Vitamin D is a fat-soluble vitamin, though its naming as a vitamin is actually factually inaccurate. Not only can most mammals synthesize vitamin D in adequate amounts, when exposed to enough sunlight, but it is synthesized in one part of the body, while acting in another. As far as obtaining vitamin D via food, very few foods actually contain vitamin D, especially in its physiologically active form, 1,25-dihydroxyvitamin D [1,25(OH)2D], which is also known as calcitriol. Originally discovered via investigation into the disease of rickets, which is the youth form of osteomalacia (softening of bones), vitamin D and recommending specific amounts, is quite a complicated issue.
There are two main issues with recommending a starting, baseline amount of vitamin D intake, whether from dietary sources or supplemental sources. The first issue is that concerns about cancer development, specifically from sunlight, cause many organizations to not recommend a specific amount of vitamin D. Secondly, since people have such varying degrees of exposure to sunlight, even within one location, over the course of different seasons, some organizations assume that no sunlight exposure happens, which could leave recommendations dangerously high, since this is very unlikely to occur in day-to-day life. But what exactly is going on in your body, when it comes to synthesizing vitamin D?
When you are exposed to sun, your body turns 7-dehydrocholesterol into vitamin D3. This happens through ultraviolet irradiation. Vitamin D3 (cholecalciferol) is then converted to 1,25-dihydroxy vitamin D3 (calcitriol) by your liver and kidneys.
Unfortunately, there is not a large amount of foods that contain vitamin D. I usually recommend to clients that they supplement with vitamin D3, or increase their sun exposure (which generally only works when they are getting almost none to begin with). That being said, “Paleo-friendly” food sources of vitamin D include: swordfish, salmon, tuna, sardines, liver, and egg. Note that swordfish, while high in vitamin D, also may contain unhealthy levels of mercury, as Chris Kresser has noted. That is because the level of selenium, which normally helps to combat the potential toxicity of mercury, is not high enough in swordfish. As Chris notes, mercury is only harmful because it binds to selenium and prevents it from performing its vital roles in the brain. So, in practice, swordfish would not be a food to consume every day. Only include it sparingly, and make salmon, tuna, sardines, liver and eggs much more frequent in the diet in order to endogenously raise levels of vitamin D.
Of course, the easiest and most natural way to raise vitamin D levels is via exposure to sunlight. Here, the research on duration of exposure is interesting. With many variations in skin color, intensity of the light, and other factors, it is quite difficult to recommend a specific, broad amount. However, generally 15 minutes per day of sun on skin seems to be a good starting place. This will cover days where you get none at all, and days where you get slightly more. In fact, some studies even recommend lower amounts:
“Although chronic excessive exposure to sunlight increases the risk of nonmelanoma skin cancer, the avoidance of all direct sun exposure increases the risk of vitamin D deficiency, which can have serious consequences. Monitoring serum 25-hydroxyvitamin D concentrations yearly should help reveal vitamin D deficiencies. Sensible sun exposure (usually 5-10 min of exposure of the arms and legs or the hands, arms, and face, 2 or 3 times per week) and increased dietary and supplemental vitamin D intakes are reasonable approaches to guarantee vitamin D sufficiency.”
As has been noted in scientific literature, vitamin D insufficiency is a common medical condition. Those that suffer from vitamin D deficiency are often found in the northern hemisphere, though that is far from all of those that may be affected. How can one tell if they have low vitamin D levels? This can be done most accurately via a blood test. You can specifically ask for a 25(OH)D level test.
Symptoms of vitamin D deficiency include tiredness, general aches and pains, or no symptoms at all. If the deficiency is very pronounced, oftentimes bone pain and infections can occur. It is important to remember that not everyone gets symptoms, even though they may be deficient in vitamin D. One study from 2010 estimated that 25-50%, or more patients seen in practices had a vitamin D deficiency.
Vitamin D is also very important in helping the body absorb more calcium, as well as aiding in regulation of cell growth. Other studies have correlated low vitamin D levels with poor mood and cognitive deficits. More recent studies have possibly shown a correlation between vitamin D deficiency and things such as restless leg syndrome. The authors of this study state:
“The present study demonstrated a possible association between vitamin D deficiency and RLS. Given the dopaminergic effects of vitamin D, 25(OH)D depletion may lead to dopaminergic dysfunction and may have a place in the etiology of RLS.”
Of course, more studies are needed and it is always best to look at causative mechanisms, not correlation. Nonetheless, I would hypothesize that in the future we will see an increasingly large number of conditions linked to vitamin D deficiency. However, many large studies show that large amounts of vitamin D actually can potentially be linked to increased mortality. In these studies, the ‘deficient’ range of people actually have the best rates of survival. However, due to the observational nature of these studies, it is hard to tell if this is the real reason as vitamin A and K2 would be immediate possible confounding variables.
Another issue, as Chris Kresser has pointed out, is that measurement of vitamin D levels are tough to pin down, at least accurately:
“It’s possible to have kind of a disconnect between your 25D levels and your 1,25D levels. They’re not always what you would expect them to be, in part because of polymorphisms and in part possibly because of disease states that cause an over-conversion of 25D to 1,25D, the active form, that happens in some autoimmune diseases. So 25D turns out to not be a very good indicator of vitamin D status.”
All this being said, I still recommend to many clients to supplement with vitamin D3, especially if they are not in the sun often. Regarding the dosage, it is tough to recommend a general amount, but 25 pounds of body weight per 1000 IUs is a good starting place. One study in those who were vitamin D deficient, showed that:
“Treatment of vitamin D deficiency for 3 months with oral cholecalciferol 5,000 IU daily was more effective than 2,000 IU daily in achieving optimal serum 25-hydroxyvitamin D (25OHD) concentrations.”
This is one of the reasons I tend to err on the higher end of supplementation. Also keep in mind that not all of the D3 listed on the label will be absorbed. It is a fat soluble vitamin, so you can take it with fat if you wish. To get more specific, there is a good study that compared the different forms of vitamin D tablets, and whether this made the vitamin D more bioavailable:
“Vitamin supplements can be ingested to improve vitamin D status. It is not known if the vehicle substance that is combined with the vitamin D tablet influences the bioavailability of vitamin D. The purpose of this review is to examine the impact of different vehicles on vitamin D bioavailability. A comprehensive literature search identified studies that directly compared the absorption of vitamin D from two or more vehicles. The change in mean serum 25(OH)D per average daily dose of vitamin D supplemented was calculated and compared among the studies. We identified four clinical studies that compared two different vehicles of vitamin D. Vitamin D in an oil vehicle produced a greater 25(OH)D response than vitamin D in a powder or an ethanol vehicle in healthy subjects. There are limited studies that have compared the influence of the vehicle substance on vitamin D bioavailability. Future studies should examine bioavailability among different vehicle substances such as oil, lactose powder, and ethanol and examine if there are any differences in bioavailability among different patient populations including those with fat malabsorption.”
As can be seen, the oil form is most preferred. I also sometimes have clients supplement with cod liver oil, which usually also includes vitamin A. The best product I have found is Green Pasture’s butter oil and fermented cod liver oil blend. This provides a good source of fatty acids, as well as more DHA than EPA. It also provides a source of CLA, or conjugated linoleic acid.
Are you getting enough vitamin D? It is a highly individualistic, and somewhat complicated, question. With murkiness in recommended daily requirements, different levels of exposure to sunlight, and no clear scientific evidence, your best bet is a blood test, and then experimentation with either more sunlight or working your way up through increasingly higher levels of supplementation.
I hope this article has provided some insight into the somewhat mysterious world of vitamin D! Feel free to leave comments or questions!