Do cooked vegetables have lower nutritional quality?

Food goes through a chemical transformation when subjected to heat. New flavors arise, some of the fresh flavors disappear and the color and texture change.

A common belief is that when cooked, vegetables, fruit and herbs lose their nutritional value.

The answer to this question will surprise you, but it is not a simple answer.

Plant food is very complex.
Plants build themselves out of water, minerals and air, using the sun as their energy source. Out of these simple building blocks plants synthesize a tremendous amount of different molecules, amongst them are the vitamins, phytochemicals, antioxidants (some of which are vitamins, but most are not) and pigments. Although food science has been exploring plants for a while we still know little about the hundreds of different molecules in every whole plant, how they work in synergy, and how they interact with the human body.

Consider the tomato. It is very tasty fresh. When cooked, a tomato gains more flowery notes, the red color deepens, and it makes a wonderful sauce. But is it as nutritious?

A tomato is rich in lycopene, ß-carotene (a precursor of vitamin A), folate, potassium, and vitamin C, and, I’m quite sure, other things, some of which we are yet to discover. Lycopene is a fat-soluble pigment that gives tomatoes, guava, pink grapefruit, watermelon and rose hip their red color, it has antioxidant activity and may play a role in preventing prostate cancer and cardiovascular disease. Lycopene from tomatoes is better absorbed if the tomato is cooked and if a source of fat is included with the meal, so a tomato sauce that includes some olive oil sounds quite ideal.

The vitamin C in the tomato on the other hand is harmed by heat.

Common sense advice would then say: Eat tomatoes both fresh and cooked.

I recently read a study by Nicoletta Pellegrini and colleagues from Universities of Parma and Naples that looked at the effects of boiling, steaming, and frying on the polyphenol, carotenoid, glucosinolate, and ascorbic acid (vit C) content of carrots, zucchini and broccoli.

The surprising results are an overall increase in antioxidants measured after boiling or steaming these vegetables, compared to the raw vegetables. Frying on the other hand caused a significant loss of antioxidants.

Vitamin C concentration decreased in all cooking methods.

Another study showing similar results was published in the American Journal of Clinical Nutrition and showed that the antioxidant content increased in cooked carrots, spinach, mushrooms, asparagus, broccoli, cabbage, red cabbage, green and red peppers, potatoes, and tomatoes, with steaming being the preferred method.

I spent the next few hours looking at other studies, showing different results. Seems like for every food different cooking methods increase or decrease the activities of its diverse constituents. The different behavior of food in the cooking process is related with the nature and molecular structures of the respective antioxidant compounds. It is suggested that foods with active polyphenolic flavonoids are more resistant than foods with vitamins and related compounds. Spices and herbs are known to retain their antioxidant activity even after very long boiling.

I definitely don’t think we need to be scientific or concerned when deciding how to handle fruits and vegetables. My common sense advice would be: Eat plenty of fresh fruits and vegetables, cook what you enjoy cooked, and don’t worry about it. Frying, which is the least preferred way to cook anyway, because it adds a lot of fat, also seems to be the most damaging to the nutrients in the food. Since vitamins leach into the water in which vegetables are boiled, you might consider saving that water for your soup.

Ayala