They are healthy, especially when cooked, because heat releases from tomato cells a substance called lycopene. This is a special carotenoid, a powerful antioxidant relevant to health. For instance, it slows down the growth of various cancer cells.
Its action against the development of free radicals from blood fats (lipids) has been observed in a group of Indian patients with coronary heart disease.
Let's be grateful to the tomato doing this good job. And let's not expect too much. While tomatoes make blood fat less dangerous because it is less oxidized, they do not make the profile of blood fats better. That is, they do not lower LDL cholesterol and triglycerides and they do not raise the beneficial HDL cholesterol. But who says that the poor cooked tomato must to the whole job? Eat soluble fiber and your cholesterol level will go down.
When it comes to beneficial effects of fruit and vegetable, single substances like lycopene never can tell the whole story. Real life is richer than simple effects. For instance tomatoes, besides lycopene, contain many more good substances such as vitamins and phenols. - (Picture by theweirdone @ Flickr)