Proteins make life possible. We need them every day, and that raises the important question: are proteins from animals or vegetables better? Or is there even a difference? In a 2004 article, “Protein – Which is Best?” Jay Hoffman and Michael Falvo examined studies to answer this question.
First, the reason we all need protein is to obtain amino acids (AAs), which compose all proteins. We need to eat nine essential AAs to stay alive, to transport oxygen in blood, to repair and grow tissues, and to make enzymes, which carry out almost every bodily process that you can name. Animal proteins almost always have all nine essential AAs, and hence are “complete,” while plant proteins often lack one or two. Fortunately, different plants lack different AAs, so by eating brown rice, which is deficient in the AA lysine, with pinto beans or green peas, which are deficient in methionine and cysteine, vegans can eat complete protein.
If we eat animal flesh itself to obtain proteins, we will also be eating cholesterol and saturated fat. Though evidence that these cause cardiovascular disease is inconclusive, there is evidence, discussed in the post on fat, that replacing some saturated fats with unsaturated fats from whole vegetable products (not refined carbohydrates) decreases our risk of cardiovascular disease. In any case, skim milk and egg whites contain almost no fat and cholesterol, and “Protein – Which is Best” says that casein, the main protein in milk, is effective at building muscle. An equivalent mass of vegetable protein, however, is not as effective, according to the article.
The article mentions several different ways to compare protein quality: Net Protein Utilization (NPU), which measures what percent of the protein we eat we actually can use to make our own proteins, and Protein Digestibility Corrected Amino Acid Score (PDCAAS), which is calculated by seeing what percentage of each essential AA the food supplies, and then taking the lowest one, where a PDCAAS score of 100% indicates a complete protein. The article gives these measurements for several proteins. The ones with the best NPUs on the list are egg (94%), milk (82%) and beef (73%), all animal proteins. Wheat gluten (67%) and soy (61%) are the top vegetable proteins, but gluten has a PDCAAS score of only 25%, whereas soy is actually a complete protein. Egg and milk are also complete; beans (PDCAAS = 75%) and peanuts (PDCAAS = 52%) are not. Thus, in general, animal proteins are not only more complete, but also better absorbed, than vegetable proteins.
Complete proteins, however, can have large amounts of cysteine and methionine, essential AAs that contain sulfur. Metabolizing them converts the sulfur to small amounts of sulfuric acid, and to neutralize the sulfuric acid, the body draws calcium out of the bones. Some studies have shown than high animal protein diets decrease bone density in elderly women, young men, and rats, but other contradictory studies have shown that increasing animal protein and decreasing vegetable protein preserves bone density. This can be explained because, firstly, dairy products have calcium, which could replace the calcium lost from bones, and secondly, some vegetable proteins actually contain as much sulfur as animal proteins. According to nutritiondata.self.com, as a percentage of total protein, Brazil nuts and walnuts contain more cysteine and methionine than do eggs and milk. Oats, wheat, and rice also contain more than milk. Thus, it is not universally true that animal proteins contain more sulfur or increase bone loss; the amount of cysteine and methionine is more important than the source of the protein, as are the other nutrients, like calcium, consumed with the protein. Also, since these sulfur-containing AAs are essential, we all need to eat them, but in moderation.
Protein supplements appear to benefit athletes in both strength and endurance training. Whey, casein, and soy are common supplements, and of these, studies have shown that casein is most effective at providing us AAs for several hours that we can best use to rebuild muscle. However, there is evidence that high protein diets increase growth factors, such as insulin-like growth factor 1, which can promote cancer growth. For those of us who are not athletes, then, it seems like a good idea to avoid excessive protein consumption.
Certain foods famous for their protein content, such as milk and soy, exert additional effects on our bodies. Soy, for example, contains isoflavones that mimic estrogen and are thought to possibly reduce breast cancer risk, but that also promote oxidation of LDL cholesterol, which leads to atherosclerosis. These additional effects, however, result from other chemicals in the food, not the protein itself. Thus, it seems like it is more important, for the purpose of health, to ask not, “Animal protein or vegetable protein?” but to ask, “What are the effects of other chemicals are in this food?” Also, because there are many differences among proteins from animals and even more differences among proteins from plants, and because some plant proteins, like soy, are even more complete than some animal proteins, like beef, we cannot generalize all plant proteins into one category and all animal proteins into another. For those of us who are dedicated athletes, it may be advantageous to eat a lot of animal protein, as we tend to absorb it better. But for most of us, as long as we fulfill our protein requirements, there should not be much difference. We can choose to be carnivores or to be vegans. But as you decide, keep in mind the other nutrients and toxins that come with the protein you eat; these are more important than whether the protein originated in an animal or a vegetable.
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