Sunday, April 26, 2009
Not All Proteins Are Created Equal
In the previous post we learned that eating 30 grams of protein per meal can increase fat loss, preserve lean muscle, prevent osteoporosis and improve the symptoms of type 2 diabetes. But protein can come from many different sources, including meat, eggs, dairy products and plants. Does the source of dietary protein make any difference?
In a word--yes. Proteins are linear molecules made of building blocks called amino acids. Proteins are synthesized within cells by an organelle called a ribosome that reads the "recipe" for each particular protein from another linear molecule called RNA. As the ribosome reads the RNA, it looks in its immediate vicinity for whichever of the twenty different amino acids is called for next in the sequence. If the ribosome comes to a point in the RNA "recipe" where the corresponding amino acid cannot be found, the ribosome falls off the RNA and synthesis of the protein stops. Until there is enough of the missing amino acid, that specific protein cannot be made.
Where does the missing amino acid come from? Some amino acids like alanine, glutamate and asparagine, can be made by our bodies. Unless there is an inborn error of metabolism, these amino acids are present in abundance. However, other amino acids like lysine, methionine and tryptophan cannot be made by the human body. These are called essential amino acids and they must be consumed as part of the diet. If any essential amino acid is not consumed in sufficient quantity, its absence shuts down much of the body's protein-synthesis machinery.
Cereal grains such as corn, millet, rice and wheat are typically low in the amino acid lysine. Even though a person consumes many grams of protein in the form of cereal grains, the low abundance of lysine will prevent his or her body from making many of the proteins it needs for growth and repair. Legumes such as beans and peanuts are low in the amino acid methionine. Eating lots of beans or peanuts will provide lots of protein, but when the plant protein is broken down into its amino acids and these are then used for human protein synthesis by ribosomes, the ribosomes will be unable to find enough methionine to produce the proteins the body needs to sustain itself. When a food source is deficient in one or more essential amino acids, it is said to contain incomplete protein. Incomplete protein can be used as a source of calories or energy, but it is inefficent in meeting the body's need for human protein synthesis.
It is possible to mix plant sources of protein such as corn and beans in order to obtain a better overall amino acid profile. However, the complementary sources must be consumed within several hours of each other or the beneficial effect will be lost. The complementarity must also be well-understood. For instance, almonds are low in lysine and methionine, so addition of cereal grains or legumes to almond protein will still result in poor protein nutrition. Another aspect to consider is the fact that plant sources of protein are often more difficult to digest than proteins found in animal sources such as whey, meat or eggs. If the plant products are refined, digestibility is improved, but nutritional quality is lost.
Animal sources of proteins typically have a much better balance of essential amino acids than plants do. When you think about it, that makes sense. Plants do not use their proteins to make blood, muscles or organs. Plant proteins are used for different funtions, and the amino acid profiles of those proteins are unique. On the other hand, animals are similar to people in many ways, and their proteins require about the same percentage of amino acids that are required to make proteins having similar functions in humans. Good sources of animal protein include whey protein, casein (cheese), eggs, meat and fish. An interesting comparison of protein quality can be found here. As the chart in that link indicates, plant protein from soy does provide a complete array of amino acids. However, because consumption of soy products may be associated with alterations in hormone levels, they should be used with caution.
Protein is an important macronutrient. It has many beneficial primary and secondary effects, but if the protein consumed is not of high quality, i.e., if it is not complete protein, the body will not be able to use it effectively to make and repair skin, nails, hair, bone and muscle.