First, a bit of a disclaimer: I'm someone who eats an almost completely plant-based diet (key word, "almost").
I founded a company that offers exclusively plant-based nutrition products. I ate exclusively vegan for the better part of a decade. Clearly, I'm a believer in eating primarily plant-based foods.
Still, there are some huge problems with vegan diets that you should be aware of (even if you're not a vegan).
1. Omega 3's
Maintaining a balance between your Omega 3 and Omega 6 levels is critical to keeping systemic inflammation in check. If your Omega 6 intake exceeds your Omega 3 intake, systemic inflammation will increase (so too will the negative secondary effects of systemic inflammation).
Unfortunately, there are far more foods that are rich in Omega 6's (vegetable oils are the biggest offender). Given this, we have to specifically include Omega 3's in our diet to maintain the optimal 1:1 balance between the two.
Omega 3's are found in plants, but often they're accompanied by even higher levels of Omega 6, meaning they aren't particularly useful for supplementation. There is one plant-based source with high Omega 3's and low-Omega 6's (flax seeds).
The problem is that there are several different types of Omega 3's and the form found in flax seeds (ALA) must be converted by the body into the different forms (DHA and EPA) before it can be utilized. The body is very inefficient at doing this conversion. Studies have shown that only around 2-3% of the ALA you eat will get converted to DHA and EPA. This means you'll have to eat somewhere around 40 grams of flax oil for your body to get 1g of usable Omega 3's.
While it is possible to consume enough flax oil to get adequate Omega 3's, it is so much less efficient than eating krill or fish oil it seems silly. Since the Omega 3's in krill and fish are already in the form of DHA and EPA, they are far more efficient sources.
Check out this past article for more on Omega 3's
2. Too Much Fruit
How many vegans do you know that don't eat copious amount of fruit?
Traditional thinking is that fruit is generally healthy, but recent evidence suggests otherwise. Fruit is essentially glorified natural high-fructose candy.
Historically, fruit provided meaningful amounts of vitamins and minerals. Today's grocery-store fruit is grown in over-farmed, depleted soils and has a small fraction of the nutrition that wild fruit did. Of course, fruit has always brought with it loads of fructose, which recent research has shown is no good for the body.
Fructose is a reactive molecule that binds with proteins in the body to form Advanced Glycation End Products (AGEs). AGEs cause cross-linking in collagen, the body's primary connective tissue.
Cross-linking reduces the flexibility of collagen, creating a number of issues including stiff joints, damaged DNA and aging skin. Fructose has also been correlated with insulin resistance (pre-diabetes), elevated triglycerides and even decreased brain function.
When we eat fructose, the body doesn't absorb it as it does with most other nutrients - it send it to the liver to be processed and turned into glycogen or fat.
Shouldn't it tell us something when the first thing our body does with something is send it to the liver to be turned into something else?
Creatine is an essential component of energy metabolism found in fish and meat, but nowhere in plant foods.
How essential is creatine? A 2003 study showed that when vegetarians and vegans supplemented with creatine, their IQ scores went up and an average of 20%. See this past article for more on creatine supplementation.
4. Ethical Arguments Against Vegan Diets
Veganism is supposed to be better for the environment and save animal lives, right? Compared to a normal American diet, veganism absolutely is a world better.
But subjected to a bit closer scrutiny, veganism no longer looks like the ethical gold-standard most vegans assume it to be.
To start with, many of the foods that vegans eat a lot of have major negative ecological impact (think soy and grains). Beyond that, even the saving animal lives argument doesn't stand up to scrutiny.
The reality is, any time you eat a food that is farmed on a large scale, you are complicit in a huge number of animal deaths that happen during the harvesting process. The industry refers to these as "tractor kills" and the animals are typically rodents and other small creatures.
On the other hand, if you ate local grass-fed, grass-finished beef every day for a year, you'd be responsible for killing about 2/3 of an animal. Kind of makes you re-evaluate when you see how a vegan can be responsible for many more animal deaths over the course of a year than someone who eats beef every day, right?
Of course, eating only local, small-scale produce is the solution here, but how many vegans actually do this?
However, don't get too excited here meat-eaters. If you eat conventional beef, that cow was fed grains, not grass. Since it takes 7 calories of feed to produce 1 calorie of meat over a cow's lifetime, you are responsible for 7 times as many tractor kills as a vegan.
5. Vitamin A/Beta-Carotene
Much like the Omega 3 problem, while you can get vitamin A from plant foods, it comes in a form that is difficult for the body to use. Carrots and other plants contain beta-carotene, which the body must convert into retinol, retinal or retinoic acid before it can be used.
This is a very inefficient process, making carrots and other vegetables a poor source of vitamin A. On the other hand, vitamin A from fish and fish oil-based supplements is already in the proper forms and thus very easy for the body to use.
6. Fat-Soluble Vitamins
Vitamin A is not the only nutrient that plants do a poor job at providing. Other fat-soluble vitamins (D and E) are difficult to get in adequate levels from plant foods.
Vitamin E is available in oil as an avegetarian supplement. Vitamin D, however, is almost always found in dry form as a vegetarian supplement.
When consumed in dry form, most of the vitamin will pass through the digestive system unabsorbed. The best vitamin D supplements come from fish oil sources.
Recently, vegetarian oil-based vitamin D supplements have become available (derived from lichen). They're quite expensive and hard to find, and I'm hesitant to endorse them until I see more research on absorption and chemical analysis. Fish-oil based sources remain your best bet.
See this article for more on vitamins and supplementation.
7. Junk Food Veganism
How many times have you been checking out at Whole Foods and the person in front of you has a shopping cart that looks like this?
Look what you're eating for goodness sake! It's heavily processed, devoid of nutrients and likely to be contaminated with mycotoxins. Your average "healthy-eater" omnivore is doing 100x better nutritionally.
8. Short-Chain Fats
There are a few highly valuable short chain fats (butyrate being the most notable one) that are simply not found in plant foods. Butyrate is a valuable fuel for the brain and has been shown to have neuro-protective effects. Butyrate also plays a critical role in maintaining a healthy gut flora and has anti-inflammatory properties.
This is a hugely beneficial nutrient you are sacrificing if you are not eating butter or ghee (the best sources). It's important to note that if you're going to eat butter or ghee, make sure it is 100% grass-fed or pastured. Traditional butter and ghee have way lower quality of fats and are often contaminated with mold toxins from grains the cows are fed.
9. Natural Plant Toxins
90% of the time, eating tons of vegetables is a completely beneficial thing to do. There are a handful plant foods out there that contain low levels of natural toxins that can be harmful if consumed in large quantities. Since a lot of these foods are staples of the vegan diet, this can particularly be a problem.
These plant toxins are produced naturally by the plants as a defense mechanism against being over-eaten by insects in their growing environments. When we consume these foods occasionally, our bodies are able to clear out the toxins and they never accumulate and cause problems.
When they are staples of the diet and they're consumed several times a week, it's an entirely different matter. If these toxins accumulate they can disrupt digestion, cause inflammation, and alter hormonal functioning.
The main offenders on the list are: legumes (tannins and phytic acid), quinoa (tannins, phytic acid, saponins), kale and other cruciferous greens (oxalates, goiterogens), nuts (phytic acid, tannins), rice (arsenic), wheat and corn (gluten, mycotoxins, phytic acid).
Vegan diets tend to be loaded with soy products. Soy farming is an ecological disaster in some parts of the world. Huge swaths of South American rainforest have been cleared in recent years to plant soy crops. If you're among those freaked out by GMO's, soy should scare you more than almost any other crop.
There are some very good health reasons to avoid soy as well. Because soy is such a huge crop and often stored and transported for long periods of time, it is a significant risk for mold and mycotoxin contamination.
Soy protein has also been shown to create GI-tract inflammation for a lot of people. Last but not least, soy contains phytoestrogens, compounds that can interact with our endocrine (hormone) system and have disruptive effects on the body. These effects have only been conclusively shown in post-menopausal women at this point, but it's logical to expect that phytoestrogens have effects on everyone.