Absence of a Time Factor in the Relationship between Level of Energy Intake and Protein Metabolism

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Abstract
Experiments are described in which additional carbohydrate (glucose) was given to dogs already receiving adequate diets. This caused a reduction in urinary N output, even when the extra carbohydrate was taken 12 hours apart from the rest of the diet. Similar experiments on human subjects, in which extra carbohydrate was taken either with the diet or 5½ hours after the last meal of the day, also demonstrated a fall in N output. In similar experiments on adult rats it was observed that extra carbohydrate given 5 hours or 12 hours apart from the dietary protein was just as effective as extra carbohydrate taken with the dietary protein. N retentions of similar magnitude were also observed when an isodynamic amount of fat was superimposed on the basal diet. From a consideration of these and previously published experiments it has been concluded that protein utilization is affected in two ways by the other energy-yielding nutrients in the diet. First, protein utilization is favorably influenced by the presence of some carbohydrate in the protein-containing meals. Close proximity in the time of eating carbohydrate and protein is necsssary in order that this interaction may take place, and fat cannot be used in place of carbohydrate. Secondly, carbohydrate and fat act interchangeably as energy sources in sparing protein; in order to exert this sparing action they do not need to be taken along with dietary protein.