|Protein-Energy Interactions (IDECG, 1991, 437 p.)|
|The metabolic basis of amino acid requirements|
Perhaps the most significant problem we have to deal with in our attempts to rationalise IAA needs is posed by the evidence relating to urea salvage in the lower gut. Urea hydrolysis in the gut has long been known to occur, but the work of Jackson and colleagues (JACKSON et al., 1990; LANGRAN et al., 1991) suggests that not only is this regulated, but it may have profound importance for IAA provision for the organism. The key question is the extent to which the nitrogen salvaged from urea is incorporated into amino acids synthesised de novo by microflora and then recycled into the body amino acid pool.
In fact, our own studies of leucine and phenylalanine metabolism (MILLWARD et al., l991b) suggest the possibility of de novo synthesis of these two amino acids. We have shown that, in normal adults fed low-protein diets, the rate of leucine oxidation and phenylalanine hydroxylation, measured with 13C and 2H, is markedly in excess of what would be expected from the rate of nitrogen excretion. While methodological considerations cannot be ruled out, it is difficult to identify any explanation other than the de novo synthesis of IAAs from urea by bacteria in the lower gut.
The implications of urea salvage and de novo synthesis of IAAs are profound. The extent implied by our results with leucine on the low-protein diet is a daily recycling of de novo synthesised amino acids equivalent to 25% of the dietary intake, and a higher fraction (at least 43%) for phenylalanine (MILLWARD et al., 1991b). This makes the concept of protein quality meaningless, with the potential for qualitative modification of the amino acid balance by the lower gut, and allows for considerable adaptation. Thus, minimum obligatory IAA requirements can only be judged to be an open question.