Research Review: The grapefruit diet - fad or science? | Precision Nutrition

Research Review: The grapefruit diet – fad or science?

By Jennifer Koslo, Ph.D.


Eat half a grapefruit before each meal and lose 10 pounds in 10 days!  Citrus does have a few powerful antioxidants with known cholesterol and blood pressure lowering effects. But can it help with weight loss? A recent study looked at the age-old claim in an effort to get some answers.

Introduction — Quack, quack goes the duck

“Who’s that waddlin’ down the street
It’s just me ‘cause I love to eat
Who’s real flabby? Yes, I am!
Every picture of me’s
Gotta be an aerial view
Now my doctor tells me
There’s just one thing left to do

Grapefruit Diet (Diet!)
Throw out the pizza and beer
Grapefruit Diet (Diet!)
Oh, get those jelly donuts out of here
Grapefruit Diet (Diet!)
Might seem a little severe
Grapefruit Diet (Diet!)
I’m gettin’ tired of my big fat rear
Blow, fatty!”

– Lyrics from Wierd Al Yankovic’s Hit “The Grapefruit Diet”

Lose up to 10 pounds in 10 days! Quack, quack, ding, ding – that statement should set off some warning bells and let you know there is some quackery afoot.

Satirized with the usual irreverence and mild cruelty by Weird Al Yankovic, fad diets will never die — not as long as we live in a world where people seek immediate gratification. Unfortunately for many vulnerable or hurried consumers, the appeal of instant results perpetuates the demand for information, products, or plans promising overnight success.

The Grapefruit Diet, also known as the Hollywood Diet, or the Mayo Clinic Diet (although it’s not even remotely associated with the real Mayo Clinic), is based on the premise that grapefruit possesses near-magical powers.

Proponents of the diet claim that eating half a grapefruit before meals high in protein and fat produces a metabolic reaction that transforms even the meekest office worker into a magnificent fat-burning machine. The thermogenic powers supposedly stem from a special fat-burning enzyme in grapefruit that acts as a catalyst to help your body incinerate high fat foods, which in turn results in fast weight loss.

Grapefruit – a thermogenic super fruit?

This is all well and good, but did you read the fine print stating that you also need to restrict your total daily calorie intake to 800 calories per day and eliminate virtually all carbohydrates, even the “good” ones like fruits, vegetables, and whole grains?

So where do the 800 calories come from? You guessed it: meat, meat, and more meat. Sounds a bit reminiscent of Atkins, doesn’t it? But actually, Atkins followed the Grapefruit Diet, and not the other way around.

Fad diet history lesson

Over eighty years. The Grapefruit Diet has been around that long. Eighty years ago puts us somewhere in the 1930s — the time when Pluto was discovered, the Mickey Mouse comic strip first appeared, and Hostess Twinkies were invented (1). Hmm, I wonder if the Twinkies had anything to do with it?

The diet made a big resurgence in the 1970s when it was promoted as the “Mayo Clinic Diet.” As you can imagine, the doctors and dietitians at Mayo weren’t too happy about that.

Fun factoid: The Cabbage Soup Diet makes similar weight loss claims to the Grapefruit Diet (lose 10 pounds in 10 days). However, the origin of that diet isn’t as clear and there are no accompanying arguments that cabbage has special fat-burning powers. It is simply a very low calorie diet that can make you really gassy. Ewww.

Science or myth?

Have any clinical research studies evaluated grapefruit’s fat-burning enzyme potential? The answer to that is yes and no.

Early studies did suggest that subjects on the grapefruit diet would lose weight, but this was most likely due to calorie restriction rather than any special fat-burning properties of the grapefruit itself.

However, before we dismiss the claims of this diet completely, let’s take a closer look.

After all, it’s possible that calorie restriction is not the only reason the diet helps people lose weight. Proponents of the diet could be in the right church but the wrong pew. Maybe they’re wrong about those magical fat-burning enzymes. But maybe grapefruit’s phytochemicals have an effect on weight loss, or maybe something in the fruit promotes satiety.

Satiety and weight loss

Let’s look at satiety and weight loss first. A recent study evaluating the satiating effects of eating or drinking something low in calories before meals compared grapefruit, grapefruit juice, and water. The subjects lost weight, although not a lot. But it didn’t matter which of the pre-meal snacks they had (2). All had some small effect.

However, a second study showed the opposite. Subjects who ate grapefruit lost more weight than those who ate a placebo (3).

Makes you want to scratch your head and wonder, doesn’t it? However, overall it seems that eating any low calorie snack before a meal may help you eat less in total. Grapefruit itself isn’t the key.

Grapefruit and phytochemicals

But what about grapefruit’s phytochemical content? Now we might be on to something.

We know that phytochemicals (plant chemicals) are absolute superstars when it comes to decreasing our risk of contracting a host of chronic diseases.

Citrus, and in particular, grapefruit, contains two of these superstars, flavonones called naringin and hesperidin. Studies conducted in mice and rats have confirmed that naringin and hesperidin act as antioxidants in the fight against free radicals, and reduce cholesterol and blood pressure. However, we can’t be as confident about these effects in humans. We’d need more studies with larger sample sizes to make that call. (2, 3).

In the meantime, scientists have also looked at the effects of concentrated doses of naringin and hesperidin on rats. And at these high doses, we see even more evidence that phytochemicals lower cholesterol and blood pressure. Not only that, but in rats, wait for it… naringin appears to stimulate fat breakdown (4).

But don’t get too excited just yet. Remember, that study focused on rats who were fed a high dose of the concentrated phytochemical, not rats being served half a grapefruit before each meal.

What to make of all this? Clearly more research is needed to see if the phytochemicals could be responsible for weight loss and other health benefits.

Thank goodness researchers are a determined, inquisitive bunch, because this week’s research review builds on the best features of the previous studies in order to see if any clear answers to these ongoing questions can be found.

Research question

This week’s research review aimed to determine if eating half a grapefruit before each meal decreases weight, blood pressure, or cholesterol levels.

Dow, C.A., Going, S.B, Chow, H-H, S., Patil, B.S., Thomson, C.A. The effects of daily consumption of grapefruit on body weight, lipids, and blood pressure in healthy, overweight adults. Metabolism: Clinical and Experimental. 2012, 61:1026-1035 (2012).


This study used a randomized controlled design in which 74 participants were enrolled in two different cohorts separated by a year (winter 2009-2010 and winter 2010-2011). The study timing corresponded to seasonal grapefruit crop production. The study lasted for a total of nine weeks: a three-week “washout” phase followed by a six-week diet intervention.

An important note here on the study design is that it was not “blinded,” meaning that subjects knew which group they belonged to. Potentially, this could produce some bias. Keep that in mind when evaluating the results.


The researchers screened almost 300 people for the study and enrolled a total of 85. Excluded from the study were smokers, post-menopausal women, individuals who had a co-morbidity like diabetes, high cholesterol, etc., those taking certain medications, those whose BMI (body mass index) was too high, and those whose weight was unstable. In other words, the researchers wanted “healthy” overweight individuals.

During the washout diet phase, they lost a few participants, so they ended up with a total of 74 subject. The control group included 25 women and 7 men and the intervention group was comprised of 32 women and 10 men.

Sometimes it’s more difficult to detect a significant difference in a healthy population, but based on some fancy statistical calculations, this sample size qualifies as large enough to determine meaningful differences between the control group and the test group, if any exist.

Diet intervention

The first three weeks of the study was a “washout phase.” Participants followed a restricted diet to clear the body of any foods that might affect the study outcomes.

Because this study was designed to determine whether or not there were active compounds in grapefruit that were responsible for potential health benefits, during the washout phase, participants were not allowed to eat any fruits or vegetables that were high in two classes of phytochemicals: polyphenols and carotenoids. Bummer – that meant no citrus, berries, spinach, carrots, tomatoes, sweet potatoes, grapes, kale, and so on.

Yikes! I would have been one of those in the “discontinued trial because diet was too strict” category.

After the washout, the 6-week intervention started. Both the control and the intervention group had to continue with the washout diet as their “background” diet, with the intervention group supplementing their diet with half a fresh Rio-Red grapefruit 15 minutes before breakfast, lunch and dinner.

How to eat a grapefruit
If you think this was self-explanatory think again. When it comes to research, attention to detail is the name of the game. Participants were given training on just how to eat said grapefruit half before each meal: peel it and eat all portions including the pith — the white part that you normally throw away. Participants were told to eat it because it contains vitamins, antioxidants and fibre. Also, they were not allowed to add sugar. Instead they used a zero-calorie sweetener


Researchers assessed a number of factors.

  • Diet assessment – A registered dietitian collected data about the participants’ dietary intake via telephone using a method of repeat 24-hour recalls. The dietitian collected three recalls during the washout phase and three during the intervention phase and then averaged the reports. While dietary recalls may not provide the most accurate data, averaging a set of results rather than depending on one report alone would increase reliability.
  • Compliance – Researchers gave participants a log book and asked them to keep a detailed record of each time they ate grapefruit during the six-week intervention. The researchers used a formula to analyze for compliance (number of grapefruit eaten before each meal/number of meals x 100).
  • Bioactive content – To get an estimate of the amount of flavonoids in the grapefruits, the researchers took the average weight of 10 grapefruits and then calculated the naringin and hesperitin content using the USDA Database for Flavonoid Content of Selected Foods.
  • Anthropometry – Height, weight, waist and hip circumference and percent body fat of the participants were measured at baseline, week 6 and week 9.
  • Blood pressure – Researchers measured blood pressure (separate measurements for systolic and diastolic), and heart rate in duplicate at baseline, week 6, and week 9. It’s always a good idea to take duplicate measures of blood pressure in order to lessen the chances of measurement error due to “white coat hypertension” (higher than normal blood pressure due to nervousness).
  • Blood collection and processing – The participants had to fast for blood collection 3 times: at baseline, week 6, and week 9. The blood was then analyzed for total cholesterol, HDL, LDL, and triglycerides.


So… does eating half a grapefruit before each meal result in weight loss? Drum roll, please!

The answer: no, at least not in this study.


But if this study didn’t produce any “knock your socks off” results, it did reveal some interesting findings related to the other research questions testing the effects of grapefruit on blood pressure and cholesterol.

  • Anthropometric and blood pressure changes – Both groups lost weight, but not a significant amount. Two measurements that did drop noticeably in the grapefruit intervention group were waist and hip circumference and systolic blood pressure. However, when compared to the control group neither of those changes proved to be a statistically significant effect of the intervention.
  • Energy intake – No surprise here, but fruit intake increased in the grapefruit eating group and decreased in the controls. (Remember, the controls had to follow the restricted diet.) Again no surprise, but vitamin C intake also increased in the intervention group. Calorie intake remained fairly constant in the grapefruit group, indicating that grapefruit eaten before a meal does not result in fewer overall calories eaten.
  • Compliance – Compliance in the intervention group was great – a full 93% recorded in their log books that they ate half a grapefruit before each meal.
  • Bioactive content – Intake of the two phytochemicals being assessed, naringenin (the active form of narginin after a conversion in the gut) and hesperidin were estimated to be 146.2 mg/l and 1.57 mg/respectively. These numbers may not mean a whole lot to most people, but they do provide an idea of the level of flavonoids that can be reached through a reasonable dietary intake. This also helps us compare the results of animal and supplement studies that typically use much higher levels.
  • Lipid profile – Within each group, total cholesterol and LDL decreased significantly by the end of the intervention. But when the two groups were compared, the changes were not significant.Still, statistical significance aside, LDL in the grapefruit group dropped an average of 18 points, which is a large enough change to provoke speculation. What could be causing it? Fibre is known to reduce cholesterol, but in this study, fibre intake in both groups remained fairly constant. That leaves the bioactive compounds of grapefruit as a possible explanation. Fat around the belly (visceral fat) is thought to be a greater predictor of high blood lipids than BMI, so the researchers proposed that the drop in waist circumference is what triggered the drop in LDL. This is definitely a plausible explanation and one worth exploring further.


One thing we have to keep in mind when interpreting data is the difference between statistical significance and clinical relevance.

The subjects in the grapefruit group may not have lost much weight, but they did lose dangerous belly fat. They also lowered their systolic blood pressure and their LDL cholesterol. Because the diet they followed was pretty restricted, it’s reasonable to conclude that the phytochemicals in the grapefruit they ate likely helped.

Another factor that may have played a role was the quantity of vitamin C in their diet. Grapefruit is, of course, rich in vitamin C, and this vitamin is an antioxidant with known beneficial effects on blood pressure. So the improvements noted in these subjects could have arisen from the combined or synergistic effects of the vitamin C and grapefruit’s phytochemicals.

The apparent LDL-reducing effects of grapefruit are particularly interesting. Typically, doctors prescribe statins to people with high LDL. Statins are effective but they come with many nasty side effects. If compounds in grapefruit could reduce the need for these drugs, that would be a good thing. Today’s study seems to suggest that grapefruit could be helpful in this regard.

All studies come with some caveats. As we’ve already noted, the subjects of this particular study knew whether they were in the control or experimental group, and this could introduce an element of bias. For example, they could have changed their level of physical activity, or they could have made other dietary changes not captured in the 24-hour recalls. Looks like a good reason for conducting more studies!

Bottom line

Grapefruit is a nutritious choice. If you enjoy it, by all means, eat it! And feel good about the possibility that it is promoting good health.

But no single food is going to magically make you skinny. Most fad diets are based on folklore. If they work at all, it’s simply because they restrict calories. Who wouldn’t lose weight if they were only eating 800 calories a day? And just how long can the people around you put up with your hunger-induced moods?

These days it seems there’s a fad diet for almost every food – grapefruit, cabbage, even Twinkies! But research investigating the effectiveness of these diets is spare at best.

Regardless, it’s pretty safe to conclude that the claims underlying most fad diets, including the grapefruit diet, are pure bunk. And while grapefruit may hold promise as a cardio-protective food, this only confirms that including ample amounts of all kinds of fruits and vegetables in your diet is your best bet for overall health.


Click here to view the information sources referenced in this article.

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