Monitoring the effect of varying the distribution of phenylalanine exchanges and protein substitute on serum phenylalanine—a preliminary study

Following a low phenylalanine diet for life has been advocated for those with phenylketonuria. The Medical Research Council (MRC) Working Party on phenylketonuria (1993a, 1993b) suggests that serum phenylalanine levels between 120 and 700 μ mol/l should be achieved and maintained during adolescence and adult life (other than preconceptionally and during pregnancy). A significant proportion of older children with phenylketonuria struggle to comply with the even distribution of protein substitute and phenylalanine exchanges throughout the day, as advised nationally (Asplin & MacDonald, 1994). This preliminary study sought to compare the control of serum phenylalanine in young people suffering from phenylketonuria who were prescribed the nationally advised regimen (Group A) and youngsters given one of two alternative regimens (Group B and Group C) known to be favoured by some youngsters. These alternative regimens provided all nutritional requirements over the day, but the distribution of protein substitute and phenylalanine exchanges differed in the two groups and was not as even as in Group A. The results show that all three study groups attained serum phenylalanine levels within the range 120–700 μ mol/l at all times blood samples were obtained. Whether or not this makes each of the three regimens acceptable is discussed. Despite careful control of the diet prescription, the amount of phenylalanine consumed can greatly exceed the prescription due to the consumption of «free foods». This does not appear to jeopardize control. The mean variation in serum phenylalanine levels over the 24‐h period was different for each group. Not all individuals within any one group gave the same 24‐h serum phenylalanine profile. This suggested that the present practice of monitoring control in phenylketonuria by one blood test at a consistent time of day is not ideal. It is not possible to predict an individual's 24‐h profile from one blood result even when the dietary prescription at each meal is known. Changes in serum phenylalanine levels occurring during an overnight fast were monitored. Further research is required to ascertain the dietary components that reduce the rate of increase in serum phenylalanine on fasting.