Does eating half a grapefruit before meals help you lose weight?

The short answer

A little — but not because grapefruit is special. The “grapefruit diet” has been around since the 1930s, and one modern trial seemed to back it: people who ate half a grapefruit before each meal lost 1.6 kg over 12 weeks, against 0.3 kg on placebo. But a larger trial then compared grapefruit with a glass of water before meals, and everyone lost the same amount. The likely explanation is simple: starting a meal with something filling and low in calories means you eat a little less of the rest.

So half a grapefruit is a good starter — about 52 kcal, mostly water, with half a day’s vitamin C — but a salad, a bowl of broth or a glass of water can do the same job. And before you start, there is one thing that matters more than any weight effect: grapefruit interacts with a long list of common medicines, including some statins and blood-pressure drugs.

What half a grapefruit gives you

Half a medium grapefruit weighs about 123 g and has about 52 kcal, 2 g of fibre, 8.5 g of natural sugar and 38 mg of vitamin C — close to half the EU and UK daily reference — according to USDA FoodData Central. Pink and red grapefruit also carry some vitamin A as beta-carotene; white grapefruit has very little. It is about 88 per cent water, which is much of why it is filling for so few calories.

For blood sugar, grapefruit is one of the gentlest fruits there is: a glycaemic index of about 25 and a glycaemic load of about 3 per 120 g, the lowest in our fruit and diabetes guide.

What the trials found

The trial that revived the grapefruit diet

In a 2006 study at the Scripps Clinic, 91 adults with obesity took one of four things three times a day before meals for 12 weeks: half a fresh grapefruit, a glass of grapefruit juice, grapefruit capsules, or a placebo capsule with apple juice. The fresh grapefruit group lost 1.6 kg, the juice group 1.5 kg, the capsule group 1.1 kg and the placebo group 0.3 kg. People eating fresh grapefruit also had lower insulin levels two hours after a sugar drink, a sign of better insulin sensitivity.

That sounds impressive, but the differences are small — about 1.3 kg over three months compared with placebo — the trial was modest in size, and the comparison drink contained sugar.

The trial that tested it against water

A 2011 trial at Vanderbilt University asked a sharper question. After two weeks of cutting calories, 85 adults with obesity took a preload of 127 g of grapefruit, grapefruit juice or water before three meals a day for 12 weeks, alongside a lower-calorie diet. All three groups lost about 7 per cent of their body weight, with falls in body fat and waist size — and there was no significant difference between them.

Put together, the trials suggest that a low-calorie “preload” before meals helps people eat less, and grapefruit is one pleasant way to do it. There is no good evidence that grapefruit burns fat or has a special effect on weight beyond that. The idea that it contains fat-burning enzymes is a myth.

A week of half a grapefruit before meals: what to expect

When	What is likely
From the first meal	You may feel fuller sooner and eat a little less of the main course. It is 52 kcal instead of whatever else you might have started with.
Through the week	A big lift in vitamin C: half a grapefruit before two meals gives more than a day’s reference amount. Little change in digestion — grapefruit is low in fibre compared with pears or prunes.
On the scales	Probably nothing you can separate from normal daily swings. In the trials, the losses built up over 12 weeks, and only alongside eating less overall.

Some people find grapefruit acidic on the stomach, or that it triggers heartburn; if so, eat it with or after food rather than on an empty stomach.

Grapefruit and medicines: read this first

Grapefruit contains natural compounds called furanocoumarins that block an enzyme in the gut wall, CYP3A4, which normally breaks down many medicines before they reach the bloodstream. With that enzyme blocked, more of the drug gets through, and blood levels can rise to the point of harm. A 2013 review in the Canadian Medical Association Journal counted more than 85 medicines that interact with grapefruit, many of them with serious possible effects.

The effect can last a day or more after a single grapefruit or glass of juice, so taking your tablets at a different time of day does not reliably avoid it. Medicines affected include:

• Some statins, such as simvastatin and atorvastatin, where higher levels raise the risk of muscle damage.
• Some blood-pressure medicines, such as felodipine and nifedipine.
• Some heart-rhythm medicines, such as amiodarone.
• Some medicines that suppress the immune system, such as ciclosporin and tacrolimus.
• Some anti-anxiety, anti-sickness, cancer and HIV medicines.

Not every medicine in these groups is affected, and the list changes as new drugs appear. The US Food and Drug Administration and the patient leaflet in your medicine’s box are good places to check; a pharmacist can tell you in a minute. Seville oranges (used in marmalade), pomelos and some grapefruit hybrids contain the same compounds.

How to do it well

• Check your medicines first. If any of them interact, choose an orange or another low-calorie starter instead.
• Eat the fruit, not the juice. The fruit is more filling and its sugars are not free sugars. Juice worked in one trial, but it is easy to drink more calories than you meant to.
• Skip the sugar. A spoonful of sugar or honey on top undoes much of the point.
• Treat it as one tool. Any filling, low-calorie starter — salad, vegetable soup, a large glass of water — works in the same way. Choose the one you will keep doing.
• Look after your teeth. Grapefruit is acidic; rinse with water afterwards and wait a while before brushing.

The bottom line

Half a grapefruit before meals is a reasonable, low-calorie way to take the edge off your appetite and lift your vitamin C — and in trials, people who used it lost a little weight over three months. So did people who drank water instead. It is not a fat burner, and a week will not show on the scales. If you take any regular medicine, check with a pharmacist before making grapefruit a habit.