Americans love to eat tomatoes. In fact, an average American consumes about 17lbs of fresh tomatoes annually and over 60lbs of processed tomatoes such as in tomato sauce and ketchup. Tomatoes are a rich source of essential nutrients such as vitamins A, C, E, and folate. In addition to these vitamins, tomatoes are also rich in a chemical compound called carotenoids. Carotenoids may sound familiar to some of you since they have been extensively studied in the prevention of heart disease and cancer on account of their antioxidants effects. The most famous carotenoid is beta-carotene which is the precursor to vitamin A. Unfortunately, beta-carotene did not live up to expectation in that several clinical trials did not show beta-carotene supplements to have heart or cancer benefit. In fact, there was some suggestion that beta-carotene was harmful. However, there is another carotenoid in tomatoes called lycopene that may yet prove to be helpful rather than harmful.
Lycopene is different from beta-carotene in that it is not a precursor to vitamin A. It is the pigment that is responsible for the rich redness of various fruits and vegetables such as tomatoes and watermelon. Because tomatoes are quite rich in lycopene, ~80% of our consumed lycopene comes from eating tomatoes and processed tomatoes. Lycopene is absorbed by the intestine and its absorption is enhanced when tomatoes are heated, processed, and homogenized. Therefore foods such as tomato juice and sauce have higher concentrations of lycopene then fresh tomatoes. Furthermore, absorption of lycopene is enhanced when eaten with some form of fat such as olive oil. Once in the circulation, lycopene is mainly transported by LDL. It is here that lycopene exerts one of its beneficial effects through the prevention of LDL oxidation.
Oxidation of LDL is a process in which the membrane of the LDL particle is damaged by free radicals. Once damaged, the proteins, fat, and cholesterol in the core are also damaged. This oxidation process makes the LDL particle more dangerous to heart arteries in that it promotes LDL invasion into arterial walls. This invasion then results in lipid wall build up, local inflammation that results in more vessel wall damage, arterial wall narrowing, and possible blood clot formation on top of a ruptured lipid plaque. This cascade ultimately can lead to a heart attack. Lycopene exerts antioxidant properties by binding to these free radicals. The free radicals are then no longer free to oxidize LDL.
Not only does lycopene appear to protect existing LDL from damage, but it also can reduce the total amount of circulating LDL and total serum cholesterol. Lycopene can directly inhibit cholesterol synthesis as well as increase the number of LDL receptors on scavenger cells called macrophages thereby promoting removal of LDL from the circulation.
A Finnish study published in the April 2007 British Journal of Nutrition was the first major study to prove the cholesterol benefit of tomatoes in humans. The authors took 21 healthy volunteers and placed them on two separate but serial tomato diets and monitored the changes in their cholesterol profiles as well as measured the amount of circulating LDL resistant to oxidation. The first diet was a low tomato one in which the subjects were not allowed to eat any tomato products as well as other lycopene containing foods such as watermelon, apricots, and guava. After three weeks, they were all then placed on a high tomato diet comprised of consuming 400ml of tomato juice and 30gms of ketchup with each meal daily for the next three weeks. The authors reported that on the high tomato diet, total cholesterol fell by 6% and LDL fell by 13%. Levels of circulating LDL resistant to oxidation also increased 13%. These changes directly correlated to increases in lycopene and other carotenoids. This study did not look at other outcomes such as heart disease risk. Other population studies have suggested that higher levels of lycopene are associated with lower rates of heart attacks.