HerbalEGram: Volume 16, Issue 7, July 2019

By Jenny Perez^a, Hannah Bauman^b, and Becky Nichols^c

^a ABC Education Coordinator

^b HerbalGram Associate Editor

^c ABC Dietetics Intern (Texas State, 2013)

Editor’s Note: Each month, HerbalEGram highlights a conventional food and briefly explores its history, traditional uses, nutritional profile, and modern medicinal research. We also feature a nutritious recipe for an easy-to-prepare dish with each article to encourage readers to experience the extensive benefits of these whole foods. With this series, we hope our readers will gain a new appreciation for the foods they see at the supermarket and frequently include in their diets. We would like to acknowledge ABC Chief Science Officer Stefan Gafner, PhD, for his contributions to this project. The original article on watermelon was published in July 2015.

Overview

The watermelon is a trailing annual with several firm, stout stems that grow up to 3 m (9 ft) in length with palmately lobed, hairy leaves, coiled tendrils, and yellow monoecious (male and female) flowers.^1,2 The watermelon fruit is a very large, round-to-oblong berry with a smooth, glossy, mottled-striped skin (rind) that is dark green to yellow in color.^1,3-4 The endocarp (flesh) of watermelons is yellowish to mostly red and contains numerous edible seeds.^1,3-4 The watermelon is considered the largest edible fruit grown in the United States; it typically weighs anywhere from a few pounds to as much as 90 pounds, with vines that can reach up to 20 feet in length.³ Fruits must be harvested when fully ripe and, unlike some other fruits, watermelon will not continue to ripen when cut prematurely from the vine.¹

Watermelons belong to the squash family, or Cucurbitaceae, along with other plants that grow on vines on the ground such as cantaloupe (Cucumis melo var. cantalupensis), pumpkin (Cucurbita pepo), butternut squash (C. moschata), and cucumber (Cucumis sativus). Plants belonging to the Citrullus genus produce seeds that are distributed throughout the edible flesh, whereas close relatives in the genera Cucurbita and Cucumis have seeds that are located at the center of the fruit, leaving the flesh free of seeds.⁴

Historical and Commercial Uses

The watermelon is native to the Kalahari Desert region in Africa and thrives in well-draining, sandy soil.³ The wild watermelon, which is smaller, less colorful, and more bitter-tasting than cultivated varieties, has been used as a food and medicine in subtropical areas of Africa for more than 4,000 years.^1-4 Genomic analysis has been used to assess the relationships among living wild and primitive watermelons from northeastern Africa, modern sweet dessert watermelons, and other Citrullus taxa.⁴ In the Kalahari Desert region, the San people use wild watermelon (known as tsama or tsamma) as an important source of water during the dry season.³ In earlier times, it was only possible to travel through this region during tsama season, when wild watermelons ripened.¹ The fruits of the wild Citrullus species are small and spherical with broad, dark stripes and firm, somewhat bitter, pale-colored, and seedy fruit flesh.⁴ Watermelons were not only abundant in energy-enhancing nutrients and replenishing electrolytes, but also a source of potable liquid in arid regions where water supplies were questionable or polluted.⁵ After the juicy flesh was consumed, watermelon gourds were often reused as canteens or for storing or even cooking berries.²

African cuisine treats the watermelon as a vegetable and uses the entire fruit: seeds, rinds, and flesh.⁶ The seeds are eaten raw or roasted as snacks after the papery seed coat is removed. The seeds are also added to dishes, ground into flour for use in baked goods, and used to thicken soups.^2,5 In tropical western Africa, watermelon seed oil is used in cooking as a substitute for peanut oil.⁵ The rind can be stir-fried, stewed, candied, pickled, or grilled. Watermelon flesh is eaten fresh or juiced, but it can also be fermented into wine.⁶

Watermelons are among the most commonly grown annual crop in tropical and subtropical.⁴ Earliest records of watermelon cultivation date back to 3000 BCE, where it was depicted in Egyptian hieroglyphics on tomb walls.³ Watermelon symbolized nourishment and was held in such high regard that it was left as a funeral offering for the dead in the afterlife.

As watermelons became domesticated, cultigens with sweet, red-colored flesh were most desirable.⁴ Cultigens are plants whose traits have been deliberately altered by humans through artificial selection, resulting in numerous cultivars or plant varieties.⁷ Sweet dessert watermelons emerged approximately 2,000 years ago.⁴ The cultivation of watermelon spread to China in the 10th century and to Europe, by the Moors, by the 13th century.³ Ultimately, the watermelon crossed the Atlantic Ocean into North America during the African slave trade.

There are more than 50 watermelon varieties and 1,200 cultivars produced worldwide varying in shape, color, and size.⁵ The four most popular cultivars are “picnic” watermelons, which weigh between 15-50 pounds; “icebox” varieties, designed to fit inside a refrigerator, which weigh between 5-15 pounds; as well as “yellow-flesh” watermelons and “seedless” watermelons. When stored in a cool, dark, or shady place, dessert watermelons can keep for weeks or even months without significant changes in quality or taste.⁴

Predominately enjoyed as a dessert, the flesh of the watermelon is eaten alone or as an addition to fruit salads.¹ In Asia and especially China, ripe watermelon seeds are dried and roasted and enjoyed as a nutritious snack. Watermelon juice can be made into sorbet, syrup, or wine. Although many people are accustomed to eating the juicy flesh, the seeds and rind of watermelon are also edible.⁸

China is the largest producer of watermelon worldwide while Spain is largest producer in the European Union.⁹ Turkey, Iran and the United States are also major watermelon producers.³ The United States ranks fifth in global watermelon production.¹⁰ Forty-four states grow watermelons, including Texas, Florida, Georgia, and California, which collectively produce two-thirds of all the watermelons domestically.¹⁰

Many cultures used watermelons as a refreshing, cleansing food that was considered a safe and dependable diuretic.⁵ Containing approximately 92% water, watermelon has many traditional uses that include hydrating and moistening tissues in the body, cleansing, eliminating impurities, and relieving inflammation.¹¹

Ancient Egyptians used watermelon to treat problems such as erectile dysfunction and prostate inflammation.³ The peoples of Russia and Central Asia used watermelon, sometimes lacto-fermented, as a diuretic and to cleanse the blood.¹² Since watermelon is digested relatively quickly, the folk traditions of the Papua New Guinea aborigines known as Onabasulu advised against eating watermelon and other juicy fruits after a heavy meal or if suffering from a stomachache.¹³

In traditional Chinese medicine (TCM), watermelon, known as xi gua, is considered sweet, cooling, and moistening, and is used to clear heat from the body while producing a diuretic effect.^11,14 Watermelon is used to alleviate thirst, treat edema, and reduce inflammation of the kidney and urinary tract.¹¹ All parts of the watermelon fruit, including the flesh (xi gua), rind (xi gua pi), and seeds (xi gua zi), are used in TCM. Watermelon juice is used to relieve dry conditions, such as thirst, constipation, and dehydration, as well as hot conditions, such as summer fevers, sunburns, and canker sores. It is also used to heal inflammation in the body.^14,15 Additionally, watermelon’s cooling properties are thought to calm irritability, restlessness, and worry.¹⁴ This is likely attributed to watermelon’s abundant supply of potassium and B vitamins.

Watermelon rind is prescribed in TCM for diabetes and in cases of alcohol poisoning.² As a traditional medicine, watermelon seed was considered more effective than pumpkin seed oil at paralyzing and expelling tapeworms and roundworms.² The seeds have demulcent, soothing properties and were also used to treat urinary tract infections and bed-wetting.²

Phytochemicals and Constituents

As its name implies, watermelon is approximately 92% water. The fruit contains only 48 calories per eight-ounce serving and has a relatively low sugar content (6%).^1-3,5 Watermelons provide a good source of B₁ (thiamine), B₆ (pantothenic acid), biotin, dietary fiber, and the electrolytes magnesium and potassium.³

Watermelon is considered a very good source of dietary antioxidants including vitamin C and the colorful carotenoids β-carotene and lycopene.³ A one-cup serving of watermelon provides nearly 20% of the RDI for vitamin C and almost 14% of the RDI for vitamin A due to its high β-carotene content.³ Watermelon’s high water and electrolyte content contributes to its diuretic properties while delivering more nutrients per calorie compared to other fruits.³

Watermelon seeds are more nutrient dense than the flesh, containing 30-40% protein and 45% edible oil.^1,2 The seed oil is an excellent energy source for humans and livestock, and provides linoleic, oleic, palmitic, and stearic acids.² Watermelon seed oil has a fatty acid profile similar to pumpkin seed oil and can be used for cooking.²

The thick skin or rind of the watermelon has a higher content of antioxidant phenolic acids and the amino acid citrulline than watermelon flesh.^16,17 In preparing fresh cut watermelon for consumer consumption, the rind is considered a waste byproduct despite being an abundant source of antioxidant polyphenols. The amount of byproduct generated by processing fresh-cut watermelon is between 30-40% of the total weight.¹⁶ Research is ongoing to discover ways in which the bioactive compounds obtained from the rind can be repurposed for food or cosmetic uses.¹⁶

Citrulline

Citrulline is a nonessential amino acid that was initially identified from the juice of watermelon.¹⁵ While citrulline has been also been found in other squash family fruits including bitter melon (Momordica charantia, Cucurbitaceae), cucumber, cantaloupe, and pumpkin, watermelon is the richest known source of citrulline.^16,17 Considered to be a major hydroxyl radical scavenger, citrulline content is highest in the wild watermelon, where its potent antioxidant ability is thought to impart drought tolerance.

Citrulline is a precursor to the amino acid arginine and is involved in the urea cycle, removing nitrogen and other toxic compounds the blood and eliminating them through urine.^16,17 Because arginine is involved in maintaining the health of numerous body systems, citrulline is of increasing scientific interest.⁹ Renal failure is often associated with impaired citrulline metabolism.¹⁷ Up to 83% of citrulline can be converted to arginine in the kidneys. Arginine is an essential amino acid that the body uses to make protein, boost muscle growth, reduce fat accumulation, improve insulin sensitivity, enhance wound healing, and stimulate the immune system.^16,17

Arginine is the precursor to forming nitric oxide (NO) in the body, a potent vasodilator that helps maintain a healthy cardiovascular (CV) system. Nitric oxide can lower resting blood pressure and increase muscle contractility, muscle blood flow, peripheral vasodilation, and glucose uptake in skeletal muscles.^18-20 Conditions such as cardiovascular diseases, erectile dysfunction, and headaches may benefit from enhanced vasodilation via increased arginine intake.¹⁶ Citrulline’s antioxidant properties coupled with its ability to generate NO make it an important option for the treatment of health conditions characterized by oxidative stress and decreased arginine availability.^16,17,20

Evidence from several studies suggests that middle-aged and older adults with CV risk factors and CV disease experienced an improvement in endothelial function when supplementing with L-citrulline for 1-8 weeks. Doses varied from 2.4 g up to 6 g daily.¹⁹ Both L-citrulline and watermelon supplementation improve blood levels of both L-arginine and NO.

Lycopene

Though the tomato (Solanum lycopersicum, Solanaceae) is better known as a source for lycopene (and, in fact, its name is derived from lycopersicum), this red-pigmented phytochemical gives fruits like watermelon, papaya (Carica papaya, Caricaceae), and pink grapefruit (Citrus × paradisi, Rutaceae) their color.²¹ Lycopene is fat-soluble and concentrated in the liver, adrenal glands, prostate gland, and fat tissue. It is considered to have more antioxidant power than both β-carotene (provitamin A) and α-tocopherol (vitamin E).²²

The lycopene contained in most foods has a 40-50% cis-isomer configuration, reducing bioavailability.²² For example, the heat processing of tomatoes induces isomerization of lycopene from all-trans to cis configuration, which converts lycopene into a highly bioavailable form with the help of carotenoid isomerase enzyme.^22,23 Watermelon actually lacks the carotenoid isomerase enzyme and, therefore, is the only known fruit that contains lycopene in an all-cis configuration, making dark, red-fleshed, fresh watermelon one of our greatest sources of readily bioavailable dietary lycopene.^16,23

Studies indicate that watermelon juice consumption increases blood levels of both β-carotene and lycopene.¹⁴ Unlike other carotenoids, such as β-carotene that are provitamin A, lycopene lacks a β-ionone ring structure and cannot be converted into vitamin A. Therefore, lycopene’s biological effects are attributed to mechanisms other than vitamin A.²² In vitro studies have discovered that lycopene’s non-oxidative mechanisms include its ability to regulate gap-junction communication, regulate gene function, modulate cytochrome P450 in the liver, and can enhance phase II drug metabolism and as such enhance elimination of undesirable substances (e.g., certain carcinogens).^22,23

Modern Research and Potential Health Benefits

The traditional uses for watermelon as a medicine are beginning to gain scientific support, particularly in regard to its applications against oxidative stress, erectile dysfunction, weight management, and kidney disease. Watermelon’s antioxidant and nutrient content defends against many different conditions. Evidence from epidemiological studies indicate that consumption of watermelon and other foods rich in lycopene is strongly associated with a lower incidence of cardiovascular disease as well as certain types of kidney and prostate cancers.¹⁶

Anti-aging and Oxidative Stress Reduction

Oxidative stress is thought to be at the root of many chronic diseases such as atherosclerosis, diabetes, cancer, and cataracts. Production of free radicals naturally occurs as byproducts of the body’s metabolic processes. However, poor diet and an unhealthy lifestyle can overwhelm the body’s detoxification processes, resulting in chronic inflammation. Consuming a diet rich in fruits and vegetables plays a significant role in protecting cells from DNA damage. Lycopene can restore antioxidant enzymes, including glutathione peroxidase and superoxide dismutase, while decreasing lipid peroxidation, which is one of the leading factors leading to atherosclerosis.²³

Skin is the largest organ in the human body and is most susceptible to damage due to the sun’s UV rays, including wrinkles, freckling, sun burn, and skin cancer.²⁴ Diets rich in colorful carotenoid compounds are directly related to skin health. Studies indicate an inverse correlation with the consumption of eggs and green leafy vegetables and the occurrence of wrinkles on the skin. Foods rich in carotenoids, such as β-carotene and lycopene, are some of the most efficient singlet oxygen quenchers. Recently, dietary carotenoids have been evaluated for use as natural photo-protective substances. Carotenoid concentrations in the skin vary by location on the body (forehead > palm > dorsal > inside arm = back of hand). The skin protecting benefits of carotenoids can be amplified through both oral and topical supplementation. Compared to topical sunscreens which require reapplication and have localized antioxidant effects, UV protection, studies indicate that the use of carotenoid-rich foods or natural products prior to and during sun exposure appears to be a systemic way of boosting blood and skin levels of these highly antioxidant carotenoids, thus reducing sunburn and skin damage.

Cardiovascular Health

Several studies have confirmed that watermelon offers cardiovascular benefits in a variety of ways. Watermelon is a great source of electrolytes, including potassium and magnesium, as well as citrulline. It also contains the most bioavailable form of lycopene. All of these compounds are involved in modulating blood pressure.^20,25 In addition to being an antioxidant, lycopene has been shown to be heart-protective and has been linked to a reduction in high cholesterol and other risk factors that can lead to cardiovascular disease.²⁶

Current research shows that citrulline in watermelons improves cardiovascular health by increasing the bioavailability of arginine, which subsequently increases the synthesis of NO (see above). Nitric oxide relaxes blood vessels, improves circulation without any side effects associated with common cardiovascular medications.¹⁸ In one clinical study, obese participants with pre-high blood pressure or stage-one high blood pressure significantly reduced their ankle and brachial systolic blood pressure, diastolic blood pressure, mean arterial pressure, and carotid wave reflection with ingestion of 6 g citrulline powdered extract derived from watermelons (equivalent to 2.3 pounds of fresh red-fleshed watermelon) over a six-week period compared to placebo.²⁷

Citrulline content is higher and more bioavailable in watermelons that have yellow-flesh and is more effective than supplementation with arginine itself because citrulline bypasses the liver and is not a substrate for arginase.^17,28 As a supplement, L-citrulline is better tolerated than L-arginine, which causes gastrointestinal side effects as high doses (more than 10 g).¹⁷ Dietary supplements containing citrulline are being used to improve sexual stamina and erectile dysfunction, however the mechanism of action is not clear.²⁸

Anti-fatigue and Ergogenic Aid

The use of watermelon juice (500 mL) or L-citrulline powder (~ 6 g) has gained popularity as an ergogenic aid to exercise training.^19,20 Human studies have been conducted to assess the effects of citrulline supplementation by athletes. The use of citrulline from watermelons can improve athletic performance primarily due to its ability to increase glucose transport in skeletal muscle as well as its use in NO synthesis. Citrulline has also been shown to accelerate the removal of lactic acid, a major contributor of muscle soreness and fatigue, allowing for more intense training and faster recovery after workouts.²⁰ Watermelon’s potassium and magnesium content also help the body recover lost electrolytes, relieve muscle cramping and maintain normal muscle and nerve function.¹⁰

A 100-g serving of watermelon contains 160 mg of citrulline.²¹ When ingested prior to exercise, citrulline can reduce the accumulation of lactic acid in skeletal muscles by accelerating metabolism of lactate; detoxify ammonia and other harmful metabolites of the urea cycle in the liver; and increase ATP production in muscles, which increases muscle strength. Citrulline is more bioavailable when delivered in a natural matrix, such as unpasteurized watermelon juice, or fresh watermelon. Regardless of whether the watermelon juice was enriched with additional L-citrulline, studies have confirmed there is a significant reduction in muscle soreness when compared with placebo.²⁰

Diabetes and Weight Management

Watermelon consumption has been evaluated for its effectiveness in glycemic control, weight management, and circulatory problems common in diabetics. Replacing conventional snacks with watermelon increases potassium intake as well as lipid-lowering phytonutrients.²⁹ To determine the effects of daily watermelon consumption, 20 overweight and obese adults participated in a four-week repeated-measures crossover study with two treatments: 2 cups freshly diced watermelon as daily snack in addition to regular diet, followed by a 2-4 week washout period, then crossed over to an iso-calorically matched low-fat cookie snack. After four weeks of low-fat cookie consumption, blood pressure, blood lipids, body weight, and BMI increased; after four weeks of watermelon snack consumption, these parameters decreased. Additionally, those consuming whole watermelon fruit snack experienced a greater feeling of fullness and felt less hungry for up to two hours afterwards.

In addition to its fiber content, watermelon consumption is associated with higher levels of adiponectin, a protein hormone involved in regulating blood glucose levels as well as the breakdown of fatty acids (FAs), which could account for its satiating effects.²⁹ Watermelon’s ability to lower blood pressure is attributed to the presence of citrulline. Improvement in blood lipid profiles was attributed to watermelon’s β-carotene and lycopene content. Due to lycopene’s lipophilic nature, excess can be stored in adipose tissue.²³ Lycopene also improves insulin sensitivity and glucose metabolism.

Cancer Preventive Effects

Lycopene’s powerful antioxidant properties have been shown to reduce the risks of prostate, lung, gastric, and colorectal cancers.^26,30 Lycopene reduces the production of pro-inflammatory mediators including interleukins, NO, and tumor necrosis factor alpha thus preventing inflammation. However, due to its antioxidant effect it seems to interfere with chemo and radiation therapy.

Research interventions have observed an inverse correlation between the consumption of lycopene-rich foods and prostate cancer.²³ Men consuming a lycopene-rich diet reported 25% lower incidence of prostate cancer and a 44% reduced risk of other cancers. Likewise, females consuming watermelon on a regular basis are five times less likely to develop cervical cancer.

Consumer Considerations

When selecting watermelons, consumers should look for fruits with smooth skin and a cream-colored underside.³ When cut, a ripe watermelon will have firm, juicy flesh and dark brown to black seeds. Watermelon is considered immature if there are white streaks in the flesh or if there are white seeds. Like most melons, watermelons spoil easily if they are not kept cool.⁸ Uncut watermelons can be kept at room temperature for no more than two weeks.⁵ When possible, refrigerate watermelons to preserve freshness, juiciness and taste.³

Melons have contact with the ground as they grow and ripen where the skin of the fruit may come in contact with undesirable substances.³ Before cutting into melons, it is advised to wash the rind thoroughly with diluted, additive-free soap or a commercial produce wash using a wet cloth or paper towel to prevent any contamination of the melon flesh. This can also help reduce pesticide residues on conventionally grown watermelons.

Nutrient Profile³¹

Macronutrient Profile (Per 1 cup diced watermelon [approx. 152 g]):

46 calories
1 g protein
11.5 g carbohydrate
0.2 g fat

Secondary Metabolites (Per 1 cup diced watermelon [approx. 152 g]):

Excellent source of:

Vitamin C: 12.3 mg (20.5% DV)
Vitamin A: 865 IU (17.3% DV)

Very good source of:

Potassium: 170 mg (4.9% DV)

Also provides:

Magnesium: 15 mg (3.8% DV)
Vitamin B₆: 0.07 mg (3.5% DV)
Thiamin: 0.05 mg (3.3% DV)
Vitamin E: 0.08 mg (3% DV)
Manganese: 0.06 mg (3% DV)
Dietary Fiber: 0.6 g (2.4% DV)
Iron: 0.4 mg (2.2% DV)
Phosphorus: 17 mg (1.7% DV)
Folate: 5 mcg (1.3% DV)
Calcium: 11 mg (1.1% DV)

DV = Daily Value as established by the US Food and Drug Administration, based on a 2,000-calorie diet.

Image credits:

Watermelon on the vine. Image courtesy of Shu Suehiro.
Watermelon illustration from Birds and Nature volume 15, January 1904.
Watermelon fruit. Image courtesy of Steve Evans.

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