Vitamin D, the reigning health topic du jour, has been linked to cancer, diabetes, obesity, the autism spectrum, unhealthy aging, and a myriad of other health issues--basically everything. (the more illnesses it is linked to, the more pills marketers can sell).
There are many dubious pundits claiming that there is an epidemic of vitamin D deficiency, with some proclaiming that as many as 75% of all people are deficient, and therefore, everyone should be supplementing with Vitamin D tablets, particularly those who get little sun exposure and the elderly.
I don't agree with the statistics regarding the number of people with deficiency because I think the blood tests don't tell the whole story, and a recent study involving obese and non-obese people makes me think I am right. Notice the emphasis on think, not know. (By the way, another recent study revealed that two out of three of the main pieces of equipment used by blood labs to measure Vitamin D often report artificially low numbers.)
Before I get to the main study, here's what everybody seems to agree on in regards to Vitamin D:
1. Vitamin D is a group of fat soluble substance most call a vitamin. (There is some disagreement if it's really a vitamin as the body can produce it on its own, unlike all other vitamins which must be derived from external sources. However, Vitamin D is often called the "sunshine vitamin" because the body can produce it from cholesterol after your skin is exposed to sun.)
2. Human beings can consume two forms of Vitamin D: cholecalciferol (vitamin D3) or ergocalciferol (vitamin D2). The liver apparently converts Vitamin D to calcidiol and the kidney converts some of it to the active form called calcitriol. Blood tests measure only calcidiol.
3. Vitamin D deficiency can cause osteomalacia (rickets in children), a disease related to softening of the bones due to problems with bone mineralization.
4. There is more vitamin D stored in fat or adipose tissue than typically circulates in the blood. That makes sense because it is fat-soluble. (There is some disagreement if they are positively or negatively correlated, eg. whether they increase and decrease together.) What is known for sure is that generally the more fat you have, the more Vitamin D is stored in such fat.
5. Routine Vitamin D blood tests only measure circulating Vitamin D (calcidiol) and not fat stored Vitamin D. (There are ways, such as liquid chromatography, to measure fat-stored vitamin D, but they are not part of normal testing.)
6. Obese people have lower levels of circulating and higher levels of fat-stored Vitamin D than non-obese people. Basically, the more fat you have, the more of your Vitamin D is stored in it and the less you have circulating and easily measurable. (This could explain why some scientists claim there is more Vitamin D deficiency today than ever before. It could be a result of an increasing weight among Americans with 2 out of 3 now deemed overweight and 36% purported to be obese.)
7. Vitamin D blood levels increase with weight loss. (It is not clear what happens to the total amount of fat-stored Vitamin D with weight loss, but presumably it decreases.)
8. Wild fish such as salmon are still a great source of Vitamin D.
Here are the questions whose answers have less agreement, if not outright disagreement.
1. What levels of blood circulating Vitamin D define deficiency and inadequacy?
2. Can the body tap into fat-stored Vitamin D when needed?
3. Why do vitamin D blood levels increase with weight loss?
4. Why do obese people have lower levels of circulating blood Vitamin D?
5. When and how much supplementation is necessary and appropriate for different levels of deficiency or inadequacy.
6. In the absence of deficiency or inadequacy, does extra Vitamin D offer any special protection against cancer, diabetes, heart disease, diabetes, etc.?
The recent study about Vitamin D, I referenced above, showed that in comparing obese young men to normal weight young men, the obese young men when exposed to tanning radiation akin to sun exposure actually seemed to produce 57% less Vitamin D than the non-overweight young men exposed to the same radiation. The results suggest that the obese get less Vitamin D from sun exposure that the non-obese.
The authors of the study contend that the reason for this result is that the fat or adipose tissue in the obese men may absorb the additional Vitamin D produced. The authors also put forth the supposition that obese people have less Vitamin D to start with because they spend less time in the sun.
This study triggered an exhaustive research effort by me to better understand the science of how vitamin D moves in and out of fat storage. More specifically, I wondered if one has a high level of Vitamin D in fat storage and a low level in blood circulation, would the Vitamin D come out of storage before a clinically evident deficiency develops. In other words, can you depend on your fat-stored Vitamin D to play a helpful role and prevent deficiency? If your fat-stored vitamin D provides the body with necessary Vitamin D, then we can dispense with all the blood tests for circulating Vitamin D because they would be unreliable indicators of deficiency and inadequacy.
The study, among others, seemed to suggest that the more obese you are, the more of your Vitamin D would go into fat storage and therefore there would be less in blood circulation. This means that the more obese you are, the more Vitamin D deficient you would be on blood measurement. If this would be a true deficency, one would expect to see cases of osteomalcia among the morbidly obese.
Guess what? After reviewing every published study available on pubmed, it was startling that there was not a single case or study reported that demonstrated bone related problems with rising obesity. In fact, it doesn't even seem to lead to osteoporosis, another bone related disease, and in fact, scientists have long believed that obesity protects against osteoporosis. (Some recent studies out of China that differentiate between obesity based on body mass index versus actual body fat composition suggest that obesity based on high fat composition may not be preventive of osteoporosis.)
Furthermore, the only studies linking osteomalacia to obesity involve obese patients who undergo a surgical procedure to lose weight called the jejunal-ileal bypass. This procedure can sometimes lead to malabsorption, a condition in which the body can't absorb certain nutrients, in which case all kinds of problems and deficiencies become more likely.
This knowledge raises a serious question regarding the validity of the widely used Vitamin D blood test. If the test does not correlate to clinical symptoms, of what value is the test? When the test shows a low level, is it because it is actually absent throughout the body or because much of it is stored in the fat, thereby undetectable by the blood test?
Although the study postulated, and other studies have also shown, that Vitamin D levels increase with weight loss, allegedly due to more outside exercise and therefore more sun exposure, I offer an alternative theory. I say theory because I can't support what I suspect is the case because I couldn't find a single study that has considered this issue. Like all theories, it needs to be tested.
I postulate that as body fat shrinks, stored Vitamin D is released into circulation. What I don't know if having more blood circulating versus fat-stored Vitamin D actually makes a health difference? The answer to this question is imperative to know definitively before we compel so many people, particularly the overweight and obese to unnecessarily supplement with Vitamin D for the sole purpose of increasing circulating levels of Vitamin D.
Until the role of fat-stored Vitamin D is elucidated, I think most of the rest of the studies on Vitamin D will be essentially meaningless because you aren't measuring all the Vitamin D in the body.
For example, another study showed that there is an increased incidence of developing metabolic syndrome and a larger waist circumference after five years in patients with low blood Vitamin D.
My questions, which this study did not fully answer, are did they really have low total body Vitamin D or was it low in the blood because most of it was stored in fat. Also, did the low Vitamin D levels put them at greater risk because they were already overweight and/or pre-diabetic or because they had low sun exposure and poor dietary intake? Finally, does low Vitamin D cause problems or indicate they already exist?
Perhaps it is not the Vitamin D blood level that matters but the presence of excess fat? Perhaps it is the excess Vitamin D that is stored in the fat that is causing the problems? We must know more about the role and activity of the fat-stored Vitamin D to answer these questions.
Maybe Vitamin D is a proxy for something else. It won't be the first time in medicine that we discovered that something was not what we thought it was. For example, we thought ulcers were caused by stress and it turned out to be mostly bacteria infections. We thought homocysteine controlled heart disease and it turned out to be only a bio-marker. We thought taking beta-carotene would prevent cancer, but it turned out that to be just the opposite. We thought statins were great for us...wait, many, but fewer still think that but maybe that will change some day as well.
From my perspective, I will no longer be recommending, in the absence of symptoms, Vitamin D supplementation for levels above 25 in overweight and obese patients. I will however, continue to strongly recommend (and offer extensive support to) overweight people to help them lose weight, not only for the sake of increasing their Vitamin D levels, but also for all the other health benefits such weight loss confers.
Also, for those without history of skin cancer, I continue to advocate for at least 15 minutes per day of sun exposure to extremities or belly, as long as reddening of the skin is avoided. Based on my current knowledge, this appears to beat supplementation almost every time, even in the elderly.
Vitamin D is obviously a complicated issue that begs for real clarity. Major studies are underway which may offer some real answers. However, if they don't consider the role of fat-stored Vitamin D, they may actually be of little value. Let's hope for the some good answers and in the interim, try to stay healthy the old fashioned way- get up, get out, and move around. Of course, watch what you eat because it is still the single most important determinant of health.