Researchers in the US have found that the watermelon contains more lycopene, the phytochemical associated with reduced prostate cancer risk and lower rates of heart disease, than tomatoes.
According to a report published in the June issue of Agricultural Research magazine, raw watermelon contains as much or more lycopene than tomatoes, even when compared to tomato juice which has undergone heat treatment - said to improve bioavailability of the chemical.
Lycopene is a red pigment that occurs naturally in certain plant and algal tissues. It gives watermelon and tomatoes their colour, but is also thought to act as a powerful antioxidant. Lycopene scavenges reactive oxygen species, which are aggressive chemicals that react with cell components, causing oxidative damage and loss of proper cell function.
Scientists have previously found that a high dietary intake of lycopene reduces the incidence of certain types of cancer. Lycopene levels in fat tissue, an indicator of lycopene consumption, have also been linked with reduced risk of heart attack.
Most clinical research dealing with lycopene has used tomatoes as the food source but US Agricultural Research Service scientists at the South Central Agricultural Research Laboratory (SCARL) in Lane, Oklahoma, and at the Phytonutrients Laboratory in Beltsville, Maryland, decided to look at lycopene levels in varieties of watermelon and to assess its bioavailability. Funding was provided in part by the National Watermelon Promotion Board.
The ARS scientists analysed 13 watermelon cultivars to establish the relative effect of genetic background on lycopene content. The 13 cultivars included different varieties of the fruit.
The researchers used tristimulus colorimeter readings to measure visible colour in the cut melons and compared the findings to the amounts of lycopene extracted from the melons. Lycopene content varied widely among cultivars and types, but the seedless ones tended to have more.
Results showed that watermelon has as much, or more lycopene as raw tomatoes and that the amount depends on both variety and growing conditions.
The ARS also assessed bioavailability of lycopene in watermelons in a study begun last year on 23 healthy adults. The scientists used tomato juice as the known benchmark for judging the relative bioavailability of lycopene.
The investigators found that lycopene concentration was similar regardless of whether subjects consumed 20 milligrams of lycopene from tomato juice or from watermelon juice, which was not heat-processed.
The investigators had expected lycopene availability to be greater from tomato juice because it had received heat treatment, which is believed to improve lycopene bioavailability.
"To our knowledge, this is the first study to show that lycopene from watermelon is bioavailable," said ARS scientist Clevidence. "Next, we would like to find out if plasma lycopene levels are higher when people eat watermelon with a meal containing fat than when they eat it by itself."
Watermelon also contains the vitamins A, B6, C and thiamin. Studies have shown that a cup and a half of watermelon contains about 9 to 13 milligrams of lycopene. On average, watermelon has about 40 per cent more lycopene than raw tomatoes. Red, ripe flesh is the best indicator of the sweetest and most nutritious watermelon.
"We think there are a lot of potential uses for watermelon that are just beginning to be explored," said Perkins-Veazie, author of the study. "It can be a so-called functional food, one that can help prevent certain diseases."
Other good sources of lycopene include red and pink grapefruit and guava.