A high intake of vegetables is associated with improve health outcomes. The exact reason for this is not known, but may relate to the presence of biologically active phytochemicals in plant tissues. Many of these chemicals are secondary metabolites of plants that are absorbed in humans and interact with out biochemistry to cause physiological changes. The flavonoids are such a group of chemicals, but others include the terpenes and the carotenoids. Some nutritionist advocate eating vegetables raw as this prevents the heat of cooking damaging the phytochemicals within the plant tissues. However, evidence shown that some plant chemicals such as lycopene in tomatoes and lutein in spinach are best absorbed following cooking. This likely relates to the fact that heat breaks down the cells walls of the plants and exposes the phytonutrients to our digestive processes. Cooking onions may also increase the bioavailability of the quercetin they contain.
However, it is important to not overcook vegetables, as this may detrimentally affect the nutrient content. A good example of this is broccoli and other cruciferous vegetables. Broccoli contains chemicals called glucosinolates. Broccoli consumption is associated with a reduced risk of cancer because the chemicals it contains can improve detoxification of carcinogens. However, the glucosinolates do not have this effect in their natural state, they much first be converted to isothiocyanates through the action of an enzyme called myrosinase in the plant tissue. This reaction can only happen if the cell wall and internal organelles of the cells are broken open to bring the glucosinolates into contact with the myrosinase enzyme. The heat of cooking is effective at performing this task, but cooking for too long destroys the enzyme and lowers the production of the cancer preventive isothiocyanates.