The glycaemic index is a measure of the speed at which glucose can be digested and absorbed to the blood from a carbohydrates food. White bread (or glucose) is used as a standard reference food and the test food is given a rating based on its relative absorption speed to this reference food. The glycaemic index is a useful research tool because it allows an understanding of how particular foods may differ in their ability to affect metabolism, particularly the insulin hormonal system and the liver. However, as well as carbohydrate foods, protein can also be eaten in various forms that differ in their speed of digestion and absorption. Further, certain amino acids are insulinogenic (stimulate the release of insulin) and therefore protein can influence plasma insulin levels. Experiments investigating the digestion rates of particular protein foods have shown differences in their absorption rates. For example, the whey protein from milk is absorbed more quickly and is more insulinogenic than the casein protein from milk.
Processing beef by mincing it has also been shown to alter its digestion and absorption rate, and this in turn has been shown to alter the metabolic effects seen from the ingested beef. For example, in one study1, researchers fed 135 grams of beef to elderly subjects as either beef steak or mince. The beef was radiolabeled in order to allow the researchers to observe its metabolic fate within the subjects. The meat proteins from minced beef appeared more rapidly in the circulation when compared to the meet proteins from the beef steak. The proteins from the minced beef also remained more available in the circulation of the subjects for 6 hours following ingestion of the food. The protein balance was significantly more positive after the minced beef than the beef steak, however, this did not increase skeletal muscle protein synthesis rates. Therefore when minced, the absorption and metabolism kinetics of beef appear to change considerably, and this may affect physiology, although it has no effects on protein synthesis.
The subjects in this study were all elderly, and it is known that chewing efficiency declines with age. Therefore the results of this experiment might not be the same had the experiment been performed on younger individuals. In fact, the researchers measured the chewing time in the subjects and reported significantly longer chewing times when consuming the beef steak compared to the steak mince. The authors used the point of complete consumption of the meat to measure plasma insulin levels, and this means that they were recorded significantly later in the those subjects eating the whole steak. This might be why the insulin levels did not differ between subjects consuming the steak and subjects consuming the mince. Therefore the processing of beef may influence its absorption rates, and this may alter physiological responses after a given amount of protein. Creating a protein index similar to the glycaemic index therefore seems justified.
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