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What's New and Beneficial About Watermelon
Alongside of tomatoes, watermelon has moved up to the front of the line in recent research studies on high-lycopene foods. Lycopene is a carotenoid phytonutrient that's especially important for our cardiovascular health, and an increasing number of scientists now believe that lycopene is important for bone health as well. Among whole, fresh fruits that are commonly eaten in the U.S., watermelon now accounts for more U.S. intake of lycopene (by weight of fruit eaten) than any other fruit. Pink grapefruit and guava are two other important fruit sources of lycopene, although in the U.S., these fruits are more often consumed in the form of juice.
Health scientists are becoming more and more interested in the citrulline content of watermelon. Citrulline is an amino acid that is commonly converted by our kidneys and other organ systems into arginine (another amino acid). The flesh of a watermelon contains about 250 millligrams of citrulline per cup. When our body absorbs this citrulline, one of the steps it can take is conversion of citrulline into arginine. Particularly if a person's body is not making enough arginine, higher levels of arginine can help improve blood flow and other aspects of our cardiovascular health. There's also some preliminary evidence from animal studies that greater conversion of citrulline into arginine may help prevent excess accumulation of fat in fat cells due to blocked activity of an enzyme called tissue-nonspecific alkaline phosphatase, or TNAP.
If you've gotten used to thinking about the juicy red flesh at the center of a watermelon as its only nutrient-rich area—and far more nutrient-rich than the more lightly-colored flesh that is farther out near the watermelon rind—it is time to change your thinking. In a recent study, food scientists compared the nutrient content of flesh from different parts of a watermelon: flesh from the center, the stem end, the blossom end (opposite from the stem), and the periphery (the part nearest to the rind). What they've discovered were impressive concentrations of phenolic antioxidants, flavonoids, lycopene, and vitamin C in all of these different areas. The exact distribution of nutrients was also highly dependent on the variety of watermelon. But there was no area in any of the watermelon varieties that came out badly in terms of nutrients, and in many of the watermelon varieties, the flesh's outer periphery contained impressive concentrations of most nutrients.
Recent studies have confirmed the nutritional importance of allowing a watermelon to fully ripen. For example, research has shown that the biggest jump in lycopene content occurs at the time when a watermelon's flesh turns from white-pink to pink. Yet when that flesh continues to ripen, resulting in a color change from pink to red, the lycopene content becomes even more concentrated. Prior to ripening, when the flesh of a watermelon is primarily white in color, its beta-carotene content is near zero. Even when allowed to ripen to the white-pink stage, a watermelon still contains very little of its eventual beta-carotene content. But as it moves from white-pink to pink to red, the beta-carotene content of a watermelon steadily increases. Like lycopene and beta-carotene, total phenolic antioxidants in a watermelon also increase consistently during ripening, all the way up until the appearance of fully red flesh. The bottom line: eating a fully ripe watermelon can really pay off in terms of nutrient benefits. Please see our section called "How to Select and Store" to learn about determining a watermelon's ripeness before you purchase it.
Nutrients in
Watermelon
1.00 cup (152.00 grams)
Nutrient%Daily Value
vitamin C20.5%
vitamin A17.2%
potassium4.8%
magnesium3.8%
Calories (45)2%
This chart graphically details the %DV that a serving of Watermelon provides for each of the nutrients of which it is a good, very good, or excellent source according to our Food Rating System. Additional information about the amount of these nutrients provided by Watermelon can be found in the Food Rating System Chart. A link that takes you to the In-Depth Nutritional Profile for Watermelon, featuring information over 80 nutrients, can be found under the Food Rating System Chart.
Health Benefits
Description
History
How to Select and Store
Tips for Preparing and Cooking
How to Enjoy
Individual Concerns
Nutritional Profile
References
Health Benefits
Anti-Inflammatory and Antioxidant Support
Phenolic compounds in watermelon—including flavonoids, carotenoids, and triterpenoids—make this fruit a choice for anti-inflammatory and antioxidant health benefits. If you had to pick a single nutrient from this anti-inflammatory and antioxidant category that has put watermelon on the map, that nutrient would be lycopene. Alongside of pink grapefruit and guava, watermelon is an unusually concentrated source of this carotenoid. Whereas most fruits get their reddish color from anthocyanin flavonoids, watermelon gets it reddish-pink shades primarily from lycopene. The lycopene content of watermelons increases along with ripening, so to get the best lycopene benefits from watermelon, make sure that your melon is optimally ripe. (See our section entitled, "How to Select and Store" for practical tips on selecting a fully ripe watermelon.) The lycopene in watermelon is a well-documented inhibitor of many inflammatory processes, including the production of pro-inflammatory messaging molecules, the expression of enzymes like cyclo-oxygenase and lipoxygenase that can lead to increased inflammatory response, and the activity of molecular signaling agents like nuclear factor kappa B (NFkB). Lycopene is also a well-known antioxidant, with the ability to neutralize free radical molecules.
Recent research has shown that the lycopene content of watermelon also remains very stable over time. When two-inch cubes of fresh-cut watermelon were stored in the refrigerator at 36°F (2°C) over 48 hours, researchers found virtually no deterioration in lycopene content. That deterioration did not start to become significant until about seven days of storage, when it decreased by about 6-11%. While we do not recommend waiting seven days before consuming fresh cut watermelon, we believe that the excellent stability of watermelon lycopene over a two-day period is great news for anyone wanting to enjoy fresh cut watermelon over the course of several days.
Cucurbitacin E is another unique anti-inflammatory phytonutrient (called a tripterpenoid) found in watermelon. Like the carotenoid lycopene, this anti-inflammatory nutrient has been shown to block activity of cyclo-oxygenase enzymes and neutralize reactive nitrogen-containing molecules. (Interestingly, cucurbitacin E does not appear to neutralize activity of reactive oxygen species—called ROS—but only activity of reactive nitrogen species, called RNS.)
Antioxidant carotenoids found in watermelon include significant amounts of beta-carotene. Like lycopene, the beta-carotene in watermelon also increases with ripening.
Red-pink fleshed watermelons typically contain far more lycopene and beta-carotene than yellow-white fleshed varieties. For example, one study we've seen showed red watermelon to contain over 600 micrograms of beta-carotene per 3.5 ounces of melon and over 6,500 micrograms of lycopene. By comparison, yellow-fleshed varieties were found to contain only 5-10 micrograms of beta-carotene and no measurable amount of lycopene. In red/pink-fleshed watermelons as a group, we've seen lycopene amounts that vary widely in a range of approximately 2,000–6,700 micrograms per 3.5 ounces of fresh melon. Beta-carotene in these red/pink-fleshed varieties also varies widely, in a range of approximately 5–325 micrograms. Because watermelon contains so many different phytonutrients—as well as key vitamins and minerals, as well as dietary fiber—your health is going to be improved by any watermelon variety that you choose. However, if you specifically want to maximize your lycopene and beta-carotene intake, you'll most likely want to stick with red/pink-fleshed varieties of watermelon.
It would be a mistake to ignore the important amount of vitamin C found in watermelon. In our Food Rating System, watermelon qualifies as an excellent source of vitamin C, even though the amount provided (about 12 milligrams per cup of fresh melon) is only 20% of the Daily Value (DV). However, due to its very high water content, the same amount of watermelon that provides us with 20% of the Daily Value for vitamin C only costs us about 45 calories, or about 2% of our total daily calories on a 1800-2000 calorie diet. That's excellent nutrient richness, and it makes watermelon a great choice for increasing vitamin C antioxidant protection.
Citrulline, Arginine, and Nitric Oxide-Related Benefits
One of the more unusual aspects of watermelon is its rich supply of the amino acid, citrulline. Citrulline is an amino acid that is commonly converted by our kidneys and other organ systems (including cells that line our blood vessels) into arginine (another amino acid). The flesh of a watermelon contains about 250 millligrams of citrulline per cup. When our body absorbs citrulline, one of the steps it can take is conversion of citrulline into arginine.
An enzyme called nitric oxide synthase (NOS)—found in many of our body's cell types—is able to take the amino acid arginine and use it to help produce a very small molecule of gas called nitric oxide (NO), which is a muscle relaxant. For example, when NO tells the smooth muscles around our blood vessels to relax, the space inside our blood vessels can expand, allowing blood to flow more freely and creating a drop in blood pressure. The relaxing of muscle tension and increasing of blood flow is also the way that NO can change erectile function in men. (The prescription medication sildenafil or Viagra (TM) works in this way.)
The amount of citrulline found in fresh watermelon is not enough to make it a food that can automatically improve blood pressure or affect other problems like erectile dysfunction. But in animal studies, intake of watermelon has been shown to help support cardiovascular function, including improvement of blood flow (through relaxation of blood vessels, or what is technically called vasodilation). In humans, intake of watermelon has been shown to increase blood levels of arginine, but only when consumed in very large amounts. For example, in one study that we reviewed, participants consumed either three cups or six cups of fresh watermelon juice daily over the course of three weeks and experienced increases in their blood arginine levels of approximately 12-22%.
Another fascinating new area of research involving watermelon and its citrulline content relates to the deposition of body fat. In animal studies, high intake of amino acid citrulline—followed by conversion of citrulline into the amino acid arginine—can result in the formation of arginine-related molecules called polyarginine peptides. These polyarginine peptides are able to block activity of an enzyme called tissue-nonspecific alkaline phosphatase, or TNAP. When TNAP activity is shut down, our fat cells (adipocytes) tend to create less fat (adipogenesis). Researchers believe that the connection between citrulline in food, arginine production by nitric oxide synthase, and fat cell metabolism may eventually provide us with additional tools for helping prevent over-accumulation of body fat.
At present, however, the best we can conclude about watermelon and its unusual citrulline content is that it's likely to provide us with some cardiovascular benefits, especially if we don't consume many foods that are high in arginine. (Some of the WHFoods highest in arginine include shrimp, spinach, sea vegetables, turkey, sunflower seeds, and sesame seeds.)
Other Health Benefits
Watermelon seeds can provide us with small but helpful amounts of both iron and zinc. We're talking about several hundred seeds (the amount contained in a typical seeded watermelon, which is not an amount that we would anticipate or suggest eating at one time) to obtain 1–2 milligrams of either mineral. Still, regular consumption of whole, seeded watermelon would provide us with nutrient benefits in this area over time. Interestingly, we've seen one study showing that the iron and zinc in watermelon seeds is surprisingly bioavailable (85-90%), despite the oxalates and phytates that are contained in the seeds. (Oxalates and phytates can sometimes bind with minerals like iron and zinc to lessen their bioavailability.)
The amount of protein in watermelon seeds is approximately 1 gram per 24 seeds. At this rate, we're likely to get several grams of protein when we eat several slices of whole, seeded watermelon. While we would not want to depend on watermelon as a key protein food, this valuable amount of protein in its seeds should at least remind us that a fruit like watermelon does have something to offer us in the way of protein benefits.
At approximately two-thirds of one gram of dietary fiber per cup, watermelon does not rank as a good, very good, or excellent source of this nutrient in our ranking system. However, you'd be receiving about 3-4 grams of dietary fiber if you enjoyed 175–200 calories of fresh watermelon in the form of several large slices, and this dietary fiber would include a nice mix of soluble to insoluble fiber. (Insoluble fibers can provide special support to the digestive system, and soluble fibers can provide special support to the cardiovascular system.) So while watermelon is not a concentrated source of fiber, we often enjoy it in larger amounts that can provide us with great fiber benefits at a low calorie cost.
