Dogs with sustained anemia and hypoproteinemia due to bleeding and a continuing low protein or protein-free diet with abundant iron are used to test the value of food proteins as contrasted with mixtures of pure amino acids.

The stimulus of double depletion (anemia and hypoproteinemia) drives the body to use every source of protein and all protein-building materials with the utmost conservation. Raiding of body tissue protein to produce plasma protein and hemoglobin is a factor when protein-building factors are supplied in small amounts.

In this severe test (double depletion) the good food proteins in adequate amounts are able to maintain body weight, a strongly positive nitrogen balance, and produce considerable amounts of new hemoglobin and plasma protein. Casein, lactalbumin, whole egg protein, liver protein are all adequate in amounts of 150 to 250 gm. protein per week.

Under comparable conditions mixtures of pure amino acids (essential for growth) do produce large amounts of new hemoglobin and plasma protein and a positive nitrogen balance but do not maintain body weight. The loss of weight is conspicuous even with large amounts of amino acids (200 to 300 gm. protein equivalent per week).

Methionine, threonine, and phenylalanine are related to nitrogen conservation in growth mixtures of essential amino acids (Paper I) but when these three are given together they have little influence on the doubly depleted dog (Table 3).

Some unidentified substance or compound present in certain proteins but absent in mixtures of the essential amino acids may be responsible for these differences in the response of the doubly depleted dog.

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