Vitamins are an essential part of the diet and are found naturally within foods such as fruits and vegetables or are added to fortified foods such as bread. Vitamins can sustain damage when exposed to elements such as air, light, water and heat. (See Reference 1)
Vitamins Highly Heat-Sensitive
According to the World Food Programme, vitamin B1, or thiamine, is highly sensitive to heat. Exposure of foods to high heat can reduce their thiamine contact by as much as 50 percent.
Vitamins Somewhat Heat-Sensitive
Other heat-sensitive vitamins, according to the World Food Programme, include vitamins A, D, C, E and pantothenic acid. For example, heat can destroy 15 to 45 percent of a food's vitamin C content.
Vitamins Not Sensitive to Heat
Some vitamins remain stable in heat. These include vitamin K, riboflavin, niacin, vitamins B6 and B12 and folic acid.
To best preserve vitamins, use foods that contain heat-sensitive vitamins fresh, without applying heat. If you do heat them, high heat for a short duration is preferable to low heat for a longer time. Steam blanching retains the most vitamins. (See Reference 2)
The extent of heat damage to a vitamin is the same whether the vitamin occurs naturally or was added during fortification. In certain forms, vitamins from synthetic sources may remain more stable when heated. (See Reference 2)