Watermelon

Watermelon (Citrullus lanatus)

Watermelon (in Hebrew “אבטיח”) is a fruit of the gourd family made up of about 91% water — and that explains the two traits that determine almost everything about it: it is very low in calories (30 per 100 grams, among the lowest of fruits), and at the same time very sweet relative to its nutrient density. But beyond water and sugar it contains two active compounds that make it more interesting than it seems: lycopene and citrulline.

Nutrition facts per 100 grams (fresh watermelon)

A typical home slice (about 280 grams) provides about 85 calories and 17 grams of sugar — that is, an amount similar to a medium piece of fruit, even though it feels like “just water.”

The two active compounds

Lycopene. Watermelon is one of the richest sources of lycopene — more than fresh tomato (about 2,570 mcg per 100 grams). It is a carotenoid that gives the red color and has been studied in connection with reducing oxidative stress, heart and blood vessel health, and in observational studies also prostate cancer. Unlike tomatoes, where cooking releases lycopene, in watermelon the lycopene is already bioavailable in the fresh fruit. Very ripe watermelon contains more lycopene than watermelon picked early.

Citrulline (L-citrulline). An amino acid that the body converts to arginine and from there to nitric oxide (NO) — a molecule that dilates blood vessels. Small studies found a modest drop in systolic blood pressure and improved blood flow following consumption of watermelon juice or a citrulline supplement. An important caveat: the doses tested in studies were high (about 3-6 grams of citrulline), while 100 grams of watermelon provide only about 100 mg. The highest concentration of citrulline is actually in the white rind, which most of us throw away.

Benefits

• Effective hydration: 91% water along with potassium, magnesium and a small amount of sodium — a combination that helps maintain fluid balance under heat stress and after exertion. Practically speaking, it is one of the most effective fruits for replenishing fluids in summer.
• Low in calories relative to volume: 30 calories per 100 grams allows a large, filling serving at a low calorie cost. This is a real advantage when in a calorie deficit.
• Muscle recovery: small studies on athletes found reduced muscle soreness 24 hours after exertion following drinking watermelon juice, probably via citrulline.
• Eye and skin health: vitamin A, beta-carotene and vitamin C, which contributes to collagen production.
• Essentially free of fat and sodium.

Downsides and considerations

• High glycemic index, low glycemic load. Watermelon has a glycemic index of 72-80 — high. But the glycemic load in a 120-gram serving is only about 4-5, because the actual amount of carbohydrate is low. What this means: a small serving is not a problem even for people with diabetes, but a huge serving — and watermelon invites exactly that — quickly adds up to a significant load. The practical solution is to pair it with a protein or fat source (feta cheese, almonds) that will slow absorption.
• Low in fiber: only 0.4 grams per 100 grams — very low for a fruit. The sugar is absorbed relatively fast, and there is none of the restraining effect that fiber provides in an apple or a pear.
• High FODMAP: watermelon is rich in free fructose and mannitol, and is considered a high-FODMAP food. It is one of the common triggers of bloating and gas in people with irritable bowel syndrome (IBS).
• Potassium in kidney disease: the amount per 100 grams is moderate (112 mg), but large servings add up. Dialysis patients or people with chronic kidney failure should limit it according to medical guidance.
• Low nutrient density: watermelon is relatively low in vitamins and minerals compared with most fruits. It is an excellent choice for hydration and volume, but no substitute for more nutrient-dense fruits.
• Cutting and storage hazard: cut watermelon kept at room temperature is a convenient medium for bacteria because of its relatively high pH and sugar content. Refrigerate within two hours of cutting.

Practical use

• A reasonable serving: 200-300 grams (2-3 slices) — about 60-90 calories.
• The white rind is edible and richer in citrulline than the red part. You can grate it into a salad or a smoothie.
• The seeds are edible and provide magnesium, zinc and protein — roasted watermelon seeds are a common snack in the Middle East and Africa.
• To choose a ripe watermelon: a yellow-cream spot on the side that rested on the ground (not white), a high weight for its size, and a deep sound when tapped.
• Frozen watermelon retains its lycopene and is great for smoothies.

Summary

Watermelon is a low-calorie fruit (30 per 100 grams) that excels at hydration, lycopene and citrulline — and on the other hand is low in fiber, has a high glycemic index, and is high in FODMAPs. In a reasonable serving of 200-300 grams it has almost no downsides; the problem arises only when the serving becomes half a watermelon, when 40-50 grams of sugar are absorbed almost unchecked. The one practical tip worth remembering: the white rind, which most of us throw away, is richer in citrulline than the fruit itself.