Intra- and interindividual variation in measurements of beta-carotene, retinol, and tocopherols in diet and plasma.

Twenty-four men were studied for 4 wk to estimate intra-and interindividual variation of retinol, a-carotene, and tocopherols in diet and plasma. Ratios of intrato interindividual variances ranged from 0.62 for a-carotene in plasma to 6.02 for vitamin E in diet. Results indicate that 3-9 independent measurements may be required to distinguish reliably even large differences (-1 SD) in these micronutrients; 28-86 independent measurements may be needed for small differences (-0.25 SD). If uncontrolled, large ratios of intra-to interindividual variance can constitute an important source of error in studies of micronutrients. Am J Clin Nuir l987;45:764-9.