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, beta-carotene, and tocopherols in diet and plasma. Ratios of intra- to interindividual variances ranged from 0.62 for beta-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 (approximately 1 SD) in these micronutrients; 28-86 independent measurements may be needed for small differences (approximately 0.25 SD). If uncontrolled, large ratios of intra- to interindividual variance can constitute an important source of error in studies of micronutrients.