Food, supplements, and fortified foods can all be appropriate sources of nutrients. It is the responsibility of nutrition educators to help consumers use those nutrient sources appropriately. Consumers should consider at least six factors when they select foods, supplements, and fortified foods: flavor, convenience, cost, nutrient content, toxic factors, and bioavailability of nutrients. Consumers are often less concerned about the latter two factors than they should be. For example, many case histories have been published on the symptoms caused by overconsumption of vitamins A, B-6, C, and D and nicotinic acid. Generally those toxic symptoms were caused by the ingestion of supplements, not foods. Current widespread use of supplements containing one or two nutrients may not only cause overt toxicity symptoms but also adversely affect the bioavailability of other nutrients. For example, ingestion of excessive amounts of vitamin C appears to interfere with copper metabolism in human subjects; ingestion of high levels of zinc impairs immune function, depresses copper absorption, and induces anemia; consumption of supplemental calcium as calcium phosphate dibasic is associated with renal calcification in rats; and ingestion of supplemental magnesium depresses the apparent absorption of calcium and retention of calcium in bone by rats.