Response

We appreciate Dr McDougall’s commitment to vegetarian diets, and we believe we have stated clearly in both our original advisory on protein intake¹ and in our response² to his first letter³ that the American Heart Association (AHA) is not opposed to vegetarian diets. A diet that is based on the AHA guidelines of 5 to 6 or more servings of whole grains and 5 or more servings of vegetables and fruits can serve as the foundation for a healthy vegetarian diet that can supply all of the amino acids necessary for well-being. However, because 6 servings of grains and 5 servings of fruits and vegetables would provide roughly only 25 g of protein, additional servings of breads, cereals, and other complex carbohydrates and fruits and vegetables will be necessary to meet protein requirements. Inclusion of legumes, soy products, and nuts will facilitate meeting protein and calorie needs of a vegetarian diet.

We must, however, stress that the evaluation of protein quality is complex, and understanding dietary protein requirements for humans continues to be a focus of research. Much scientific progress has been made and is still ongoing since the work done by Rose in the 1950s.⁴ More recent studies indicate that the evaluation of protein quality must consider the biological value or protein efficacy ratio, which is a measure of protein retention or utilization by the body, the chemical score based on its essential amino acid content, and protein digestibility or availability of the individual amino acids.⁵ An example of research in the field that has recently added to our knowledge of the human lysine requirement and the nutritional value of wheat protein is the demonstration of an adaptive capacity in some individuals that allows wheat protein to be utilized more efficiently than expected.⁶ Numerous other studies have been reported, but it is beyond the scope and purpose of this letter to summarize and document their findings. As a start, we refer the reader to the article by Young and Pellet⁵ that provides 50 references and concludes that a “mixture of plant proteins can serve as a complete and well-balanced source of amino acids for meeting human physiological requirements.” Additionally, an article by Millward⁷ summarizes 59 articles and reviews the “meat or wheat debate” and concludes that plant-based diets (but not one food) can supply protein in the amount and quality needed. Furthermore, there are several micronutrients that may be limited in diets without animal proteins—not just vitamin B₁₂, as Dr McDougall mentions in his letter, but also calcium, zinc, and omega-3 fatty acids.

As all groups involved in nutrition counseling have pointed out, it is possible to achieve adequate nutrient intake on a vegetarian diet, but it requires careful planning to include plant sources that provide adequate amounts of all amino acids and the above listed micronutrients, and some special consideration for vitamin B₁₂. The AHA guidelines are meant to be applicable to all adults in the US population, where average consumption of vegetables/fruits is only about 3 servings per day and whole grain intake is also lower than recommended at 6 servings per day.

We don’t want individuals to be unnecessarily concerned about vegetarian diets, but we do want to be sure that they receive appropriate guidance to assure that nutrient intake meets their needs and that nutrients are derived from whole foods rather than from supplements.