Vegetarians have long encountered the cultural bias that diets without animal flesh necessarily lack protein and thus its building blocks, amino acids like methionine and lysine. Until the 1990s, when they began documenting vegetarianism's health benefits, nutrition researchers too generally assumed that "a population following a vegetarian diet is at higher risk for developing nutrient deficiency diseases than a population following a meat-based diet," as Joan Sabate observed in 2003 in the "American Journal of Clinical Nutrition." Well-balanced vegetarian diets in fact supply all necessary nutrients, including methionine and lysine.
According to the United Nations Food and Agriculture Organization, the body needs protein "to build and maintain muscle, blood, skin and bones and other tissues and organs" and sometimes to obtain energy. The human body makes an astounding variety of proteins by combining 20 different amino acids, most of which it can synthesize itself. However, the nine essential amino acids, including methionine and lysine, must be present in the diet. Lysine helps the body absorb calcium; form collagen, an essential component of the skeletal and connective tissue system; and synthesize carnitine, which helps it metabolize fatty acids and regulate blood lipids. Methionine is critical to blood lipid transport. It supplies sulfur atoms and methyl groups -- configurations of one carbon atom and three hydrogens each -- essential to numerous biochemical reactions in the body.
Requirements and Deficiencies
According to the World Health Organization, adults generally need a daily protein intake of at least 0.83 g per kg of body weight. This should include at least 10 mg/kg methionine and 30 mg/kg lysine. Severe protein deficiency causes disorders like kwashiorkor, a syndrome that leads to muscle wasting, failure to grow and bulging bellies in children. According to the University of Maryland Medical Center, lysine deficiency can result in "fatigue, nausea, dizziness, loss of appetite, agitation, bloodshot eyes, slow growth, anemia and reproductive disorders." Methionine deficiency is implicated in steatohepatitis, or inflammation and scarring caused when part of the liver turns fatty. However, well-planned and varied vegetarian diets prevent protein and amino acid deficiencies, including methionine and lysine deficiencies.
Vegetarian and Vegan Sources
Ample sources of lysine and methionine are readily available to all vegetarians, people who refrain from animal flesh, and even to vegans, vegetarians who refrain from all foods of animal origin, including dairy and eggs. Eggs and dairy products are generally rich in both of these amino acids, as are legume foods like tofu and tempeh. Pistachios, amaranth and quinoa have good lysine content. Good sources of methionine include all nuts; vegetables like potatoes, broccoli and spinach; and fruits like oranges and avocados.
As biologist J. Stein Carter explains, protein combining is the practice of “combining plant protein sources” -- and sometimes eggs or dairy -- “to achieve a better amino acid balance than either would have alone.” Examples of protein-combined meals include cow milk and oatmeal or refried beans and tortillas. Despite widespread confusion among vegetarians, amino acid balance need not be achieved at each meal. A varied diet throughout the day will do the job much more easily.
- “American Journal of Clinical Nutrition”: Plant proteins in relation to human protein and amino acid nutrition; V. Young and P. Pellett; May 1994
- Food and Agriculture Organization of the United Nations: Get the Best from Your Food
- University of California-Clermont College: Complementary Protein and Diet; J. Stein Carter; 2004
- VeganHealth.org: Protein; December 2010
- Vegetarian Resource Group: Protein in the Vegan Diet; Reed Mangels
- World Health Organization: Protein and Amino Acid Requirements in Human Nutrition; 2007