When it comes to heating food, some people worry about "the loss of vitamins." There are even "health gurus" who make startling claims: vitamins break down above 40 degrees, so you have to eat them raw for them to be effective.
The effect of heating on different vitamins varies, and the temperatures and times for "heating" various foods also differ, so "how many vitamins are lost from cooked food" is not an easy question to answer.
Vitamin C is very water-soluble, and some B vitamins, such as folate, B6, and B12, are also water-soluble, so they can be easily lost when boiled. From the perspective of preserving water-soluble vitamins, steaming is a better method than boiling. These unstable vitamins, along with vitamins E and K, are also lost when exposed to light. Furthermore, acidity and alkalinity also affect their stability. Certain metals, such as iron and cobalt, can cause vitamins E and B12 to lose their activity. Strictly speaking, a vitamin "losing its activity" is not a matter of "life and death," but rather a question of how much is lost.
The U.S. Department of Agriculture's database contains the vitamin content of various common foods. Excluding the influence of changes in water content, the general loss percentages of vitamins after heating are as follows: Vitamins A and E are not greatly affected by temperature, with a loss of about 10% after cooking. Vitamin C is lost by about 16%, and Vitamin B1 is the least stable, with a loss of 26%. The other vitamins are mostly within this range. Overall, cooking food results in a vitamin loss of roughly 10% to 25%.
However, heating also has positive effects on vitamins. Some foods contain what are called "antivitamin substances," which can bind with vitamins, and heating can destroy these substances, thereby increasing the absorption rate of vitamins. Beta-carotene is another example. It is found in many vegetables, such as carrots, spinach, sweet potatoes, broccoli, and more. Inside the human body, it can be converted into vitamin A and is the primary way for vegetarians to obtain this vitamin. However, the absorption rate of beta-carotene from raw vegetables is very low, and heating can significantly increase it. Interestingly, with ultra-high-temperature and prolonged heating, it can convert from its biologically active trans structure to an inactive cis structure, so the loss of beta-carotene in these vegetables after canning is relatively large.
Considering the potential problems of eating raw vegetables, such as pathogenic bacteria and some toxins that need to be destroyed by heating, it's hard to say whether vegetables should be eaten raw or cooked. If the foods you eat are quite varied, the total amount of various vitamins will likely be higher, so losing a bit doesn't matter. After all, what our bodies need is for the various vitamins to reach a certain requirement, not for one or two specific vitamins to be as high as possible.