I’m going to be honest about this up front (so you don’t have to keep reading if you don’t want to), I do not think the vegan diet is a great choice for athletes. My dislike of the vegan diet for athletes is not because of something I have read or because I belong to a meat-eating cult (for what it’s worth, I’m also not a fan of the carnivore diet). It’s not because I tried it and it didn’t work for me (because I have never tried it). It’s because I have not yet seen a patient (athlete or not) who follows a vegan diet and is truly healthy and feels amazing.
If you’re a vegan and you think that your diet is truly working for you, it might be, but you may also be using the wrong metrics to measure whether or not it’s working. My take on this topic as a nurse practitioner is not due to personal experience with a vegan diet (I have never tried it before). As a healthcare professional, my job is to share information based on science and clinical experience, not always from personal experience. This is a topic I get asked about a lot (and I know others do too), so what I am sharing is my opinion from an evidence-based and clinical perspective.
Also, please do not misinterpret this as an attack on veganism and vegetarianism. It is not. I know plenty of people who are not my patients who eat this way and I have never once tried to convince them otherwise. But if you come to me looking for advice on how to feel better, and you eat this way, chances are I’m going to encourage you to change your mindset and your eating habits a bit in order to achieve the results you want.
Athletes and Veganism
A lot of people who adopt a vegan or vegetarian lifestyle, are those who are coming from a way of eating that is pretty unhealthy - lots of processed foods, sugar, diet soda, and unhealthy fats. These are people who feel terrible (because they are eating terribly), looking to lose weight, and just want to feel better. Even if you are not eating copious amounts of junk, switching to a vegan or vegetarian diet probably (hopefully) has you eating more vegetables and less sugar.
Once someone has switched to a vegan or vegetarian diet they initially feel great because they have eliminated most of the unhealthy and inflammatory foods they were previously consuming. Their diet is now lower in processed foods, they aren’t drinking soda, and hopefully they are eating more vegetables. But this tends to be a temporary improvement. After three or six months of being vegan, the nutrient deficiencies start to add up (especially if you haven’t done your research before making the switch) and they tend to start a slow descent into not feeling so great again. This is not based on what I’ve read, this is based on what I’ve seen with actual patients in my clinic.
protein & Amino acids
The most common problem I see with vegan athletes is protein deficiency. If you are an athlete you should be eating around 1.5-2g/kg of protein (potentially more depending on a lot of individual factors). 
Vegan protein sources tend to be grain based, incomplete (they must be paired with something else to form a complete protein), and less bioavailable than animal protein sources.  Non-grain based vegan protein sources include legumes, which are actually higher in carbohydrate content than protein, which means that most vegan diets tend to be high in carbs. This isn’t necessarily a bad thing but may be problematic if you are looking to be a LCHF vegan (for athletic performance).  
Meat and poultry has about 25 grams of protein per 100 grams and fish contains around 20 grams of protein per 100 grams. Most vegan sources of protein (vegetables, beans, soy, etc.) have anywhere from 5-20 grams per 100 grams. Beans have the most protein at around 20 grams per 100 grams, but 100 grams of beans also contains over 60 grams of carbohydrate.
So while it’s not impossible to get adequate protein on a vegan diet, it’s going to be a challenge to get enough while also simultaneously not getting too many carbohydrates. It’s similar to finding a training program that only takes you 15 minutes per day, but gets you amazing results vs. spending an hour a day training to get the same results. It’s possible, it’s just more work, and in my experience if it requires more work it typically doesn’t happen.
In general, quality of protein on a vegan diet is also inferior to that of an omnivorous diet.  I realize that soy made the list of high quality proteins, see the section below for my thoughts on soy (it’s not the best, but it’s not the worst). Other plant-based options have lower protein quality and digestibility. Combine inferior protein quality with the difficulty getting adequate protein, and you’ve got a recipe for protein deficiency, especially in the athlete population.
In the same category as overall lack of protein, there’s also the issue of individual amino acids. There are several amino acids that are virtually impossible for vegans to get and are considered “conditionally essential” for humans, proline and taurine are the most significant. This means that while they are not essential (the term for amino acids that you must get from your diet) because your body will produce a small amount, most people have some sort of condition that requires a larger amount of these amino acids than their body can make on it’s own. Thus it becomes important to get it from your diet.
As far as I know there are no significant vegan sources for proline or taurine. Prolene is found in meat, eggs, and bone broth and taurine is mostly in shellfish and meat (and eggs to a much lesser extent). Brewer’s yeast is the only vegan source of taurine that I’m aware of and I was unable to find how much taurine is actually in brewer’s yeast vs. animal sources. Taurine is important for cardiovascular health , to prevent inflammation , and to improve exercise performance .
In general, I’m not a fan of soy and soy products. Soy contains isoflavones, which are estrogen-like compounds. These substances can interfere with sex hormones and even potentially contribute to infertility (it lowers testosterone in men).  Soy also contributes to nutritional deficiencies, thyroid issues (it inhibits your thyroid’s ability to use iodine which increases TSH), and may be linked to breast cancer.  Soy protein isolate is often contaminated with aluminum and the processing it undergoes during its creation destroys many of the amino acids (proteins).
Soy also tends to be one of the main offenders when it comes to food sensitivities and gut irritants. I’ve worked with many people to determine what foods were causing their fatigue, brain fog, skin issues (specifically eczema and acne), joint pain, etc. and soy and dairy are two of the most common culprits.
There is some evidence that fermented soy products (such as natto, miso, and tempeh) are be safe in small amounts. The fermentation process degrades some of the phytic acid, but it does not eliminate the problematic isoflavones. 
Many of the studies that I found that discuss the beneficial effects of soy are actually sponsored by the soy industry or written by someone who has a financial interest in the soy industry. This doesn’t necessarily make the studies invalid, but it has a strong potential to skew the results to make it appear that soy is beneficial when it really may not be.
Foods like soy milk and soy based meats are highly processed foods and should be limited. Most soy based meat products also contain highly inflammatory vegetable and seed oils which contribute to inflammation and leaky gut. This article by Mark Sisson is an excellent look at the pros and cons of soy in its various forms.
Vitamins, Minerals, & nutrients
Nutrient deficiencies are very common in vegans. Specifically iron, B12, omega-3s, vitamin K, vitamin A, and zinc since the best sources of these nutrients come from animal based products. Being a healthy vegan is hard work (and even harder if you’re an athlete) because instead of relying on the inherent nutrient density in foods like beef, eggs, salmon, and seafood, you have to be very specific and intentional about what you eat in order to get sufficient nutrients from your food.
Rather than getting all the necessary zinc and iron from something like oysters, vegans need to be sure to eat the right amount of specific nuts and seeds to make sure that they don’t also get too much calcium and copper in the same ela (calcium binds iron and copper interferes with zinc).
Lack of Vitamin K (combined with issues with absorbable calcium) creates a problem for bone health. Vitamin K1 is found in plant sources, but vitamin K2 is found in butter, cheese, milk, eggs, meat, and liver. Vitamin K1 is poorly absorbed by your body, one study found that less than 10% of the K1 found in plant foods is absorbed by your body.  Since vitamin K is a fat soluble vitamin, the reason K2 may be better absorbed by the body might be due to the fact that it’s typically found in foods with fat, however that is not clear. Vitamin K2 also stays in the blood longer allowing for it to be better utilized by tissues. 
Vitamin A is also an issue, again because there is a difference between the plant version and the animal version. The plant versions of vitamin A, such as beta-carotene, are actually converted into the animal form by your body.  The problem is that some people are very efficient at converting the plant version of vitamin A into the animal version, but this process is highly variable depending on the person. If you are a person who is unable to convert beta-carotene into a retinoid, then you may be at risk for vitamin A deficiency.
Probably the biggest vitamin deficiency seen in vegans is B vitamins, specifically B12 because they are found exclusively in animal food sources. Oysters, sardines, wild salmon, beef, lamb, eggs, and milk are all sources of B12. You can supplement with B vitamins, but B12 is often poorly absorbed in the gut and while B12 injections may be helpful, it is also possible that you don’t have the appropriate co-factors in your body to absorb it at all (cofactors for particular vitamins are found in the foods that contain those vitamins).
It’s also difficult to get the right fats in a vegan diet. One of the main missing fats for vegans is CLA, which is most abundant in butter, eggs, and beef. One of the most common vegan sources of CLA is safflower oil, which is actually incredibly high in omega-6 PUFAs. 
Algae is the only vegan source of DHA (commonly found in salmon, sardines, and mackerel). If vegans do not supplement with algae or algae oil they have to make DHA out of ALA which is an incredibly difficult and inefficient process for the body. , 
The other problem with nutrients is that most vegans and vegetarians I’ve encountered are not actually eating that many vegetables (hence #morevegetablesthanavegetarian). Their diets tend to be made up of a lot of grains, legumes, tofu, and other processed vegetarian/vegan products like veggie burgers (which are mostly soy, grains, and PUFA/omega-6 oils), and fake meats (which are mostly soy and grains).
If you do choose to adopt a vegan diet, you absolutely must supplement with B12 because the plant-based foods do not provide B12 and fortified foods are generally inadequate sources (even more true if you are an athlete). Iron is also a big problem because all of the vegan sources of iron are non-heme iron which is not as easily utilized by your body as heme iron is.
can you be a healthy vegan?
Honestly, I don’t know if the truly healthy vegan exists. Almost all of the vegan patients I’ve seen insist that they are the healthiest they’ve ever been, but then we go on to talk about many unpleasant symptoms they have. They tell me they keep getting injured or sick, or they have no energy and are barely able to make it through their workouts. I do understand that these complaints are common amongst the population in general, but the point is if you tell me that you’re perfectly healthy but then go on to make statements about being sick or injured…you might not be as healthy as you think you are.
Remember that health is not simply defined as the absence of disease. I see a lot of people who think they are healthy, but they actually could vastly improve their health. Do you sleep well at night and wake up feeling rested? Do you take any medication? Do you suffer from lack of energy or mental clarity during the day? Do you ever experience gas or bloating after a meal? Do you have a daily bowel movement?
If you answered yes to any of those questions, then you are definitely not as healthy as you could be. And this applies to everyone, vegan, vegetarian, Paleo, carnivore, or otherwise.
Nutritional deficiencies are probably the biggest concern on a vegan diet, but they are also a concern if you are following a Standard American Diet (SAD). The average person already doesn’t get enough of many of the vitamins and nutrients that I discussed earlier as being a concern for deficiencies in vegans. It’s important to remember too that just because you are not deficient in something by the FDA standards for adequate intake does not mean that you have enough to feel good. The recommended daily intake for nutrients is meant to be enough for 97% of the population not to be deficient, but that automatically leaves 3% of the population as not getting enough and does not address the difference between adequate and optimal.
But what about all the amazing athletes out there who are vegans?
Perhaps the whole time you’ve been reading this you’ve been thinking about all the amazing athletes out there who are vegans (at the moment the only two I can think of are Scott Jurek and Venus Williams). But thinking that their vegan diet is responsible for their stellar athletic performance is the wrong way to look at it. There are also athletes out there who are eating McDonalds and drinking Red Bull and Diet Coke and are crushing it, but that doesn’t mean that their diet is responsible for their performance. You also have no idea if they are suffering from any sort illness or health problems. So attributing their superior athletic performance to their diet is unfair.
The Environmental and Ethical Impact of Veganism
A lot of people cite the environment as the reason why they are choosing to go vegan (athletes and non-athletes alike), but the truth is that a Paleo type diet (which includes pastured and grass-fed meats and sustainable sources of seafood) is actually more environmentally friendly than veganism. I don’t want to spend a lot of time on this piece of the puzzle, so I’m going to hit some high points and then direct you to some other articles and resources.
Eating pasture raised beef is not only more energy efficient (think of how many meals you can get out of one cow vs. how many meals you can get out of one acre of soybeans) but also has zero (or potentially negative) effect on contributions to greenhouse gas emissions (pasture raised cows are actually important in the carbon sequestering process).
Eating vegetables does not mean that you have not harmed living animals. Lierre Keith is a great source of information and perspective when it comes to reconciling respect for living things with eating animals.
Dietician and farmer Diana Rogers is currently making a documentary called “Kale vs. Cow” which looks at many of the environmental and moral issues related to vegetarian/vegan diets compared to a Paleo type diet.
final thoughts on vegan athletes
My advice to anyone who asks me about diet is that you should follow a diet that works for you (determining what that is takes some work on your part). It’s also incredibly important not to assume that you are eating your ideal diet based on something you read or what someone else is doing. We all have unique genetics, biochemistry, lifestyles, and activity levels and need to match and fine tune our diets to those factors.
If you've switched from eating a Standard American Diet or a diet full of processed foods and sugar to a vegan diet and felt awesome, that’s great! But assuming that because you feel better after some initial dietary changes means that the way you are eating now is the end of your journey to optimal health is misguided thinking.
Have you tried adding some high-quality protein and fat sources from animal-based products? If the answer is no then you can’t possibly know that you feel better when you aren’t eating those things. Because comparing the way you felt while you were eating animal products in the context of the SAD to the way you feel not eating them while eating a vegan diet is comparing apples to oranges. I have this very same discussion with people who tell me that they tolerate gluten and dairy just fine but have never tried removing them from their diets and then adding them back in.
I’m not saying that being an optimally healthy vegan athlete isn’t possible, but I certainly haven’t seen it happen yet. I encourage you to do your research and don’t make a decision about the way you eat based on what a book tells you or what you see someone else doing. I encourage you to experiment with your diet and critically evaluate the way you feel during that process in order to figure out what’s truly right for you.
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