Functions of protein:
- • essential component for many vital cell constituents
- • carriers in blood for vitamins and other compounds
- • hormones, cell membrane receptors, antibodies
- • enzymes
- • fluid balance
- • supporting structures
- • bull glucose synthesis (gluconeogenesis); this is a survival mechanism and will occur when carbohydrate intake is not sufficient to maintain blood glucose
- • supply energy; this typically only occurs at the end of endurance exercise when blood glucose is low.
Protein has a wide variety of important functions. Although it does supply energy at times, protein does not typically do so, contrary to conventional wisdom. Protein, in contrast to carbs and lipids, contains nitrogen.
Proteins are composed of twenty amino acids. Ten of these amino acids are essential, which means they cannot be synthesized at an adequate rate and need to be re-obtained through one’s diet. Eight of these, in turn, are essential all the time. The remaining two, arginine and histidine, are known as being “conditionally essential” because we require them only in times of rapid growth. On the other hand, we can produce nonessential amino acids whenever we need them.
The content of essential amino acids determines whether we categorize foods rich in protein as being either “high” or “low” quality.
A high quality protein is also known as a complete protein because it contains all of the essential amino acids. Meat and dairy provide the majority of high quality proteins. Eggs are the highest quality proteins. Soybeans and quinoa are both plant and complete proteins.
Lower quality proteins, in contrast, are called incomplete proteins because they lack one or more of the essential amino acids. Plant protein and lower quality protein are one and the same, comprising all starch (such as grains, potatoes, legumes) and vegetables. With low quality proteins, we may receive all of the essential amino acids by combining two different types of food products. This is the only way to accomplish protein synthesis, for without all of the essential amino acids the synthesis will not take place. For this reason, there were those who worried about the need to combine sources of plant protein in order to verify that amino acids were being consumed at appropriate levels. Nonetheless, as the number of vegans and vegetarians has increased without any evidence of mass protein deficiency appearing, this anxiety has since been allayed.
Extra dietary proteins are not stored as protein, but as fat. Whenever we consume more protein than we need, those amino acids are catabolized and either become components for energy or are converted into triglycerides and then stored as fat. Consuming a large amount of protein, therefore, will contribute to excess body fat.
Dietary requirements for protein assume that one’s energy needs are met. If these needs are not met, then amino acids are used for energy. In addition, these requirements also assume that one has consumed a sufficient amount of carbohydrates. If not, then amino acids will be relied upon to make glucose through gluconeogenesis. Although protein requirements are based mainly on the content of lean tissue in the person, body weight is used more conventionally. If someone has a higher than average amount of body fat, then the “ideal” body weight for that person should be used. For a healthy adult, the protein required is about 0.8 g/kg.
Although the protein requirements for athletes may be slightly higher than for non-athletes, the literature on this subject is far from clear. Since the percentage of lean tissue in athletes is higher than in non-athletes, a larger requirement is possible. Yet many of the athletes who participated in these studies were college-aged males. Their protein intake may have been higher as a consequence of not being fully formed. Meanwhile, different food sources can yield different results.
In the event that there is a requirement for extra protein, it should be consumed shortly after an intensive workout, forty-five minutes being the typical window. During this period, muscle enzymes take amino acids from the diet up into the muscle tissue, thereby replenishing them. For an athlete, the typical requirement of proteins is thought to be about 1.2 to 1.6 g/kg.
When energy levels are adequate, but protein intake is low, especially when it is abruptly lowered, say, during a famine, then one can develops kwashiorkor. Kwashiorkor results in stunted growth, skin lesions, and decreased plasma albumin. In young children, it typically presents itself as a swollen belly on an otherwise emaciated body. A lack of protein, which prevents fluid from being balanced, causes the swelling.
Marasmus is a state in which overall energy is low for extended periods of time. During Marasmus, the reduction in energy is greater than the reduction in protein. It takes the more general form of starvation and was seen most hauntingly in patients with certain types of cancer as well as AIDS.
Although common in the US, taking excess amounts of protein is not without consequence. Excess protein increases fluid needs–itself determined by the dietary content of protein, sodium, and fiber. Consuming fluid insufficiently, on top of taking in excessive amounts of protein, increases kidney stone risk. As said before, the human body does not store additional proteins and amino acids as protein. Additional amino acids will be converted to fatty acids, then triglycerides, and finally stored as fat. Excessive catabolism of amino acids leads to more calcium to be lost through the urine, increasing the risk of both kidney stones and osteoporosis. Vegan diets contain sufficient protein. Therefore, excess protein intake would begin with adding animal products.