Variability in man of the levels of some indices of nutritional status over a 60-d period on a constant diet

Abstract

The biological variability of the values of various biochemical indices used for the assessment of the nutritional status of man was investigated in 4 healthy male volunteers (mean age 23 years). They were given a constant diet for 60 d, with the nutrient intake at the level of the Dutch recommendations. Blood was collected twice a week and 24-h urine samples on all 60 d; furthermore, on one day blood was collected every 2 h. Two months after the 60-d constant diet period the intra-individual variability of the various indices was studied once more, this time under habitual dietary practices. The intra-individual variability of urinary excretion of the vitamins B1 and B2 turned out to be rather high (coefficient of variation (CV) ranging from 11 to 25 per cent). Despite a constant sodium intake throughout the study, the variability of urinary Na excretion was also high (mean CV 16 per cent); the level of CV did not improve when the excretion was expressed on the basis of creatinine. Levels of serum HDL- and total cholesterol were fairly constant, both over the 60-d period and during the day (CV 2.5-7 per cent). The CV within subjects for vitamin B1, B2 and B6 concentrations in blood ranged from 6 to 12 per cent, the variations of the corresponding erythrocyte enzyme activities were smaller. It is remarkable that the CV within subjects of most variables studied was not higher during the period on the habitual diet. We conclude that casual blood sampling is insufficient for an evaluation of the vitamin B status of an individual. The high intra-individual variability of 24-h vitamin B1 and B2 urinary excretions indicates that in the assessment of the vitamin status or intake of an individual the importance of these excretion figures is limited. The 'a-values' of the B vitamins may predict vitamin status better than the concentrations of the vitamins in blood or the activities of the corresponding enzymes. The variability of the parameters of vitamin B status over the day (CV 3-11 per cent) suggests that the values of these parameters depend on the time of the day at which samples are taken.