Lycopene (from the neo-Latin Lycopersicum, the tomato species) is a bright red carotene and carotenoid pigment and phytochemical found in tomatoes and other red fruits and vegetables, such as red carrots, watermelons, gac, and papayas, although not in strawberries or cherries. Although lycopene is chemically a carotene, it has no vitamin A activity. Foods that are not red may also contain lycopene, such as asparagus and parsley.
In plants, algae, and other photosynthetic organisms, lycopene is an intermediate in the biosynthesis of many carotenoids, including beta-carotene, which is responsible for yellow, orange, or red pigmentation, photosynthesis, and photoprotection. Like all carotenoids, lycopene is a tetraterpene. It is insoluble in water. Eleven conjugated double bonds give lycopene its deep red color. Owing to the strong color, lycopene is a useful as a food coloring (registered as E160d) and is approved for use in the USA, Australia and New Zealand (registered as 160d) and the European Union.
Lycopene is a symmetrical tetraterpene assembled from eight isoprene units. It is a member of the carotenoid family of compounds, and because it consists entirely of carbon and hydrogen, is also a carotene. Isolation procedures for lycopene were first reported in 1910, and the structure of the molecule was determined by 1931. In its natural, all-trans form, the molecule is long and straight, constrained by its system of eleven conjugated double bonds. Each extension in this conjugated system reduces the energy required for electrons to transition to higher energy states, allowing the molecule to absorb visible light of progressively longer wavelengths. Lycopene absorbs all but the longest wavelengths of visible light, so it appears red.
Plants and photosynthetic bacteria naturally produce all-trans lycopene. When exposed to light or heat, lycopene can undergo isomerization to any of a number of cis-isomers, which have a bent rather than linear shape. Different isomers were shown to have different stabilities due to their molecular energy (highest stability: 5-cis ≥ all-trans ≥ 9-cis ≥ 13-cis > 15-cis > 7-cis > 11-cis: lowest). In human blood, various cis-isomers constitute more than 60% of the total lycopene concentration, but the biological effects of individual isomers have not been investigated.
Carotenoids like lycopene are found in photosynthetic pigment-protein complexes in plants, photosynthetic bacteria, fungi, and algae. They are responsible for the bright orange–red colors of fruits and vegetables, perform various functions in photosynthesis, and protect photosynthetic organisms from excessive light damage. Lycopene is a key intermediate in the biosynthesis of carotenoids, such as beta-carotene, and xanthophylls.
The unconditioned biosynthesis of lycopene in eukaryotic plants and in prokaryotic cyanobacteria is similar, as are the enzymes involved. Synthesis begins with mevalonic acid, which is converted into dimethylallyl pyrophosphate. This is then condensed with three molecules of isopentenyl pyrophosphate (an isomer of dimethylallyl pyrophosphate), to give the twenty-carbon geranylgeranyl pyrophosphate. Two molecules of this product are then condensed in a tail-to-tail configuration to give the forty-carbon phytoene, the first committed step in carotenoid biosynthesis. Through several desaturation steps, phytoene is converted into lycopene. The two terminal isoprene groups of lycopene can be cyclized to produce beta-carotene, which can then be transformed into a wide variety of xanthophylls.
Lycopene is the pigment in tomato-containing sauces, turning plastic cookware orange and is insoluble in water. It can be dissolved only in organic solvents and oils. Because of its non-polarity, lycopene in food preparations will stain any sufficiently porous material, including most plastics. To remove this staining, the plastics can be soaked in a solution containing a small amount of household bleach.
Absorption of lycopene requires that it be combined with bile salts and fat to form micelles. Intestinal absorption of lycopene is enhanced by the presence of fat and by cooking. Lycopene dietary supplements (in oil) may be more efficiently absorbed than lycopene from food.
Lycopene is not an essential nutrient for humans, but is commonly found in the diet mainly from dishes prepared from tomatoes. The median and 99th percentile of dietary lycopene intake have been estimated to be 5.2 and 123 mg/d, respectively.
|Source||mg wet weight|
|Raw tomato||4.6 per cup|
|Tomato juice||22 per cup|
|Tomato paste||75 per cup|
|Tomato ketchup||2.5 per tablespoon|
|Watermelon||13 per wedge|
|Pink grapefruit||2 per half grapefruit|
Fruits and vegetables that are high in lycopene include autumn olive, gac, tomatoes, watermelon, pink grapefruit, pink guava, papaya, seabuckthorn, wolfberry (goji, a berry relative of tomato), and rosehip. Ketchup is a common dietary source of lycopene. Although gac (Momordica cochinchinensis Spreng) has the highest content of lycopene of any known fruit or vegetable (multiple times more than tomatoes), tomatoes and tomato-based sauces, juices, and ketchup account for more than 85% of the dietary intake of lycopene for most people. The lycopene content of tomatoes depends on species[clarification needed] and increases as the fruit ripens.
Unlike other fruits and vegetables, where nutritional content such as vitamin C is diminished upon cooking, processing of tomatoes increases the concentration of bioavailable lycopene. Lycopene in tomato paste is up to four times more bioavailable than in fresh tomatoes. Processed tomato products such as pasteurized tomato juice, soup, sauce, and ketchup contain a higher concentration of bioavailable lycopene compared to raw tomatoes.
Cooking and crushing tomatoes (as in the canning process) and serving in oil-rich dishes (such as spaghetti sauce or pizza) greatly increases assimilation from the digestive tract into the bloodstream. Lycopene is fat-soluble, so the oil is said to help absorption. Gac has high lycopene content derived mainly from its seed coats. Cara cara navel, and other citrus fruit, such as pink grapefruit, also contain lycopene. Some foods that do not appear red also contain lycopene, e.g., asparagus, which contains approximately 30μg of lycopene per 100 gram serving (0.3μg/g) and dried parsley and basil, which contain approximately 3.5-7 μg of lycopene per gram.
In humans, the Observed Safe Level for lycopene is 75 mg/day, according to one preliminary study.
Lycopene is non-toxic and commonly found in the diet, mainly from tomato products. There are cases of intolerance or allergic reaction to dietary lycopene, which may cause diarrhea, nausea, stomach pain or cramps, gas, and loss of appetite. Lycopene may increase the risk of bleeding when taken with anticoagulant drugs. Because lycopene may cause low blood pressure, interactions with drugs that affect blood pressure may occur. Lycopene may affect the immune system, the nervous system, sensitivity to sunlight, or drugs used for stomach ailments.
In a review of literature on lycopene and its potential role as a dietary antioxidant, the European Food Safety Authority concluded that there was insufficient evidence for lycopene having antioxidant effects in humans, particularly in skin, heart function or vision protection from ultraviolet light.
Although lycopene from tomatoes has been tested in humans for cardiovascular diseases and prostate cancer, there was no effect on any disease. The US Food and Drug Administration (FDA), in rejecting manufacturers' requests in 2005 to allow "qualified labeling" for lycopene and the reduction of various cancer risks, provided a conclusion that remains in effect as of 2017:
"...no studies provided information about whether lycopene intake may reduce the risk of any of the specific forms of cancer. Based on the above, FDA concludes that there is no credible evidence supporting a relationship between lycopene consumption, either as a food ingredient, a component of food, or as a dietary supplement, and any of these cancers."
A 2010 review concluded that research has been insufficient to establish whether lycopene consumption affects human health. Lycopene has been studied in basic and clinical research for its potential effects on cardiovascular diseases and prostate cancer, although results through 2017 have not changed the prevailing FDA view that evidence of benefit remains inconclusive. A 2011 review showed insufficient evidence to support the use of lycopene for the prevention of prostate cancer.
Lycopenemia, characterized by an orange discoloration of the skin, has been observed with high intakes of lycopene-containing foods. One case study reported the incidence of lycopenemia in a 61-y-old woman who had consumed ∼2 L of tomato juice daily for several years (10). Although there was evidence of lycopene and fatty deposits in the liver, there was an absence of measurable hepatic dysfunction. After 3 wk of consuming a diet free of tomato juice, the orange discoloration faded.
The OSL risk assessment method indicates that the evidence of safety is strong at intakes up to 20mg/d for lutein, and 75 mg/d for lycopene, and these levels are identified as the respective OSLs. Although much higher levels have been tested without adverse effects and may be safe, the data for intakes above these levels are not sufficient for a confident conclusion of long-term safety.
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