Body proteins are being synthesized and degraded continuously (1). The estimated turnover is ∼210 g/day (2). Amino acids resulting from protein degradation can be recycled (reused for synthesis), but this is incomplete. Therefore, dietary protein is necessary for maintenance of lean body mass. Also, dietary protein is required to replace protein lost from the shedding of skin, hair, nails, cells in the gastrointestinal tract, and protein-containing secretions. However, the actual losses are estimated to be only 6–8 g/day (3).
Overall, approximately ∼32–46 g of high-quality dietary protein/day is reported to be required to maintain protein balance (2). This is considerably less than amounts of protein reportedly consumed by American adults (∼65–100+ g/day) (4). The excess food-derived amino acids then are oxidized as fuel directly or indirectly after conversion to glucose.
In 1915, using a phlorhizinized dog preparation, Janney (5) demonstrated clearly that the deaminated amino acids (carbon skeletons) present in dietary proteins could be used to produce glucose endogenously. For most common proteins, 50–80 g of glucose can be derived from 100 g of ingested protein. Nevertheless, as early as 1913, Jacobson (6) reported that ingestion of proteins did not raise the blood glucose.
Later, in 1924, MacLean (7) fed 50 g of meat protein to two subjects, one with and one without mild diabetes. The …