Of all the nutrients contained in tomatoes, one of the most studied and important is lycopene.
Lycopene is a carotenoid (similar to the orange pigment found in carrots), with a bright red color, and it's mainly found in red fruits. In western countries, 85% of the consumption of lycopene is represented by tomatoes, even though the fruit with the highest concentration is the vietnamese "gac" (containing 70 times the amount of lycopene of tomatoes!).
Many plants and algae use lycopene to protect themselves from excessive sunlight exposure during photosynthesis, and lycopene is an important intermediate in the biosynthesis of beta-carotene and other orange-red-yellow pigments.
It is not soluble in water, but being hydrophobic it can be dissolved in oils and alcoholic substances: because of this, consuming tomatoes with olive oil, for example, leads to a much higher absorption of lycopene.
Lycopene levels vary according to the different species of tomatoes, with increased amounts in deep, bright red species. Processed tomato foods actually have a higher bioavailability of lycopene, making salsas and ketchup an even better source of lycopene than raw, uncooked tomatoes.
In humans, lycopene is not a necessary nutrient, but it is generally consumed with red or orange fruits and vegetables: it is absorbed together with short chain fatty acids and transported into the bloodstream with LDL or VLDL lipoproteins, accumulating in organs such as testes, adrenal glands and liver.
Because of its peculiar chemical structure, lycopene is currently the most potent carotenoid singlet oxygen antioxidant known, with an antioxidant power approximately a hundred times greater than Vitamin E (one of the most powerful fat-soluble antioxidants).
Antioxidants such as lycopene greatly reduce the amount of DNA damage received by our cells, which is essentially the reason why we age over time, and they have classically been linked with better health, slower aging, and a certain degree of protection from cancer (which also arises from damaged DNA). Lycopene appears, according to very preliminar studies, to reduce the risk of developing diseases such as cardiovascular disease, cancer, diabetes, osteoporosis, and male infertility.
Despite its proven potency as an antioxidant, and even if preliminary studies showed an inverse correlation between lycopene consumption and prostate cancer, there are not many scientific studies on lycopene, and past studies have not proven with sufficient degree of certainty the reduction in relative risks.
As such, the FDA has not yet approved claims that lycopene reduces prostate cancer.
There's a very interesting study published in the American Journal of Clinical Nutrition: in patients with colorectal adenomas, a benign tumor that is often a precursor of colorectal cancer, blood levels of lycopene were 35% lower compared to subjects with no adenomas (beta-carotene also was be 25.5% lower, although according to researchers, this difference was not significant). In their final (multiple logistic regression) analysis, only low levels of plasma lycopene (lower than 70 microgram per liter) and smoking significantly increased the likelihood of colorectal adenomas by, respectively, 230% and 302%.