In my last post in this series on veganism, I looked at anthropological evidence to establish that as a species in our natural habitats, humans are omnivores, and we thrive on an omnivorous diet. But although we are still humans (most of us, anyways), we don’t quite live in our natural habitats anymore, so what applied to the natives that Weston Price studied doesn’t necessarilyapply to us. In a modern-day setting with greater access to a variety of foods, can humans thrive on a vegan diet without needing to supplement? Is it biologically possible for humans to get what they need on a diet devoid of animal products?
In this two-part post, I’ll try to answer that question by looking at some of the nutrients that are hardest to come by on a vegan diet. In part one, we’ll look at three nutrients that are vital to bone health: vitamin D, calcium, and vitamin K2.
One potential risk of a vegan diet is inadequate vitamin D, which could lead to osteoporosis and dental problems. While lacto-ovo vegetarians don’t appear to have significantly lower bone density than omnivores, a few studies of vegan women show that their bone density is lower than that of omnivores and other vegetarians, and their risk of fracture is higher. (1)
Vitamin D is a bit of an unusual case, because there aren’t many foods (animal-derived or otherwise) that supply significant amounts of vitamin D. However, the few foods that do contain vitamin D tend to be animal-based, including fish, liver, and egg yolks. Additionally, the most usable forms of vitamin D are vitamin D3 and its metabolite 25-hydroxyvitamin D, which are only found in animal products. (2)
Luckily for vegans, the body can produce vitamin D3 when the skin is exposed to sunlight. Of possible concern is the availability of vitamin D precursors such as cholesterol, which also tend to be less prevalent in a vegan diet, but in a healthy person this shouldn’t be too much of an issue.
It’s worth noting that meat has been shown provide unique protection against rickets and osteomalacia (vitamin D deficiency diseases) that its vitamin D content alone cannot fully account for. (3) Meat doesn’t contain that much vitamin D (an average of 7 μg vitamin D/kg), but a study of Asians showed that a diet including ‘high intakes’ of meat, fish, and eggs was completely protective against softening of the bones, even though the diet as a whole was low in vitamin D and they lived at a high latitude with low exposure to sunlight. Diets high in dairy – but not meat – did not have the same protective quality, despite the fact that vitamin D and calcium intake were the same between groups.
As this is an observational study, it’s possible that the findings resulted from factors other than meat consumption, but it appears that small amounts of meat could be necessary to maintain bone integrity at higher latitudes where exposure to sunlight is limited. A vegan diet alone isn’t enough to cause softening of the bones, but a vegetarian or vegan diet along with limited sunlight appears to be detrimental to bone integrity.
In other words, it’s vital for vegans to get plenty of sun exposure, and if that’s not possible due to the environment, vegans probably need to supplement with vitamin D3. It’s easy to get your vitamin D levels tested with a quick blood draw, and Denise Minger recommends supplementing with a vegan form of D3 to get your blood levels up to at least 35 ng/mL. However, recent research suggests that levels between 20 and 30 ng/mL might be most optimal, so if you’re otherwise healthy and get plenty of sunlight, a level as low as 20 is probably fine.
Calcium can be a thorn in the side of strict paleos and vegans alike, due to the popular belief that one can only obtain sufficient calcium levels by consuming dairy. However, unlike those on a paleo diet who can fall back on bone broth (or just straight-up bones) as a source of calcium, the vegan diet appears to be sorely lacking in this much-publicized mineral.
First, I’d like to point out that calcium probably gets more airtime than it really deserves. Yes, calcium is important, but other nutrients such as magnesium and vitamin K2 are just as, if not more, important to bone health than calcium, but they don’t get nearly as much attention.
As I mentioned above, studies indicate that long-term vegans have lower bone density than omnivores and lacto-ovo-vegetarians. (4) However, that association disappears when vegans are consuming adequate levels of protein and calcium. In this case, the association disappears at 525mg of calcium per day, which I think is a very reasonable goal.
A possible problem is that many vegan sources of calcium aren’t particularly well absorbed, due to compounds such as phytates, oxalates, and fiber that are also present in plants. However, several studies show that the calcium bioavailability for cruciferous vegetables such as kale, cabbage, and mustard greens is even better than that of milk! (5) Keep in mind that these vegetables were boiled for a few minutes before testing, which probably has an effect on the bioavailability of their nutrients. If you’re a raw vegan and don’t plan on cooking your kale, you might not be getting as much calcium as you think you are.
Also, even though the calcium in kale and other cabbage-family veggies appears to be well absorbed, they still don’t provide that much calcium per serving. For instance, 1 cup of cooked kale will only net you about 45mg of absorbable calcium, and the same amount of bok choy should give you about 85mg. (6) According to the same analysis, nuts and legumes are poor sources of calcium due to higher levels of phytates and oxalates, although it’s unclear whether the samples studied were properly prepared by soaking or sprouting.
In general, I think that getting adequate calcium on a vegan diet isn’t nearly as big of an issue as some people make it out to be, as long as grains, legumes, nuts, and seeds are prepared properly and plenty of greens are eaten. Plus, it looks like calcium supplementation often does more harm than good, so I think I’d avoid that option if possible.
Vitamin K2 works with vitamin D and calcium to ensure bone health, and it’s a nutrient that many people (not just vegans) are deficient in. Vitamin K2 was relatively unknown – or at least misunderstood – until recently, but more and more health professionals are acknowledging how important it is. Its primary role is to ensure that calcium is deposited in the correct locations of your body, such as your bones and teeth, rather than in soft tissue like arteries. You can read more about it here.
Luckily for vegans, the richest food source of vitamin K2 is natto, which is a completely vegan, fermented, soy…um, thing. (Product? Condiment? I’m actually not quite sure how to classify natto, and while I’ve never tried it myself, there’s definitely sliminess involved. Here’s a picture. And another one, just for kicks and giggles.)
If you’re a fan of natto, then your vitamin K2 status (and dental health) probably rivals that of the lauded hunter-gatherer populations studied by Weston A. Price. However, if you’re a vegan and don’t eat natto, you may be deficient in vitamin K2.
Vegans get plenty of K1 in the form of leafy greens and other plant foods, but other than natto, the only good sources of vitamin K2 are animal products. Cheese and butter from grass fed cows are particularly good sources, as are organ meats. Animals can convert K1 into K2, and a couple studies indicate that humans can as well, but the evidence that vitamin K2 is more therapeutic than vitamin K1 indicates that the conversion process in most humans isn’t efficient enough to supply adequate K2. (7, 8, 9) Thus, vegans who don’t regularly consume natto probably need to supplement with vitamin K2.
Okay folks, that’s it for part one. You can probably see where the evidence is going with regards to the question we’re trying to answer, but I’ll leave that for the conclusion of the next post. Check back for part two on Monday! *UPDATE: click here for part two!*