This study aimed to assess the differences in nutrient composition of two diets, as estimated by a food composition database developed at the University of Crete, Greece, and by chemically analyzing duplicate portions of the diets. For this purpose, one healthy monk was asked to collect all foods consumed during a fasting (as indicated by Greek Orthodox Christian rituals) and a non-fasting week. Weekly samples were homogenized and frozen for chemical analysis. At both fasting and non-fasting weeks, food consumption was recorded using 7-day weighed food records. The food composition database provided a rough estimate of dietary intake, since differences greater than 15% between the two methods were found in both diets for total fat, dietary fibre, dietary cholesterol, plant sterols, iron, sodium and vitamins E and B-12. The discrepancies between the two methods can be minimized by regularly updating the food composition database and by incorporating reliable nutrient values for local foods. This will improve the reliability of studies examining the effect of dietary patterns on health, allow nutrient and dietary pattern comparisons and data exchange between countries and permit the incorporation of the nutrient composition of local products in European food composition networks.