Biological variation of ascorbic acid and α-tocopherol

Accumulating evidence that free radicals may contribute to various diseases, has sparked epidemiological and experimental studies of the correlation between plasma levels of antioxidant vitamins and risk to develop cancer, ischaemic heart disease and stroke. These studies often do not take into account the random biological fluctuation of the antioxidant concentration, which occurs in each individual. The weekly and monthly variability of the antioxidants ascorbic acid and α-tocopherol was studied in 12 healthy volunteers (4 women and 8 men) aged 23-45 years. Vitamin levels were determined using high performance liquid chromatography. Over 12 weeks the mean plasma concentration of ascorbic acid was 42± 12 μmol/I and of α-tocopherol was 31±3 μmol/I. The intraindividual coefficients of variation (estimated using analysis of variance techniques) were 26% (ascorbic acid) and 12% (α-tocopherol). The analytical goal for imprecision was achieved for both vitamins, i.e. it was less than one-half of the measured intraindividual variation. Both antioxidants showed marked individuality, indicating that an individual's reference values are more useful than population-based data. The critical difference required for significance of changes in serial results is smaller for α-tocopherol (34%) than that for ascorbic acid (72%).