This article is more than 5 years old. Information may no longer be current.
Vitamin D continues to be scrutinized, poorly understood
Click Here to Manage Email Alerts
Vitamin D has been a somewhat controversial and, at least for a time, overblown issue. There are several things that are poorly understood.
Five or 6 years ago, vitamin D was touted as wonderful for all sorts of things, over and above skeletal issues. There are suggestive studies that if you live in an area where people have higher levels of vitamin D you might have better bone density, and you’d also be less likely to get breast cancer, prostate cancer, rheumatoid arthritis, hypertension, type 1 diabetes, asthma and multiple sclerosis, as well as a variety of other bad things.
People started measuring vitamin D, probably in circumstances where it wasn’t appropriate. For many years now, measurement of 25-hydroxyvitamin D has been among the most common special chemistry tests ordered from reference labs in the United States, and vitamin D supplementation has been widely recommended.
Nelson Watts
There may be extra-skeletal benefits of vitamin D. One that seems relatively secure is that older people who have low levels of vitamin D are more likely to fall, which often results in fractures. Giving those elderly people who are vitamin D-deficient supplements of vitamin D to raise their blood levels makes it less likely that they will fall. Other than that, it’s really hard to find evidence that giving vitamin D does anything, but there are several problems.
Inconsistent assessments
First, laboratory assays for vitamin D are highly variable. The laboratory draw stations collect blood, it gets sent from an institution’s lab to a reference lab, and the result may come back with different numbers. Some people think a level of 20 ng/mL is acceptable, but in the bone field, we shoot for a level of at least 30 ng/mL. For the same patient with the same blood sample, getting two levels that are 50% different makes it awfully hard to know where you are and where you need to go. That’s actually a common story because the labs have different methods for measuring, and they don’t always measure the same things. One of challenges for anyone who is interested in vitamin D for individual patient care or for research is going to have to deal with this kind of variability in laboratory testing.
Second, vitamin D, like calcium, is a threshold nutrient, which makes the issue of supplementation awfully hard to assess. Once you get enough, there is no value in getting more.
We think if people take in at least 1,200 mg calcium a day from their diet (or supplements if their diet is not sufficient), they get all the benefit they would achieve, and there absolutely is no value in adding more calcium. Likewise, if somebody has an adequate level of vitamin D — and we could argue whether a blood level of 20 ng/mL or 30 ng/mL would be adequate — there is no value in giving them more vitamin D.
One of the challenges with clinical trials that have tried to show whether calcium or vitamin D supplements help is that participants have not been restricted to those who were deficient. Therefore, they included perhaps a substantial number who were already getting enough calcium or vitamin D and couldn’t possibly benefit from the intervention. Another potential flaw in these studies is the dose of vitamin D may not be adequate to bring the really deficient patients up to the threshold level.
Two years ago, the US Preventive Services Task Force issued a report that it could find no evidence that supplements of calcium or vitamin D were helpful in reducing the risk of fractures. I agree with that, but it can’t find any evidence that they don’t help prevent fractures in people who are deficient because those studies haven’t been done.
An ideal trial
A trial would need to be done looking at vitamin D in people who are getting enough calcium but who are vitamin D deficient and signed up in sufficient numbers, so that whatever events we’re looking at — be it heart attacks, or multiple sclerosis, or falls or fractures — we are able to draw conclusions.
They would be divided randomly into at least two groups: a control group getting no vitamin D supplementation and an intervention group that was getting a large enough dose of vitamin D to be fully effective.
It’s more of a challenge with calcium because there’s a considerable variation from one person to another in what’s called fractional calcium absorption. If we have eight or 10 people in a room and give each 1,000 mg calcium, nobody is going to absorb 1,000 mg. Fractional calcium absorption would range from 10% to 30%, which means the best calcium absorbers might take in 300 mg of that, and the rest passes through the digestive system.
If the person who absorbs 10%, the lowest efficiency, gets that 1,200 mg a day, that will be enough for them to reach that threshold. If the person absorbs 30%, they don’t need 1,200 mg calcium to reach that amount and could probably get by with 800 mg calcium a day.
If we were going to do a study looking at calcium supplements, we would need to look at people who are not getting enough, but who are adequate in vitamin D and everything else we think is important. We would have to measure not only their dietary calcium, but we would also have to know their fractional calcium absorption.
That’s not an easy thing to do, and I don’t think we would be able to do that in the thousands of subjects we need to go through to find the ones who are calcium deficient to enroll in a trial, in which, again, some of them would get placebo, some would get calcium-rich foods or calcium supplements sufficient to bring their absolute calcium absorption up to that threshold where they’re getting a maximum effect.
To add to the confusion, people think you can get all the vitamin D from the sunshine. Binkley and colleagues conducted a study in Hawaii in which they measured 25-(OH)D levels in 93 healthy young men and women who spent on average 29 hours a week in sun and didn’t use sunblock. Half of them had vitamin D levels ≤30 ng/mL, which by bone standards are deficient, and one of them had a level as low as 10 ng/mL.
So, while sunshine can be a good source of vitamin D, it’s not a reliable way to get adequate levels of vitamin D, and my dermatology colleagues would be angry if I suggested that people go out in the sunshine without using their sunblock.
Today’s hurdles
Where we stand today, it’s clear that calcium is the raw material for bone, and that vitamin D is essential for the absorption and assimilation of calcium. If you have enough calcium but don’t have enough vitamin D, it’s hard to see how you could maintain or rebuild good bone.
Throwing the baby out with the bathwater was implied by the US Preventive Services Task Force recommendation. It’s incorrect and premature, and I advise my patients to try to get 1,200 mg calcium per day, from diet if possible, adding a calcium supplement for those whose diet consistently falls short of goal.
I measure 25-(OH)D in my patients where I’m concerned about bone health, especially those where I’m going to be giving a prescription medication. I would like to practice evidence-based medicine, but I can only give logic for this: It’s hard to think patients are going to get their money’s worth out of their prescription medication if they don’t have enough calcium and vitamin D, and there’s a wide margin of safety there.
For my patients where I’m concerned about bone health, where their vitamin D levels are lower than ideal, I recommend an over-the-counter supplement of vitamin D, and for my friends and family who ask advice, but where I’m not authorized to test, I recommend an OTC supplement of vitamin D, 2,000 IU a day.
The average healthy person in population surveys in the United States has been shown to have a 25-(OH)D level around 20 ng/mL to 22 ng/mL. Adding a supplement of 2,000 IU a day will raise the blood level an average of 20 points, using a target level of 30 ng/mL to 50 ng/mL (50 ng/mL and 60 ng/mL is about as high as you would get if you spent the summer working as a lifeguard). If you look at people near the equator, and have lifestyles similar to those before the industrial revolution, they have vitamin D levels in the 40 ng/mL to 45 ng/mL range.
There’s a suggestion that moving to northern latitudes and staying inside with sunblock has probably put us at an overall vitamin D disadvantage compared with our remote ancestors, and to get to an adequate level requires either sunshine, which is not reliable or necessarily safe for everybody, or a vitamin D supplement.
A committee of the Institute of Medicine in 2010 set a safe upper limit for vitamin D of 4,000 IU a day, so my suggestion to my friends and family that they take 2,000 IU a day of vitamin D is well within that range that should be safe for virtually anybody.
Regenstrief Institute. Common LOINC Laboratory Order Codes. Available at: loinc.org/usage/orders. Published June 3, 2011. Accessed May 9, 2014.
Disclosure: Watts reports no relevant financial disclosures.