Protein

Protein is found throughout the body—in muscle, bone, skin, hair, and virtually every other body part or tissue. It makes up the enzymes that power many chemical reactions and the hemoglobin that carries oxygen in your blood. At least 10,000 different proteins make you what you are and keep you that way.

The Institute of Medicine recommends that adults get a minimum of 0.8 grams of protein for every kilogram of body weight per day (or 8 grams of protein for every 20 pounds of body weight). (1) The Institute of Medicine also sets a wide range for acceptable protein intake—anywhere from 10 to 35 percent of calories each day. Beyond that, there’s relatively little solid information on the ideal amount of protein in the diet or the healthiest target for calories contributed by protein.

• In the United States, the recommended daily allowance of protein is 46 grams per day for women over 19 years of age, and 56 grams per day for men over 19 years of age. (2)

Around the world, millions of people don’t get enough protein. Protein malnutrition leads to the condition known as kwashiorkor. Lack of protein can cause growth failure, loss of muscle mass, decreased immunity, weakening of the heart and respiratory system, and death.

All Protein Isn’t Alike

Protein is built from building blocks called amino acids. Our bodies make amino acids in two different ways: Either from scratch, or by modifying others. A few amino acids (known as the essential amino acids) must come from food.

• Animal sources of protein tend to deliver all the amino acids we need.
• Other protein sources, such as fruits, vegetables, grains, nuts and seeds, lack one or more essential amino acids.

Vegetarians need to be aware of this. People who don’t eat meat, fish, poultry, eggs, or dairy products need to eat a variety of protein-containing foods each day in order to get all the amino acids needed to make new protein.

The Protein Package

• Some high-protein foods are healthier than others because of what comes along with the protein: healthy fats or harmful ones, beneficial fiber or hidden salt. It’s this protein package that’s likely to make a difference for health. For example, a 6-ounce broiled porterhouse steak is a great source of protein—about 40 grams worth. But it also delivers about 12 grams of saturated fat. (3) For someone who eats a 2,000 calorie per day diet, that’s more than 60 percent of the recommended daily intake for saturated fat.
• A 6-ounce ham steak has only about 2.5 grams of saturated fat, but it’s loaded with sodium—2,000 milligrams worth, or about 500 milligrams more than the daily sodium

6-ounces of wild salmon has about 34 grams of protein and is naturally low in sodium, and contains only 1.7 grams of saturated fat. (3) Salmon and other fatty fish are also excellent sources of omega-3 fats, a type of fat that’s especially good for the heart. Alternatively, a cup of cooked lentils provides about 18 grams of protein and 15 grams of fiber, and it has virtually no saturated fat or sodium. (3)

Protein and Chronic Diseases

Proteins in food and the environment are responsible for food allergies, which are overreactions of the immune system. Beyond that, relatively little evidence has been gathered regarding the effect of the amount of dietary protein on the development of chronic diseases in healthy people.

• However, there’s growing evidence that high-protein food choices do play a role in health—and that eating healthy protein sources like fish, chicken, beans, or nuts in place of red meat (including processed red meat) can lower the risk of several diseases and premature death. (2, 4-8, 25)

Cardiovascular disease

Research conducted at Harvard School of Public Health has found that eating even small amounts of red meat, especially processed red meat, on a regular basis is linked to an increased risk of heart disease and stroke, and the risk of dying from cardiovascular disease or any other cause. (4, 6, 8) Conversely, replacing red and processed red meat with healthy protein sources such as poultry, fish, or beans seems to reduce these risks.

• One investigation followed 120,000 men and women in the Nurses’ Health Study and Health Professionals Follow-Up Study for more than two decades. (6) For every additional 3-ounce serving of unprocessed red meat the study participants consumed each day, their risk of dying from cardiovascular disease increased by 13 percent.
• Processed red meat was even more strongly linked to dying from cardiovascular disease—and in smaller amounts: Every additional 1.5 ounce serving of processed red meat consumed each day—equivalent to one hot dog or two strips of bacon—was linked to a 20 percent increase in the risk of cardiovascular disease death.
• Cutting back on red meat could save lives: the researchers estimated that if all the men and women in the study had reduced their total red and processed red meat intake to less than half a serving a day, one in ten cardiovascular disease deaths would have been prevented.

In terms of the amount of protein consumed, there’s evidence that eating a high-protein diet may be beneficial for the heart, as long as the protein comes from a healthy source.

• A 20-year prospective study of over 80,000 women found that those who ate low-carbohydrate diets that were high in vegetable sources of fat and protein had a 30 percent lower risk of heart disease compared with women who ate high-carbohydrate, low-fat diets. Diets were given low-carbohydrate scores based on their intake of fat, protein, and carbohydrates. (9) However, eating a low-carbohydrate diet high in animal fat or protein did not offer such protection.
• Further evidence of the heart benefits of eating healthy protein in place of carbohydrate comes from a randomized trial known as the Optimal Macronutrient Intake Trial for Heart Health (Omni Heart). A healthy diet that replaced some carbohydrate with healthy protein (or healthy fat) did a better job of lowering blood pressure and harmful low-density lipoprotein (LDL) cholesterol than a similarly healthy, higher carbohydrate diet. (10)
• Similarly, the “EcoAtkins” weight loss trial compared a low-fat, high -carbohydrate, vegetarian diet to a low-carbohydrate vegan diet that was high in vegetable protein and fat. Though weight loss was similar on the two diets, study participants on the high protein diet saw improvements in blood lipids and blood pressure. (11)
• A more recent study generated headlines because it had the opposite result. In that study, Swedish women who ate low-carbohydrate, high-protein diets had higher rates of cardiovascular disease and death than those who ate lower-protein, higher-carbohydrate diets. (12) But the study, which assessed the women’s diets only once and then followed them for 15 years, did not look at what types of carbohydrates or what sources of protein these women ate. That was important because most of the women’s protein came from animal sources.

Diabetes

Again, protein quality matters more than protein quantity when it comes to diabetes risk (23)

• A recent study found that people who ate diets high in red meat, especially processed red meat, had a higher risk of type 2 diabetes than those who rarely ate red or processed meat. (7) For each additional serving a day of red meat or processed red meat that study participants ate, their risk of diabetes rose 12 and 32 percent, respectively.
• Substituting one serving of nuts, low-fat dairy products, or whole grains for a serving of red meat each day lowered the risk of developing type 2 diabetes by an estimated 16 to 35 percent.
• Another study also shows that red meat consumption may increase risk of type 2 diabetes. Researchers found that people who started eating more red meat than usual were found to have a 50% increased risk of developing type 2 diabetes during the next four years, and researchers also found that those who reduced red meat consumption lowered their type 2 diabetes risk by 14% over a 10-year follow-up period.
• More evidence that protein quality matters comes from a 20-year study that looked at the relationship between low-carbohydrate diets and type 2 diabetes in women. Low-carbohydrate diets that were high in vegetable sources of fat and protein modestly reduced the risk of type 2 diabetes. (13) But low-carbohydrate diets that were high in animal sources of protein or fat did not show this benefit.

For type 1 diabetes (formerly called juvenile or insulin-dependent diabetes), proteins found in cow’s milk have been implicated in the development of the disease in babies with a predisposition to the disease, but research remains inconclusive. (14, 15)

Cancer

When it comes to cancer, protein quality again seems to matter more than quantity. Research on the association between protein and cancer is ongoing, but some data shows that eating a lot of red meat and processed meat is linked to an increased risk of colon cancer. (2)

• In the Nurse’s Health Study and the Health Professionals Follow-Up Study, every additional serving per day of red meat or processed red meat was associated with a 10 and 16 percent higher risk of cancer death, respectively. (6)
• A 2014 study showed that higher consumption of red meat during adolescence was associated with premenopausal breast cancer, suggesting that  choosing other protein sources in adolescence may decrease premenopausal breast cancer risk. (22)
• People should aim to reduce overall consumption of red meat and processed meat, but when you do opt to have it, go easy on the grill. High-temperature grilling creates potentially cancer-causing compounds in meat, including polycyclic aromatic hydrocarbons and heterocyclic amines. You don’t have to stop grilling, but try these tips for healthy grilling from the American Institute of Cancer Research: Marinate meat before grilling it, partially pre-cook meat in the oven or microwave to reduce time on the grill, and grill over a low flame.

In October 2015, the World Health Organization (WHO)’s International Agency for Research on Cancer (IARC) announced that consumption of processed meat is “carcinogenic to humans,” and that consumption of red meat is “probably carcinogenic to humans.” (24)

• The IARC Working Group, comprised of 22 scientists from ten countries, evaluated over 800 studies.
• Conclusions were primarily based on the evidence for colorectal cancer. Data also showed positive associations between processed meat consumption and stomach cancer, and between red meat consumption and pancreatic and prostate cancer. (24)

Osteoporosis

Digesting protein releases acids into the bloodstream, which the body usually neutralizes with calcium and other buffering agents. Eating lots of protein, then, requires a lot of calcium – and some of this may be pulled from bone.

• Following a high-protein diet for a long period of time could weaken bone. In the Nurses’ Health Study, for example, women who ate more than 95 grams of protein a day were 20 percent more likely to have broken a wrist over a 12-year period when compared with those who ate an average amount of protein (less than 68 grams a day). (16) This area of research is still controversial, however, and the findings have not been consistent. Some studies suggest that increasing protein increases risk of fractures; others have linked high-protein diets with increased bone-mineral density, and thus stronger bones. (17-19)

Protein and Weight Control

The same high-protein foods that are good choices for disease prevention may also help with weight control. Researchers at Harvard School of Public Health followed the diet and lifestyle habits of 120,000 men and women for up to 20 years, looking at how small changes contributed to weight gain over time. (20)

• Those who ate more red and processed meat over the course of the study gained more weight, about one extra pound every four years, while those who ate more nuts over the course of the study gained less weight, about a half pound less every four years.
• One study showed that eating approximately one daily serving of beans, chickpeas, lentils or peas can increase fullness, which may lead to better weight management and weight loss. (21)

There’s no need to go overboard on protein. Though some studies show benefits of high-protein, low-carbohydrate diets in the short term, avoiding fruits and whole grains means missing out on healthful fiber, vitamins, minerals, and other phytonutrients.

References

1. Institute of Medicine, Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). 2005, National Academies Press: Washington, DC.
2. Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. 2007,World Cancer Research Fund, American Institute for Cancer Research.: Washington, DC.
3. Agriculture, U.D.o., USDA Nutrient Database for Standard Reference, Release 14. 2005.
4. Bernstein, A.M., et al., Major dietary protein sources and risk of coronary heart disease in women. Circulation, 2010. 122(9): p. 876-83.
5. Aune, D., G. Ursin, and M.B. Veierod, Meat consumption and the risk of type 2 diabetes: a systematic review and meta-analysis of cohort studies. Diabetologia, 2009. 52(11): p. 2277-87.
6. Pan, A., et al., Red meat consumption and mortality: results from 2 prospective cohort studies. Arch Intern Med, 2012. 172(7): p. 555-63.
7. 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.
8. Bernstein, A.M., et al., Dietary protein sources and the risk of stroke in men and women. Stroke, 2012. 43(3): p. 637-44.
9. Halton, T.L., et al., Low-carbohydrate-diet score and the risk of coronary heart disease in women. N Engl J Med, 2006. 355(19): p. 1991-2002.
10. Appel, L.J., et al., Effects of protein, monounsaturated fat, and carbohydrate intake on blood pressure and serum lipids: results of the OmniHeart randomized trial. JAMA, 2005. 294(19): p. 2455-64.
11. Jenkins, D.J., et al., The effect of a plant-based low-carbohydrate (“Eco-Atkins”) diet on body weight and blood lipid concentrations in hyperlipidemic subjects. Arch Intern Med, 2009. 169(11): p. 1046-54.
12. Lagiou, P., et al., Low carbohydrate-high protein diet and incidence of cardiovascular diseases in Swedish women: prospective cohort study. BMJ, 2012. 344: p. e4026.
13. Halton, T.L., et al., Low-carbohydrate-diet score and risk of type 2 diabetes in women. Am J Clin Nutr, 2008. 87(2): p. 339-46.
14. Akerblom, H.K., et al., Environmental factors in the etiology of type 1 diabetes. Am J Med Genet, 2002. 115(1): p. 18-29.
15. Vaarala, O., et al., Removal of Bovine Insulin From Cow’s Milk Formula and Early Initiation of Beta-Cell Autoimmunity in the FINDIA Pilot Study. Arch Pediatr Adolesc Med, 2012.
16. Feskanich, D., et al., Protein consumption and bone fractures in women. Am J Epidemiol,1996. 143(5): p. 472-9.
17. Darling, A.L., et al., Dietary protein and bone health: a systematic review and meta-analysis.Am J Clin Nutr, 2009. 90(6): p. 1674-92.
18. Kerstetter, J.E., A.M. Kenny, and K.L. Insogna, Dietary protein and skeletal health: a review of recent human research. Curr Opin Lipidol, 2011. 22(1): p. 16-20.
19. Bonjour, J.P., Protein intake and bone health.Int J Vitam Nutr Res, 2011. 81(2-3): p. 134-42.
20. Mozaffarian, D., et al., Changes in diet and lifestyle and long-term weight gain in women and men. N Engl J Med, 2011. 364(25): p. 2392-404.
21. Li SS, Kendall CW, de Souza RJ, Jayalath VH, Cozma AI, Ha V, Mirrahimi A, Chiavaroli L, Augustin LS, Blanco Mejia S, Leiter LA, Beyene J, Jenkins DJ, Sievenpiper JL. Dietary pulses, satiety and food intake: a systematic review and meta-analysis of acute feeding trials. Obesity, 2014. Aug;22(8):1773-80.
22. Farvid MS, Cho E, Chen WY, Eliassen AH, Willett WC. Adolescent meat intake and breast cancer risk. Int J Cancer, 2014.
23. Ley, S. H., Sun, Q., Willett, W. C., Eliassen, A. H., Wu, K., Pan, A., … & Hu, F. B. (2014). Associations between red meat intake and biomarkers of inflammation and glucose metabolism in women. Am J Clin Nutr, February 2014.
24. Bouvard V, Loomis D, Guyton KZ, Grosse Y, Ghissassi FE, Benbrahim-Tallaa L, Guha N1, Mattock H, Straif K; International Agency for Research on Cancer Monograph Working Group (2015). Carcinogenicity of consumption of red and processed meat. Lancet Oncoldoi: 10.1016/S1470-2045(15)00444-1. [Epub ahead of print]
25. Song, M., Fung, T.T., Hu, F.B., et al. Association of Animal and Plant Protein Intake With All-Cause and Cause-Specific Mortality. JAMA Intern Med. Published online August 01, 2016.