If you look on the Internet you will find several references to the so-called ‘Grapefruit diet’.
It’s also called the 'Hollywood diet' by some and others have erroneously called it the 'Mayo Clinic diet', a title which the Mayo Clinic has tried to shake off for many years.

I did a search using more than one search engine that accesses public medical libraries and there still does not seem to be much hard evidence in the scientific libraries to support all the claims made for the grapefruit diet.
One preliminary study that did support the grapefruit diet was done over 12 weeks by the Scripps Clinic in the USA. I will mention what their findings were but take note that this study does not appear to have been published and this is purely a report by the head researcher of the study.

The 12-week pilot study, led by Dr. Ken Fujioka, monitored weight and metabolic factors (such as insulin secretion) of the 100 men and women who participated in the Scripps Clinic “Grapefruit Diet” study. On average, participants who ate half a grapefruit with each meal lost 3.6 pounds, while those who drank a serving of grapefruit juice three times a day lost 3.3 pounds. (However, many patients in the study lost more than 10 pounds)

I’m still taking the abovementioned study with a pinch of salt because the body of scientific literature regarding weight loss supports a calorie restricted diet composed of mainly raw fruits and vegetables, avoiding refined foods and exercising at least 4- 5 times per week for 30 minutes.

What I did find in the literature is that grapefruit is, as we all know, a VERY healthy food to consume.
Here are some other benefits that are reported in scientific literature

1) Grapefruit contains antioxidants which protect the human body in a variety of ways.
2) Grapefruit contains phytochemicals, especially phenolics, which are suggested to be the major bioactive compounds that cause fruits and vegetables to have their health benefits.
3) Dietary grapefruit pectin supplementation inhibits hypercholesterolemia and appears to be proportionately protective against atherosclerosis.
4) Some studies suggest that the active ezyme in grapefruit called naringenin may possess therapeutic potential as an anti-proliferative agent suppressing the growth of cancer cells.
5) Flavones like naringenin and hesperetin reach relatively high concentrations in blood plasma after ingestion of orange juice or grapefruit juice. Considerable health effects could ensue in individuals consuming them regularly. These benefits are associated with their anticarcinogenic and antioxidative properties.
6) Naringenin, one of the active flavones in grapefruit, has powerful plasma lipid lowering properties which means it can reduce LDL cholesterol and thus reduce total cholesterol.
This flavone also increases antioxidant activity in the bloodstream which is essential for health.

The bottom line:

a) There are amazing benefits in consuming 5 – 9 portions of raw fruits and vegetables on a daily basis including grapefruit.
b) Although some natural substance like grapefruit may assist your body in the weight loss process, at the end of the day burning more energy than what you consume makes you lose weight.

Regards
Dr Anton