Safe Upper Limit
There is concern that long term consumption of excess protein in adulthood, especially from animal sources, raises cancer risks and all-cause mortality . Some diets, such as the high meat version of the paleo diet, advocate higher consumption of protein than the current RDA. It is controversial whether that is a good idea long term because of the longevity issues. But even in the short term, it is important from a health perspective to know the safe upper limit for protein. As part of metabolizing protein, your liver has to convert nitrogen in the blood to urea. Exceeding the upper limit the liver can handle can lead to various symptoms of protein toxicity. Your kidneys have to handle the urea load from the digestion of proteins, so excess protein can impair kidney function. The theoretical safe upper limit to avoid protein toxicity is 3.5-4.6 grams per Kg of body weight (1.6-2.1 grams per pound of body weight. Throwing a bit of safety factor in, a recommended upper limit is 2 to 2.5 grams per Kg of body weight (0.9-1.1 grams per pound of body weight . This is more than twice the RDA, even with the upward revision discussed in the last post, so there is some wiggle room between the recommended amount and what is considered a safe upper limit (at least in the short term).
Can Vegans Get Enough Protein?
This is a question vegans get so tired of answering that I’ve seen the picture above on T-shirts. There are two main issues: does plant food provide complete protein (a balanced set of amino acids), and does plant food have enough protein density so you can get your daily requirement from eating a reasonable amount of calories? It’s been known that the answers to both of these is yes since as early as 1971, when Frances Moore Lappe’s book Diet for a Small Planet came out. So I was disappointed to see these same old issues raised in Dr. Mark Hyman’s recent book Food: What the Heck Should I Eat?.
Plant Protein Completeness:
There are 9 amino acids that the body cannot produce (therefore called essential), that we need to get from protein in food. Animal products tend to have a good amount of each of these, so are considered “complete”. Plant sources tend to be deficient in one or more, but by eating a combination of sources as suggested in healthy vegan or wfpb diets, we are assured of a complete supply . Back when Ms. Lappe wrote her book, it was thought you had to do the combining at each meal, and she gave instructions on how to do so. She also pointed out that traditional populations that could not afford a lot of animal products have all come up with solutions for this in their diets, for example combining rice and tofu, or tortillas and beans. But it is now known that our body is smart enough to do the combining throughout the day, as long as a variety of plant foods is eaten . It is also claimed that meat is a better source of the amino acid leucine than plant sources , but there is actually concern that excess consumption of leucine from meat sources contributes to type II diabetes, cancer and aging [5,6].
Getting Enough Protein From Plants:
Here is where Dr. Hyman got the math wrong. He cites this example: 100 grams of beef has 36 grams of protein, while 100 grams tofu has 9 grams of protein. So this seems like you need to eat a lot more tofu to get your protein. But that amount of beef is 217 calories, while that amount of tofu is 70 calories. To get the same amount of protein as you get from beef, you’d need to eat 400 grams of tofu, but that is only 280 calories, not so much more than the beef. There are even denser sources of protein. White button mushrooms have 3.6 g protein/100g but only 23.8 calories, so 36 grams of protein is 238 calories. Nutritional yeast has 9 grams of protein for a 60 calorie serving.
There is also a surprising amount of protein in vegetables, for example: 1 avocado has 10 grams, 1 cup broccoli – 5 grams, 1 cup spinach – 5 grams, 2 cups cooked kale – 5 grams, 1 cup cooked sweet potato – 5 grams.
Although I believe you can get enough protein from plant foods, it helps be knowledgeable about the concentrated sources like tofu, tempeh, and seitan (which was traditionally called fu in Japan). Some authors say don’t worry about protein, just eat a variety of plant foods and you’ll get plenty. I think that may be going too far. Some traditional people that are quite healthy on a near-vegan diet are short of stature, but it is not genetic. An example is the Mexican Pimas still eating their traditional diet and those in Arizona eating closer to the SAD. As discussed in a previous post, those on the traditional diet are much healthier, but those on the modern diet are taller . The moral is not to switch to the modern diet or that you can’t get enough protein without meat, but you need to audit your diet to see assure that you’re really getting adequate protein (like the recommend daily allowance discussed above), and maybe add concentrated sources if necessary. As I discussed under meat alternatives, for those like me that are not good cooks, there are some healthy brands of meat alternatives like Tofurky and Sweet Earth. Just chop them up and throw them in stir-fry or salad and you have a delicious meal with plenty of protein. There are also good plant-based supplements in powder form.
I can’t resist throwing in this about nutritional yeast: A favorite Sci Fi author of mine was Dr. Isaac Asimov. He wrote a great novel, The Caves of Steel, in 1953 that was both sci-fi and a detective story involving a human cop partnered with a robot (the movie I-Robot was partially based on it). It depicted a future society short on resources where protein needs were met by growing vast quantities of yeast
- Song, M, et al, “Association of Animal and Plant Protein Intake With All-Cause and Cause-Specific Mortality,” JAMA Internal Medicine, 2016
- Bilsborough S, Mann, N, “A Review of Issues of Dietary Protein Intake in Humans”, International Journal of Sport Nutrition and Exercise Metabolism, 2016
- McDougall J., “Plant foods have a complete amino acid composition”. Circulation, 2002
- Hyman, M, Food: What the Heck Should I Eat?, Little, Brown and Company, 2018
- Melnik B, “Leucine signaling in the pathogenesis of type 2 diabetes and obesity.”, World J Diabetes.2012
- Wang, X, Proud C, “Nutrient control of TORC1, a cell-cycle regulator”, Trends Cell Biol. 2009
- Schulz, L, Chaudhari, L, “High-Risk Populations: The Pimas of Arizona and Mexico”, Curr Obes Rep. 2015