Protein Combining – Is it necessary?

The myth that plant proteins must be combined at every meal to be of any use to the body was popularized in the early 70’s by the book Diet for a Small Planet by Frances Moore Lappé. The author has since been retracting the statement frequently. “In combating the myth that meat is the only way to get high-quality protein, I reinforced another myth,” she said. Unfortunately, the protein combining myth has taken root in the public and even among a few doctors.

The concern that vegetarians, especially vegans and vegan athletes, may not consume an adequate amount and quality of protein is unsubstantiated. Vegetarian diets that include a variety of plant products provide the same protein quality as diets that include meat.[i]  Protein consumed from a variety of plant foods also supplies an adequate quantity of essential amino acids when caloric intake is met.[ii]

Combining two or more incomplete protein foods (those low in one or more essential amino acids, such as rice and beans, peanut butter and whole grain bread, tortillas with beans, and cooked beans with cornbread) is not required in every meal as long as variety is present over a few days.[iii] The reason for this lies in the pool of indispensible amino acids (IAAs) maintained by the body,[iv] [v] that can be used to complement dietary proteins.

The pool of amino acids come from four sources:[vi] [vii] [viii]

  1. Enzymes secreted into the intestine to digest proteins.
  2. Intestinal cells sloughed off in the intestine.
  3. Intracellular spaces of the skeletal musculature
  4. Synthesis of amino acids by intestinal microflora.

Thus patients can consume beans at dinner and have a grain based breakfast the next morning, and still have adequate protein.

References

[i] Position of the Academy of Nutrition and Dietetics: Vegetarian Diets. Journal of the Academy of Nutrition and Dietetics May 2015 Volume 115 Number 5, 801-810

[ii] Position of the Academy of Nutrition and Dietetics: Vegetarian Diets. Journal of the Academy of Nutrition and Dietetics May 2015 Volume 115 Number 5, 801-810

[iii] Position of the Academy of Nutrition and Dietetics: Vegetarian Diets. Journal of the Academy of Nutrition and Dietetics May 2015 Volume 115 Number 5, 801-810

[iv] Nasset ES. Amino Acid Homeostasis in the Gut Lumen and its Nutritional Significance. World Rev Nutr Diet 1972 ; 14:134-153

[v] Nasset ES. Role of the digestive tract in the utilization of protein and amino acids  JAMA 1957; 164:172-177

[vi] Fuller MF, Reeds PJ. Nitrogen cycling in the gut. Ann Rev Nutr 1998; 18:385-411

[vii] Badloo et al. Dietary protein, growth and urea kinetics. J Nutr 1999;129:969-979

[viii] Millward et al The nutritional value of plant based diets in relation to amino acid and protein requirement. Proc Nutr Soc 1999;58:249—260