Tuesday, November 12, 2013

Underestimated Vitamin D Sources: Especially Eggs, But Also Chicken, Pork, Fish & Dairy Contain an Overlooked, Physiologically Relevant Amount of Ready-Made 25OHD

What do you need for a high 25OHD picnic at the beach? Eggs!
Regular SuppVersity readers know: The slowly abating vitamin D hype is driving me up the walls. Whenever you search a database for recent articles with the word "vitamin" in it, you are flooded with papers on vitamin D - many of them simplistic adulations without any new data or information. Others are totally irrelevant experiments on cell lines or non-significant epidemiological analyses, where no one can tell you whether the low vitamin D levels are mechanistically or corollarily involved in whatever the scientists are trying to tell you vitamin D was beneficial for.

Among all this mess, you can still find a handful of interesting papers. You just have to look close enough to spot gems such as a review by Ovesen, Brot, and Jakobsen (2013).

Are Eggs the Best Dietary Vitamin D Source We Have?

"Eggs? The best vitamin D source?" I don't have the hubris to say that eggs are the absolute #1, but considering the fact that eggs are the #1 source of "actual", preformed 25OHD, aka 25-hydroxyvitamin D*, in our diets, you are probably going to agree that eggs may well be the most underrated source of vitamin D in our diet (25OHD is what doctors and scientists will measure in your blood stream; most essays measure total 25OHD so I will not differentiate the different forms here).
Preformed vitamin D? Isn't that dangerous? No, quite the opposite: Pharmacologic doses of 25OHD do not change or may even decrease plasma levels of 1,25(OH)D, aka calcitriol, which can potential cause calcification (Trummel. 1669; Heaney. 1997). When you are D-deficient, though, your body will retain the 25OH. This is particularly interesting for people with intestinal malabsorption, for whom 'regular' vitamin D supplements, i.e. vitamin D3, aka cholecalciferol, or vitamin D2, aka ergocalciferol, don't work (Francis. 1983)
When it comes to vitamin D, eggs, fish, dairy, meat, and offal are quite unique. All of them contain "vitamin D" in the still-to-be-converted D3 form you all know. What only a few people know is that they also contain the celebrated vitamin D metabolite 25-hydroxyvitamin D (25OHD). This is not only the form your doctor will measure when you ask for a "vitamin D test", it's also the "vitamin D" that has been linked to all sorts of health benefits in the aforementioned epidemiological studies.

If you want to increase or maintain your 25OHD levels, it is obviously an advantage if you don't have to rely on your body to transform the dietary D2 + D3 into 25OHD. It is even better though, if the efficacy of this "supplement" (as of now I have not seen 25OHD in caps, so you better stick to eggs) has a higher bioavailability than its non-polar precursor, vitamin D3, which is absorbed mainly into the lymph (Thompson. 1966; Blomstrand. 1967).
"[T]he more polar metabolite 25OHD at physiological concentrations is also absorbed directly, and more rapidly and efficiently, from the proximal jejunum into the portal vein independent of fat absorption. These findings are consistent with results from clinical studies, which have found better absorption of 25OHD versus vitamin D in patients with fat malabsorption. Also, in healthy subjects and in patients with bone disease  25OHD is absorbed better and faster than vitamin D." (Ovesen. 2013)
I would hope that you are by now at least considering to eat an egg instead of popping dozens of vitamin pills. For those who still need actual data before they subscribe to the egg-o-logical approach to vitamin D 'supplementation', I have compiled a couple of figures in Table 1:
Table 1. 25OHD content (µg/100g) of chicken & egg, pork, beef, fish, dairy (various sources; cf. Oveson. 2013)
With respect to fish, it's worth mentioning that the exact concentrations vary from species to species. Moreover, the 25OHD content of fish is distributed relatively evenly in muscle, fat and organ mass of the fish - much in contrast to vitamin D3, by the way, which is contained mostly in the organs, specifically the liver. For the average human being, this means that he or she will actually get his / her share of 25OHD with every serving of fish. For vitamin D3 that's clearly not the case, because few of us actually eat cod liver on a regular base.

The sentence "You Eat What You Feed" is not new to SuppVersity regulars - it's also the title of an older article discussing how you can use food additives to increase the Omega-3 content of your steaks, milk and other animal products | read more
Another thing we should not forget is the influence of the diet. As you as a SuppVersity reader know, it's very difficult to predict the exact amount of almost every nutrient you will get from the products you buy at the super market, in this day and age of food supplements and synthetic chow (see "You Eat What You Feed: How Much Omega-3s Can You Possibly Pack into a Single Steak? The Impressive Effects of a "Grass(+)" Diet on Raw Meat & Meat Products" | read more).

If you ate the bacon of the pigs in the Thompson study from the late 1960s, for example, you would get a whoppy dose of 0.7–2.0µg 25OHD per 100g and thus max. 10x more than from regular bacon. Why? Easy: The sows were on a(n extremly) high vitamin D diets (total D-intake was 2–3mg/day for 2-3 weeks; Thompson. 1966).
What's the "minimal" vitamin D level: If we take the most recent study by Tepper as our guide, you need much less vitamin D to keep your heart disease risk, or rather the markers that have been associated with the latter in check.
In their study, the scientists measured the vitamin D levels of 400 healthy men (age 25-65 years) and identified their relationship to fasting plasma insulin (FPI), fasting plasma glucose (FPG), triglycerides (TG), high sensitive C-Reactive Protein (hs-CRP), blood pressure and triglycerides and found that (a) vitamin D status is related to cardiometabolic indicators in healthy men and that (b) their data would suggest that 11−14 ng/ml should be defined as the minimal vitamin D threshold. So, if you are below that mark you are in for trouble and supplementation actually necessary.
Egg yolks instead of pills? The question, whether you can trash your vitamin D3 (or D2) supplements, as long as you make sure you get your daily dose of egg yolks (the 25OHD is in the yolk, not the egg white!), is difficult to answer. The 'potency' of oral 25OHD is estimated to be 1.5-5x higher than the one of 'regular vitamin D3' the form of 'D' you will find it in the majority of OTC supplements. Since we don't even know how much D3 a given individual actually needs the best thing you can do is to get tested, even if the estimated equivalent values and bioactivity were accurate.

The optimal dietary / supplemental intake is not the only thing that's still highly debated, though. The "optimal" serum levels are similarly dubious. While there appears to be a consensus that you would be ill advised not to take action, when your 25OHD levels come back in the deficiency zone from 30ng/ml-0ng/ml, the answer to the question whether it makes sense to opt for the 'magic' 80ng/ml depends on the expert you ask.

In view of the emerging importance of free vs. total vitamin D, the 25OHD number (=total) on your blood test may soon be deemed completely irrelevant, anyways. There are after all numerous recent studies that support the hypothesis that free 25OHD and not total 25OHD is, where the magic happens. In fact, I have only recently covered two of them in the SuppVersity Facebook news. News-item (1) deals with vitamin D binding and insulin resistance (read more) and news-item (2) addresses the correlation between the free vitamin D and lipid levels in the blood.

  • Blomstrand R, Forsgren L: Intestinal adsorption and esterification of vitamin D31,2-3H in man. Acta Chem Scand 1967;21:1662–1663. 
  • Francis RM, Peacock  M, Storer JH, Davies AEJ, Brown WB, Nordin BEC: Calcium malabsorption in the elderly: The effect of treatment with  oral  25-hydroxyvitamin  D3.  Eur  J  Clin Invest 1983;13:391–396. 
  • Heaney RP, Barger-Lux MJ, Dowell MS, Chen TC, Holick MF: Calcium absorptive effects of vitamin D and its major metabolites. J Clin Endocrinol Metab 1997;82:4111–4116. 
  • Ovesen L, Brot C, Jakobsen J. Food contents and biological activity of 25-hydroxyvitamin D: a vitamin D metabolite to be reckoned with? Ann Nutr Metab. 2003;47(3-4):107-13. Review. 
  • Thompson GR, Lewis B, Booth CC: Absorption of vitamin D3-3H in control subjects and patients with intestinal malabsorption. J Clin Invest 1966;45:94–102. 
  • Trummel CL, Raisz LG, Blunt JW, DeLuca HF: 25-Hydroxycholecalciferol: Stimulation of bone resorption in tissue culture. Science 1969; 163:1450–1451.