Vitamin levels in food are reduced during preparation and processing and over time. Only vitamins D and Biotin seem to be unaffected by storage, processing and cooking.

Vitamins B2, E and K are light-sensitive. For that reason foods contain these vitamins should be stored in dark places to minimise a reduction of these vitamins through storage.

Vitamin C as well as vitamin E work as anti-oxidants, so their levels are reduced when they are exposed to the oxygen in the air. The amount of vitamin decreases the longer food is stored before preparation.

Freezing of the food has little effect on vitamin content. Cooking before freezing, however, can reduce the amount of vitamins. Vitamins, especially vitamins B and C, get lost in cooking water, regardless whether the cooking is done on the stove or in the microwave. Using small amounts of water and shortening the cooking time can reduce vitamin loss.