There is a misconception in Western society that in order to obtain adequate amounts of protein, one must consume animal-based foods. Proteins are composed of building blocks called amino acids, and just like animal protein, plant-based protein sources contain all of the essential amino acids that cannot be made in the body. In fact, many animals get all of their protein exclusively from plants.
Animal vs. Plant Protein
Ounce per ounce, animal-based foods are more concentrated sources of protein than plant-based foods. The amino acids in animal-based proteins are in a similar number and proportion to the proteins in human muscle, thus making them easier to digest and assimilate during the process of building protein in the body.
However, even if we consume plant-protein with amino acid compositions that our different from our own, our bodies are able to break these down into their amino acid building blocks and reassemble them in the appropriate proportions. In people who get all their protein from plant-based sources, it’s important to eat a variety of foods to ensure enough of all the amino acids are available to produce the necessary human proteins.
In general, animal proteins are 95% digestible in our small intestine. Some plant-based proteins such as soy, wheat gluten, and wheat flour are 90% digestible, while other plant-based sources range from 50-80% digestibility (1).
Animal proteins have a greater content of sulfur-containing amino acids which results in a more acidic environment in our bodies. This requires buffering by calcium which can be extracted from our bones and ultimately may result in poor bone strength.
While all animal proteins may contribute to chronic disease, processed meats are the most dangerous. In fact, foods such as sausage, bacon, jerky, lunch meats, and hot dogs, are considered by the World Health Organization to be a Group I carcinogen (unprocessed red meat is classified as Group 2A, “probably carcinogenic to humans”), sharing company with things like tobacco, alcohol, and radiation.
One study showed that consuming just under 2 ounces of processed meat daily has been found to increase the risk of developing type 2 diabetes by 50% (2). This was also demonstrated in the Nurses Health Study which followed over 90,000 women for 8 years. Women who consumed processed meat 5 or more times per week were found to have a higher incidence of diabetes (3). Consumption of red meat and animal protein were also found to significantly increase risk of diabetes in the large Adventist Health and EPIC population studies. (4, 5).
Saturated Fat and Cholesterol
Animal proteins are essentially the exclusive source of dietary cholesterol in a Western diet. In fact, eggs (25%), chicken (12.5%), and beef (11%) together account for more than 50% of the cholesterol consumption in the United States (6). Additionally, except for coconuts and palm oil, nearly all dietary saturated fat comes from consumption of animal protein. Foods that are particularly high in saturated fat include processed meats such as salami and sausages, as well as higher fat cuts of beef and pork, and some cheeses.
Saturated fat consumption has a greater influence on our blood cholesterol than cholesterol consumption. According to the landmark 2013 Baylor Conference (7), cholesterol is the main risk factor for heart disease. The primary conclusion from that conference was that, if the total cholesterol is between 90-140mg/dL, there is no evidence that other risk factors such as smoking, high blood pressure, diabetes, etc., will produce plaque buildup in the arteries. Other studies confirm that progression of plaque ceases when total cholesterol is less than 150mg/dL (8).
Studies by Drs. Ornish (9), Esselstyn (10), and Frattoroli (11) demonstrated benefits of low fat (<10% of calories) diets in addition to other lifestyle changes including exercise and stress management, in reducing symptoms of angina and even reversing coronary artery disease plaques.
The American Heart Association currently recommends that less than 5-6% of consumed calories come from saturated fat, and reinforces that one is unlikely to reach an ideal LDL (bad) cholesterol level of 50-70mg/dL in the absence of a whole-food, plant-based, primarily vegetarian diet. (12)
TMAO (Trimethyl-amine N-oxide)
Trimethyl-amine (TMA) is a metabolite produced by gut bacteria following ingestion of choline and carnitine, which are abundant in meat, poultry, fish, dairy, and egg yolks. The specific bacteria capable of producing TMA are absent in the microbiome (population of gut bacteria) in vegans. TMA is converted to TMAO by the liver. TMAO has been associated with increased inflammation, blood clotting, and reduced clearance of cholesterol. These mechanisms can contribute to development and instability of cholesterol plaques in the arteries, ultimately resulting in heart attack and stroke.
A 2017 study in the Journal of the American Heart Association found a more than 60% increased risk of all cause death and cardiovascular events in people with high TMAO levels.
Heme iron is iron absorbed from animal rather than plant foods, and is found in especially high concentrations in beef, clams, and oysters. Because the body can absorb heme iron vs. non-heme iron more efficiently (35% vs. 5%), it can accumulate in excess, and cause inflammation and oxidation. It can lead to DNA damage through production of a free-radical called hydroxyl and can cause oxidation of the bad cholesterol, LDL, which makes it more likely to cause blood vessel damage. No such association has been found with non heme-iron from plant-based sources.
A meta-analysis of 21 studies involving almost 300,000 people showed a 57% increase in the risk of developing heart disease in those that consumed the most vs. the least heme iron. (livescience.com. Too Much Iron from Meat May Raise Heart Risks). Another review of over 130,000 people showed a 31% increase in the risk of coronary heart disease. (13) A population study of nearly 39,000 men in 2013 showed a higher risk of stroke associated with heme iron intake (14). A population analysis of over 500,000 people showed 18% higher rates of colon cancer in those with the highest vs. lowest intake of heme iron. (15).
Advanced Glycation End Products (AGEs)
These are products that are formed by a reaction between a protein and a sugar. The two most common AGEs, Heterocyclic Amines (HCAs) and Polycyclic Aromatic Hydrocarbons (PAHs), are found almost exclusively in animal protein-rich foods, especially if they are cooked at high temperatures, for a prolonged time, in a dry environment. Cooking methods such as grilling, roasting, broiling, searing, and frying are associated with the highest levels. AGEs are pro-oxidants and proinflammatory, and have been associated with diabetes, vascular disease, and kidney disease. AGEs can also affect DNA and increase the risk of developing cancer.
Dysbiosis and Increased Intestinal Permeability
In those that consume a high-fat, high-animal protein, low-fiber diet, there tends to be a much lower diversity of bacterial species in the gut microbiome. This can lead to a syndrome called “leaky gut”, where toxins from the colon can leak into the blood stream. This mechanism is felt to be responsible for the underlying inflammation associated with many modern day chronic illnesses, including cardiovascular disease, cancer, and autoimmune disease.
- Tome, D., Digestibility issues of vegetable versus animal proteins: protein and amino acid requirements-functional aspects. Food Nutr Bull, 2013. 34(2): p. 272-4.
- Pan, A., et al., Red meat consumption and risk of type 2 diabetes: 3 cohorts of US adults and an updated meta-analysis. Am J Clin Nutr, 2011. 94(4): p. 1088-96.
- Schulze, M.B. et al., Processed meat intake and incidence of Type 2 diabetes in younger and middle-aged woemn. Diabetologia, 2003. 46 (11): p. 1465-73.
- Snowdon, D. A. and RL Phillips, Does a vegetarian diet reduce the occurrence of diabetes? Am J Public Health, 1985. 75(5): p. 507-12.
- Sluijs, I., et al., Dietary intake of total, animal, and vegetable protein and risk of type 2 diabetes in the European Prospective Investigation into Cancer and Nutrition study. Diabetes Care, 2010. 33(1): p 43-8.
- Greger, M., 2C Nutrition: Foods Overconsumed and Underconsumed, in Lifestyle Medicine Core Competencies. 2015, ACPM: ACPM Education.
- Benjamin, M.M. and W.C. Roberts, Facts and principles learned at the 39th annual Williamsburg Conference on Heart Disease. Proc (Bayl Univ Med Cent), 2013. 26(2): p 124-36.
- Roberts, W. C., Preventing and arresting coronary atherosclerosis. Am H Journal, 1995. 130(3): p. 580-600.
- Ornish, D., et al., Can lifestyle changes reverse coronary heart disease? The Lifestyle Heart Trial. Lancet, 19990. 336(8708): p. 129-33.
- Esselstyn, C.B., Jr., Updating a 12-year experience with arrest and reversal therapy for coronary heart disease. Am J Cardiol, 1999. 84(3): p. 339-41, a8.
- Frattaroli, J., et al., Angina pectoris and atherosclerotic risk factors in the multisite cardiac lifestyle intervention program. Am J Cardiol, 2008. 101(7): p. 911-8.
- Trumbo, P.R. and T. Shimakawa, Tolerable upper intake levels for trans fat, saturated fat, and cholesterol. Nutr Rev, 2011. 69(5): p.270-8.
- Eur J Nutr 2014;53(2):395-400
- Stroke. 2013;44:334-39
- Cancer Prev Res. 2011;4(2); 177-84