In recent years, there has been a reduction in caloric intake in populations with decreased energy demands. This has place a greater emphasis on the bioavailability of nutrients in foods because the total intake of nutrients is generally closely linked with total caloric intake. An assessment of the adequacy of dietary intakes of nutrients requires not only knowledge of the nutrient content of the foods ingested but also the extent to which the nutrient present in the diet is available for absorption and utilization. Nutrients ingested but not released during the digestive process for absorption are of no nutritional value. Bioavailability may be considered the relative absorption of a nutrient from the diet. An index of bioavailability may be extended to include the relative accumulation of a nutrient into various tissues. Various nutrients and dietary components interfere with the bioavailability of vitamins. Hence, requirements for vitamins cannot be considered independently, but must be evaluated in relationship to other nutrients and compounds consumed by an individual. An overview has been presented as to the factors that influence the bioavailability of vitamins in the human food supply.