It is assumed that the "highest quality protein" is protein that supports maximal growth of a young animal. Because rats grow quickly and have limited protein stores and a high metabolic rate, deficiencies and imbalances in amino acid patterns in young growing rats can be easily detected in a short period of time. The protein efficiency ratio (PER) is defined as the weight gained (in grams) divided by the amount of test protein consumed (in grams) by a young growing rat over several days. Obviously, duration of diet, age, starting body weight, and species of rat used are important variables. Typically, 21-day-old male rats fed 9 to 10% protein (by weight) for 10 days to 4 weeks are used. In one series of tests, casein produced a PER of 2.8; soy protein, 2.4; and wheat gluten, 0.4. Clearly, a casein diet produced much better growth (2.8 g of carcass weight increase for every gram of casein consumed) than gluten (0.4 g of carcass weight increase for every gram of gluten consumed). Such an approach has been useful in defining the relative efficacies of clinical formulas used in enteral and parenteral nutrition ( 176). The formula that provides the optimal mixture of essential and nonessential amino acids should induce the most rapid growth. Results from this method will be skewed in application to humans, depending upon the extent that human requirements for individual amino acids differ from those of the rat. However, the method has been very useful in comparing a new protein source against reference proteins, such as egg protein, and does evaluate other factors such as relative digestibility.
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Metabolism. There isn’t perhaps a more frequently used word in the weight loss (and weight gain) vocabulary than this. Indeed, it’s not uncommon to overhear people talking about their struggles or triumphs over the holiday bulge or love handles in terms of whether their metabolism is working, or not.