ay be that the lack of carbohydrate in this diet or the quality of the protein used were factors.2
A big factor in the protein diet is the source of the dietary amino acids. In many of the liquid protein diets, the source of amino acids was collagen. Collagen is a tough connective tissue comprising body parts such as tendons and ligaments. Collagen is usually a meat packing industry by product that is only sold as food when it is liquified to gelatin. When collagen has been further reduced to its amino acid constituents for liquid protein diets, the product is deficient in the essential amino acid methionine. Hence, the collagen protein is not a complete protein source.3
What this means is that body protein must be broken down to supply the missing amino acid, methionine. As a result, the protein from this liquid protein diet can build up body tissues. Instead, the body tissues are catabolized or the collagen derived protein is converted to glucose. While it would have seemed a simple matter to add methionine to liquid protein diets, this was not done because of the expense and the fact that methionine produces a strong, unpleasant taste. It is the taste appeal and low price that made the liquid protein diet a popular fad diet among the American people.4
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