Nutritional value, cooling properties, and food safety of watermelon: Who should pay attention to portion size?

In the sweltering summer, a slice of chilled watermelon is almost everyone's first thought of fruit. This fruit, originally from Africa and also known as the "cold melon" or "summer melon," has the English name "Watermelon," which literally translates to "watery melon"—its name itself speaks volumes about its value. Common varieties in Taiwan include the large red watermelon, the yellow-fleshed small jade watermelon, seedless watermelons, and the Black Beauty watermelon, all known for their refreshing taste and high water content, making them one of the most popular summer foods.

However, there are many debates surrounding watermelons: Which is more nutritious, red or yellow flesh? Does lycopene need to be consumed with oil for absorption? Watermelons are so sweet, can diabetics eat them? What is the "watermelon-eating-at-dusk-day-off-happiness"? This article will break these down one by one, and also fill in the most practical, yet most often overlooked, part—How to store it safely after cutting?。

I. Why is watermelon a popular summer fruit for cooling off?

Watermelon flesh approximately 91% is moisture.This ratio is higher than most fruits, making it especially thirst-quenching on hot days—it provides both hydration and a touch of natural sugar, making it easier to drink than just water. According to...U.S. Department of Agriculture FoodData CentralEach 100 grams of watermelon pulp contains approximately 30 kcalIt contains approximately 112 mg of potassium, as well as small amounts of vitamin C, magnesium, and lycopene.

"How much to eat per day" can also be very specific. The Hong Kong Department of Health recommends...Daily Two Plus Three"This means at least two servings of fruit and three servings of vegetables per day; and"Department of Health's "Living a Healthy New Direction"Lie Ming,Half a bowl of fruit chunks (such as watermelon, wrinkled melon, or honeydew melon) equals one serving of fruit.In other words, casually eating three or four slices of watermelon after a meal is almost equivalent to two servings of fruit—this will be explained further in the "Calories" section later.

In traditional Chinese medicine theory, watermelon is cold in nature and sweet in taste. Ancient texts describe it as having a "cold nature and sweet taste."Natural White Tiger SoupKnown as "watermelon rind," it is used to clear heat, relieve summer heat, and quench thirst; watermelon rind itself is also a traditional Chinese medicine, called "watermelon rind."Watermelon rindIt is sweet and cool in nature, and in traditional Chinese medicine it is used to clear heat, quench thirst, and promote urination (see...).The School of Chinese Medicine, The Chinese University of Hong KongIt needs to be clarified here:The above is a description of the theoretical system of Traditional Chinese Medicine.This helps explain why eating watermelon is especially comforting in summer; however, if you experience any discomfort, you should still consult a doctor or a registered Chinese medicine practitioner, rather than trying to treat yourself with food therapy instead of seeking medical attention.

II. What are the nutritional differences between red-fleshed and yellow-fleshed watermelons?

Red-fleshed and yellow-fleshed watermelons have different nutritional focuses, with the biggest difference being...LycopeneResearch by the U.S. Department of Agriculture's Agricultural Research Service (ARS) indicates that lycopene is mainly found in...Red meatThe variety of orange-fleshed watermelon is only available in small quantities.There are almost no yellow-fleshed varieties.Seedless watermelons generally have a higher lycopene content than seeded varieties (see...). USDA ARS｜Perkins-Veazie & Collins 2006Therefore, if the goal is lycopene, choosing red meat is more direct.

A comparison of common nutrients is as follows (per 100 grams of fruit pulp, source: USDA FoodData Central):

Nutrients	watermelon	Comparison
calorific value	Approximately 30 kcal	Low-calorie fruit
Moisture	Approximately 91 grams	Higher than most fruits
Lycopene	Approximately 4,532 micrograms (red meat)	Higher than raw tomatoes (approximately 2,573 micrograms)
beta-carotene	Approximately 303 micrograms	It is higher than orange (about 71 micrograms) and similar to papaya (about 276 micrograms).
Vitamin A (RAE)	Approximately 28 micrograms	Higher than orange (approximately 11 micrograms)
Vitamin C	Approximately 8.1 mg	Significantly lower than kiwi (approximately 93), strawberry (approximately 59), and orange (approximately 53).
鉀	Approximately 112 mg	It is considered medium to low in quality among fruits.
Dietary fiber	Approximately 0.4 grams	Low content

There are two points worth remembering here. First,Red watermelons do indeed have a higher lycopene content than raw tomatoes.—Many people only remember tomatoes, neglecting the very affordable source of watermelon. Secondly,Watermelon is actually not high in vitamin C.This isn't bad news, just a reminder: if you want to supplement your vitamin C intake, kiwis, strawberries, and oranges are more effective; watermelons excel at hydration and providing lycopene, but these have different roles and don't need to replace each other.

3. Is it true that "lycopene needs to be combined with oil to be absorbed"?

This statement is correct in its direction, but it's often presented too rigidly. Lycopene belongs to...fat solubleCarotenoids are generally absorbed more efficiently when eaten with fatty foods—as noted by the National Cancer Institute (NCI) when discussing tomato products.Cooked and oilyBest absorption occurs at this time (see) NCI）。

However, watermelons have one thing that's different:Lycopene in watermelon is mainly in the "cis" structure.This structure is more easily absorbed by the body than the trans structure commonly found in tomatoes, so it doesn't require heating to improve its utilization rate like tomatoes do. The practical conclusion is simple:There's no need to deliberately stir-fry watermelon with oil, nor is it necessary to change how you eat it.Eating watermelon before or after a regular meal already meets the condition of "with oil"; the saying that "watermelon is better absorbed after a meal" comes from this perspective.

As for whether it's good to eat on an empty stomach, the key point is not lycopene, but the stomach and intestines and sugar – see Section 7.

4. Don't rush to throw away the watermelon rind: the white part contains citrulline.

The white part that's peeled off when cutting a watermelon (the layer between the flesh and the green rind), called "watermelon rind" in Traditional Chinese Medicine, can be used in cooking. It contains dietary fiber and a special amino acid—CitrullineUSDA ARS data shows that citrulline is present in all parts of a watermelon, while...The concentration is relatively high in the rind.。

Citrulline is converted into arginine in the human body, and arginine is a precursor to the body's production of nitric oxide. This pathway is related to vasodilation, which is also the main reason why watermelon has been studied for its cardiovascular health benefits in recent years.

So, is the claim that "eating watermelon lowers blood pressure" credible? It can be understood this way: most current studies showing improvements in vascular function use...Watermelon extract or high-dose citrulline supplementsFor example, a study of 14 adults with prehypertension or stage 1 hypertension had them consume 6 grams of watermelon extract (containing approximately 4 grams of citrulline) daily for six weeks. The results showed improvements in arm and ankle blood pressure and carotid wave reflex (see...). Figueroa et al. 2012, Am J HypertensAs forFresh watermelon in a normal serving sizeThe results of the studies were inconsistent, with some studies not observing a significant reduction in blood pressure (see the summary of relevant studies). Volino-Souza et al. 2022, Nutrients）。

To be fair: Watermelon contains components beneficial to vascular health and can be a refreshing option in a balanced diet. However, eating watermelon regularly does not replace blood pressure medication or blood pressure management. If you have high blood pressure, you should still take medication and monitor your blood pressure as directed by your doctor. For dietary suggestions, please refer to [link/reference needed].<Prevention of Cardiovascular Problems>。

In terms of preparation, the white part tastes like cucumber with a mild flavor, and is commonly prepared in Chinese cuisine.Peel off the green outer skin, shred it and use it in cold dishes, or cut it into strips and use it in soups.This method avoids wasting ingredients and also results in a serving of watermelon containing more fiber than eating red meat alone. It's important to note that:The watermelon rind must be thoroughly washed with running water first.Cut again, because once the knife cuts through the skin, it will bring the surface microorganisms into the flesh (see Section 10).

5. It's not high in calories, but it's easy to overeat.

Watermelon contains about 30 kcal per 100 grams, which is not considered a high-calorie fruit. The problem lies in its "easy to eat" nature—its high water content and refreshing taste make it easy to unknowingly consume more than intended.

Based on the official conversion mentioned earlier:Half a bowl of watermelon chunks = a serving of fruitGenerally, three to four slices of watermelon constitute about 1.5 to 2 servings of fruit, containing approximately 90 to 120 kilocalories. This number itself isn't high, but if eaten "casually" after a main meal, it adds extra calories and sugar to the meal. To eat it lightly, treat watermelon like a...A serving of fruitArrange it this way—use it to replace the day's fruit portion, instead of adding to the original portion.

VI. High sweetness does not equal rapid glycemic index: the GI and glycemic load of watermelon

"Watermelons are so sweet, must their glycemic index be very high?" This is the most common question, and the answer is more complex and practical than you might imagine.

To state the conclusion first:Watermelon does have a high glycemic index (GI), but a very low glycemic load (GL).According to data from the Human Nutrition Unit at the University of Sydney, the GI of watermelon is approximately... 72(GI ≥ 70 is considered high), but a 120-gram serving of watermelon contains only about 6 grams of available carbohydrates, resulting in a glycemic load of only about [missing information]. 4 (GL ≤ 10 is considered low)The reason is simple: watermelons are over 90% water.The amount of carbohydrates is naturally lower for the same weight.。

The division of labor between GI and GL is as follows: GI describes "how quickly carbohydrates raise blood sugar," while GL also takes into account "how much carbohydrate you actually consume." Take watermelon as an example: it raises blood sugar quickly, but the increase is limited each time.The key is the portion size.A large bowl of watermelon chunks versus a small slice of watermelon can have a GL (gross quotient) that differs by several times.

Therefore, for people who need to monitor their blood sugar, the practical approach is not to "completely avoid" it, but rather:Control portion sizes (e.g., half a bowl to a bowl of fruit chunks), eat with protein or fat-rich foods, and avoid eating large amounts on an empty stomach.Regarding the overall dietary principles and individualized goals for blood sugar control, it is recommended to discuss them with a doctor or registered dietitian; for the role of fruits in daily life, please refer to [link to relevant information].<Sugar and Fructose Metabolism in Fruits>。

7. "Watermelon Reaction at Midnight": The Cold Nature of Watermelon in Traditional Chinese Medicine and the Modern Fructose Mechanism

There's a saying in Taiwan, "Eating watermelon at midnight can cause illness," meaning that eating too much watermelon on an empty stomach can easily lead to abdominal pain and diarrhea in the middle of the night. Traditional Chinese medicine explains this by saying that watermelon is cold in nature, and people with weak stomachs or a cold constitution are more likely to feel uncomfortable eating it on an empty stomach.

Interestingly, this long-standing experience...There are corresponding mechanisms in modern research.Watermelon is considered a high-FODMAP food in terms of nutrition, mainly for two reasons:

1. It contains more fructose than glucose.When the human small intestine absorbs fructose, it needs glucose to be present for efficient absorption. When the proportion of fructose is higher than that of glucose, the excess fructose is not easily absorbed and will enter the large intestine where it will be fermented by bacteria, producing gas and causing bloating.
2. Contains polyols.These types of sugar alcohols can draw water into the intestines through osmosis, increasing the water content of the intestinal contents, which may cause loose stools or even diarrhea.

In other words, "eating a large amount of watermelon on an empty stomach" delivers a large amount of fructose and sugar alcohols into the intestines at once—which aligns with the traditional advice of "not eating on an empty stomach and eating in moderation." Here, it's important to clarify one point:It is not fiber that causes bloating and diarrhea.A cup of sliced watermelon contains only about 0.6 grams of dietary fiber, mainly due to its sugar content combined with a large amount of water and volume. People with irritable bowel syndrome or fructose malabsorption will experience particularly pronounced effects.

Conversely, if the diarrhea is caused by "leaving it out for too long after cutting," then it is not a matter of constitution, but a matter of food safety—see Section 10.

8. Who should pay attention to portion sizes?

Watermelon is a safe everyday fruit for most people, but the following groups should pay attention to portion sizes:

ethnic groups	Key Points
Patients with poor kidney function who need to restrict potassium	Watermelon actuallyLow potassium fruit(Approximately 112 mg per 100 grams), the UK NHS's low-potassium diet guidelines for kidney patients list watermelon as "low-potassium," recommending a serving size of...a small pieceThose who need to restrict potassium should still include watermelon in their daily potassium intake and adjust it according to medical advice; watermelon juice is easy to drink in large quantities at once, so extra care should be taken.
Those taking potassium-sparing diuretics or ACEI/ARB antihypertensive drugs	These types of medications can raise blood potassium levels, and the UK Medicines and Healthcare products Regulatory Agency (MHRA) has warned of the risk of hyperkalemia when consuming them with potassium-rich foods. Watermelon doesn't need to be avoided, but the amount and blood potassium monitoring should be discussed with your doctor.
People who need to control their blood sugar	As discussed in Section 6, the focus is on portion sizes and meal planning, not on completely abstaining from food. Consult your doctor or registered dietitian for an overall dietary plan.
Patients with sensitive stomach and irritable bowel syndrome	Watermelon is a high-FODMAP food, so it is recommended to start with small portions and observe your tolerance (see Section 7).
Toddlers	When giving watermelon to children,Remove the seeds and cut into small pieces.It's important to note that while sunflower seeds won't germinate in the body after being swallowed, small, round, hard foods pose a choking risk. Removing the seeds and cutting them into small pieces is for safety.
Postpartum, post-surgery, or those with a cold constitution	Traditional Chinese medicine considers watermelon to be cold in nature, so people with this condition should avoid eating too much at once, especially on an empty stomach. Since postpartum and post-operative diets need to be adjusted according to individual circumstances, it is recommended to consult medical professionals or registered TCM practitioners first.

You should seek medical attention as soon as possible if the following situations occur, rather than continuing to observe on your own:If diarrhea persists for more than 48 hours without improvement; signs of dehydration appear (significantly reduced urine output, excessive thirst, dizziness); bloody or tarry stools; persistent severe abdominal pain or high fever. If you have kidney disease, heart disease, or diabetes, and experience significant discomfort or worsening of your condition after eating, you should contact your doctor as soon as possible.

9. What is the relationship between watermelon frost and watermelon?

Many people are curious whether commercially available "watermelon frost" is made from watermelon. The answer is:It is indeed related to watermelon, but the main ingredient is not the pulp.

According to the School of Chinese Medicine, The Chinese University of Hong KongThe article "The Miraculous Watermelon Frost"The explanation is that watermelon frost is made by...Watermelon and Glauber's salt (natural sodium sulfate decahydrate crystals)The white powder formed after processing together. The traditional method is to cut open the watermelon, remove the flesh, fill it with Glauber's salt, put the watermelon back on and fix it, and place it in a cool and ventilated place. After a period of time, a white powder will be released from the outside of the watermelon rind, which can be collected to make watermelon frost.

Here we need to correct a common misconception:Watermelon frost is not "frost that grows on a watermelon rind after it has been left out for a long time and has become moldy".Its formation is related to the crystallization of Glauber's salt, and its main component is sodium sulfate; while commercially available traditional Chinese medicine preparations (such as sprays and lozenges) contain not only watermelon frost itself, but also other medicinal materials such as borax, phellodendron bark, coptis, rhubarb, and borneol, so the finished product is usually not pure white.

In Traditional Chinese Medicine (TCM) theory, watermelon frost is salty and cold in nature, and is used externally to clear heat and purge fire, reduce swelling and relieve pain. It is traditionally used for conditions such as sore throat, oral ulcers, and swollen gums. It is important to emphasize that:This falls under the category of traditional Chinese medicine, not as an everyday food, and should not be regarded as a method for treating oral or throat problems at home.If you feel unwell, you should consult a doctor or a registered Chinese medicine practitioner. The original text from the Chinese University of Hong Kong's School of Chinese Medicine also clearly recommends that "if you feel unwell, you should not buy Chinese medicine or prepared Chinese medicine to treat it yourself. It is recommended to seek treatment from a professional Chinese medicine practitioner."

10. How to store watermelon? There's a big difference between storing it whole and cutting it.

One of the most common misconceptions about storage is that "watermelons are not suitable for the refrigerator." This statement...Only applicable to whole, uncut watermelonsOnce cut open, the rules are completely reversed.

Whole and uncut: Room temperature is conducive to lycopene accumulation.

A study by the U.S. Department of Agriculture stored whole red-fleshed watermelons at 21°C (approximately room temperature), 13°C, and 5°C for a total of 14 days, and found that...Watermelons stored at 21°C can have their lycopene content increase by up to 40%, and their beta-carotene content also shows a significant increase.The nutrient content of the refrigerated group remained largely unchanged, but the low temperature inhibited this ripening process, and prolonged refrigeration (about a week) was more likely to cause chilling injury and accelerate spoilage (see...). Perkins-Veazie & Collins 2006, J Agric Food ChemTherefore, it is reasonable to buy a whole watermelon and place it in a cool, ventilated place for a few days; if you want it to be chilled, put it in the refrigerator for a few hours before eating.

After cutting: It must be refrigerated, and attention should be paid to the time limit.

Once cut open, the fruit loses its protective skin, and the knife, cutting board, and hands all increase the chance of contamination.The preservation principles are the same as for other perishable foods.Hong Kong Centre for Food SafetyFood Safety Tips for Fruits and Fruit ProductsList:

• Cut or peeled fresh fruit, if left at room temperatureLess than 2 hoursCut fruit can be refrigerated and kept for later use, or consumed within a total time limit of 4 hours; refrigerated cut fruit should be kept within [time period missing]. Within 1 to 2 daysFinished eating.
• If it has been placed at room temperatureMore than 2 hours but less than 4 hoursIt should be consumed within the total time limit of 4 hours.And it should not be put back in the refrigerator.。
• If it has been placed at room temperatureMore than 4 hoursIt should be discarded immediately.

Three other practical tips:Before cutting it open, thoroughly rinse the entire melon (including the skin that you don't want to eat) with running water.The hard skin can be gently brushed with a clean fruit brush, but soap or detergent should not be used;Use a dedicated fruit knife and cutting board.Do not use the same tools as those used for handling raw meat or fish.Buy as much as you want, eat as much as you want.Ideally, cut and eat immediately. If you can't finish it, wrap the cut surface tightly with cling film and refrigerate. To determine if a cut watermelon is spoiled, rely on its smell, texture, and appearance (sourness, stickiness, seepage, abnormal discoloration), rather than its "okay-looking" appearance—pathogenic bacteria don't necessarily make food smell bad immediately, so if in doubt, throw it away.

XI. Frequently Asked Questions

Can watermelon provide vitamin C?

Yes, but it's not very efficient. Every 100 grams of watermelon contains only about 8.1 milligrams of vitamin C, significantly less than kiwifruit, strawberries, and oranges. You can eat watermelon to replenish water and lycopene, but if you want to supplement vitamin C, the latter is a more direct option.

Is it better to have red or yellow watermelon?

It depends on your goals. Red meat has a much higher lycopene content (yellow meat has almost no lycopene), and it also contains both beta-carotene and vitamin A; yellow meat has a sweeter taste and is also hydrating. If you want to get more lycopene, choose red meat varieties.

Should watermelon be eaten with oil?

No need to do it deliberately. Lycopene is fat-soluble, and eating it with oil can improve its absorption. However, the lycopene in watermelon is mainly in the more easily absorbed cis structure. It is sufficient to eat it with a normal meal, without the need to stir-fry it with oil.

Is it okay to eat watermelon on an empty stomach?

For people with healthy digestive systems, eating watermelon in moderation is fine. However, eating a large amount of watermelon on an empty stomach is equivalent to consuming a large amount of fructose and sugar alcohols at once, which can easily lead to bloating, flatulence, or even loose stools. People with sensitive digestive systems are advised to eat it after meals and control the portion size.

Watermelons are so sweet, can diabetics eat them?

Yes, it's possible, but the key is portion size and food pairing. While watermelon has a high glycemic index, its glycemic load is low. Half a bowl to a bowl of fruit chunks is a reasonable reference portion size, and it can be eaten with protein or fatty foods. Please discuss your individual blood sugar goals and dietary plans with your doctor or registered dietitian.

Can eating watermelon help with hangovers?

This is a common saying, mainly because watermelon is hydrating, while alcohol metabolism itself causes dehydration. But it needs to be clarified:Alcohol is primarily metabolized by the liver, and eating watermelon will not speed up this process.While hydration after drinking alcohol can be helpful, the truly effective approach is to control the amount of alcohol consumed. Those taking medication should especially avoid alcohol; see details below.<Drug-Alcohol Interactions>。

Is it okay to eat watermelon rind?

The white part is fine; it tastes like cucumber and is commonly prepared as a cold dish or soup. However, the outer skin must be thoroughly washed with running water before cutting. In Traditional Chinese Medicine, this layer is called "watermelon rind" and is considered a medicinal herb.

Is watermelon frost made from watermelon?

Partially correct. Its raw material comes from watermelon, but it needs to be processed together with Glauber's salt to crystallize and produce the product. It is not "watermelon rind getting moldy after being left out for a long time." Watermelon frost falls under the category of traditional Chinese medicine and should not be purchased or used without consulting a doctor or registered traditional Chinese medicine practitioner.

Is it still edible if a cut watermelon has been left out for a few hours?

Consider the time and temperature. Food stored at room temperature for less than 2 hours can be refrigerated and used later; food stored for more than 2 hours but less than 4 hours must be eaten within 4 hours and should not be put back in the refrigerator; food stored for more than 4 hours should be discarded. Be even more conservative in hot summer weather.

When should you see a doctor?

If diarrhea lasts for more than 48 hours, or if signs of dehydration appear, or if stools are bloody or black and tarry, or if there is persistent severe abdominal pain or high fever, seek medical attention as soon as possible. Individuals with pre-existing kidney disease, heart disease, or diabetes should contact their doctor promptly if they experience significant discomfort after eating.

XII. Sources of Information

• U.S. Department of Agriculture FoodData Central — FoodData Central(Nutritional data per 100 grams of watermelon)
• Hong Kong Department of Health's "Living a Healthy New Direction" Common questions about healthy eating: The amount of fruit to serve
• Hong Kong Centre for Food Safety — Food Safety Tips for Fruits and Fruit Products
• The School of Chinese Medicine, The Chinese University of Hong Kong (Dr. Mo Dilin) — Magic Watermelon Frost
• The UK's NHS (North Bristol NHS Trust) — Low potassium diet: information for kidney patients
• U.S. Department of Agriculture Agricultural Research Service (ARS) — Watermelon, phytochemicals and health
• Perkins-Veazie P, Collins JK. Carotenoid changes of intact watermelons after storage. J Agric Food Chem. 2006;54(16):5868–5874. — PubMed
• Volino-Souza M, Oliveira GV, Conte-Junior CA. Current Evidence of Watermelon (Citrullus lanatus) Ingestion on Vascular Health. Nutrients. 2022;14(14):2913. — PubMed
• Figueroa A, et al. Watermelon extract supplementation reduces ankle blood pressure and carotid augmentation index. Am J Hypertens. 2012;25(6):640–643. — PubMed
• Kavanaugh CJ, Trumbo PR, Ellwood KC. The US FDA's evidence-based review for qualified health claims: tomatoes, lycopene, and cancer. J Natl Cancer Institute. 2007;99(14):1074–1085. — PubMed
• The National Cancer Institute (NCI) — Prostate Cancer, Nutrition, and Dietary Supplements

Health Disclaimer:The content of this article is for general health information only and does not constitute medical advice for diagnosis, treatment, or prevention of any disease, nor can it replace a professional assessment by a doctor or registered dietitian. Any dietary adjustments or use of supplements, especially if you have a chronic illness, are taking medication, are pregnant or breastfeeding, or have infants or elderly family members, should be discussed with a doctor or relevant professional first.

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