the average person. As we age, we produce less vitamin D in the skin, and people with diabetes especially tend to have lower levels than average, so we may need to supplement even if we get a lot of sun exposure, especially as we age.
Our face gets the most sunshine, so to reduce the risk of skin cancer on the face, exposing arms or legs would mean less overexposure to one part of the body. Exposing skin to sunshine is difficult in northern climes in the winter, so then supplementation makes sense. I don’t take vitamin D in the summer as I work outside a lot and don’t like sunscreen. In the short days of winter I take 1000 units a day. My 25-hydroxy levels of vitamin D stay around 47 ng/mL.
What works for me might not work for you. But so far, the preponderance of the evidence is that vitamin D has more good effects than bad when levels are kept within limits, and toxicity is low. The evidence for some other supplements, alas, is often not as clear.