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Are You Getting Your Protein?

Combining the right foods to get the correct amount and type of protein can be tricky. Mauro Di Pasquale, M.D. gives you tips on what foods you should concentrate on.

    9/25/2003 - Combining the right foods to get the correct amount and type of protein can be tricky. Mauro Di Pasquale, M.D. gives you tips on what foods you should concentrate on.

In general, proteins of animal origin contain adequate amounts of the essential amino acids and hence they are known as first class proteins. On the other hand, many proteins of vegetable origin are relatively deficient in certain amino acids, notably lysine and the sulphur-containing amino acids.

Mixtures of plant proteins can serve as a complete and well-balanced source of amino acids for meeting human physiological requirements. However the combining of right foods is necessary to obtain the necessary levels of both the essential or indispensable and conditionally indispensable amino acids.

The essential amino acid lysine is consistently at a much lower concentration in all major plant-food protein groups than in animal foods. Since lysine is the limiting amino acid, the addition of limited amounts of lysine to cereal diets improves their protein quality. Studies in Peru and Guatemala have demonstrated that growing children benefited by this addition. In addition, the sulphur-containing amino acids are distinctly lower in legumes and fruits and threonine is lower in cereals compared with amounts found in proteins of animal origin.

Complementary Proteins

There are important differences among and between food products of vegetable and animal origin including the concentrations of proteins and indispensable amino acids that they contain. The concentration of protein and the quality of the protein in some foods of vegetable origin may be too low to make them adequate, sole sources of proteins. In some of the poorer parts of the world, diets are based predominantly on a single plant (e.g. corn) and they frequently lead to malnutrition.

Fortunately, the amino acid deficiencies in a protein can usually be improved by combining it with another so that the mixture of the two proteins will often have a higher food value than either one alone. For example, many cereals are low in lysine, but high in methionine and cysteine. On the other hand, soybeans, Lima beans, and kidney beans are high in lysine but low in methionine and cysteine. When eaten together these types of proteins gives a more favorable amino acid profile.

Another example would be the combination of soybean, which is low in sulphur-containing amino acids, with cottonseed, peanut and sesame flour, and cereal grains, which are deficient mainly in lysine. In general oil-seed proteins, in particular, soy protein, can be used effectively in combination with most cereal grains to improve the overall quality of the total protein intake. A combination of soy protein, which is high in lysine, with a cereal that contains a relatively good concentration of sulphur containing amino acids results in a nutritional complementation; the protein quality of the mixture is greater than that for either protein source alone.

Some examples of complementary food proteins include:

  • Beans and corn (as in tortillas)
  • Rice and black-eyed peas
  • Whole wheat or bulgar
  • Soybeans
  • Sesame seeds
  • Soybeans
  • Peanuts
  • Brown rice
  • Bulgar wheat

This kind of supplementation works when the deficient and complementary proteins are ingested together or within a few hours of each other. Various nutritional responses are observed when two dietary proteins are combined. These have been classified by Bressani et al into one of four types.

Type I is an example where no protein complementary effect is achieved. For example, this occurs with combinations of peanut and corn, where each of the protein sources have a common and quantitatively similar lysine deficiency and are both also deficient in other amino acids.

Type II response is observed when combinations are made of two protein sources that have the same limiting amino acid, but in quantitatively different amounts. Corn and cottonseed flour, for example, are both limiting in lysine but cottonseed is relatively less inadequate than is corn.

The third type of response (Type III) demonstrates a true complementary effect because there is a synergistic effect on the overall nutritive value of the protein mixture; the protein quality of the best mix exceeds that of each component alone. This type of response occurs when one of the protein sources has a considerably higher concentration of the most limiting amino acid in the other protein. An example of this response, based on studies in children, is observed when corn and soy flour are mixed so that 60% of the protein intake comes from corn and the remainder from soya protein.

Finally, the Type IV response occurs when both protein sources have a common amino acid deficiency. The protein component giving the highest value is the one containing a higher concentration of the deficient amino acid. Combinations of some textured soy proteins and beef protein follow this type of response.

These nutritional relationships have been determined from rat bioassay studies. However, the more limited results available from human studies with soy and other legumes confirm the applicability of this general concept in human nutrition. This knowledge helps us to understand and evaluate how nutritionally effective combinations of plant protein foods can be achieved.

Even when combinations of plant protein foods are used there is still the concern of timing of ingestion of complementary proteins. Is there a need to ingest different plant proteins at the same time, or within the same meal, to achieve maximum benefit and nutritional value from proteins with different, but complementary, amino acid patterns? This concern may also extend to the question of the need to ingest a significant amount of protein at each meal, or whether it is sufficient to consume protein in variable amounts at different meals and even different days as long as the average daily intake meets or exceeds the recommended or safe protein intakes.

According to FAO/WHO/UNU, estimates of protein requirements refer to metabolic needs that persist over moderate periods of time. However, the body does not store much protein outside of a meager free amino acid pool, and begins certain catabolic processes in the postabsorptive phase making the ingestion of regular amounts of protein critical for maximizing the anabolic effects of exercise.

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There is a limited database that we can consult to make a definitive conclusion on the timing of consumption of complementary proteins or of specific amino acid supplements for proteins that are deficient in one or more amino acids. Earlier work in rapidly growing rats suggested that delaying the supplementation of a protein with its limiting amino acid reduces the value of the supplement. Similarly, the frequency of feeding of diets supplemented with lysine in growing pigs affects the overall efficiency of utilization of dietary protein. Studies in human adults showed that overall dietary protein utilization was similar whether the daily protein intake was distributed among two or three meals.

In general, especially under conditions where intakes of total protein are high, it may not be necessary to consume complementary proteins at the same time. Separation of the proteins among meals over the course of a day would still permit the nutritional benefits of complementation.

However in athletes trying to maximize protein synthesis and muscular hypertrophy it is necessary to have a full complement of amino acids present for every meal in order to maximize the anabolic effects of exercise.