Grapefruit

Photo Credit: Jenny Niedenfuehr

Grapefruit has been a staple for weight-loss diets for decades, but to what extent does grapefruit consumption really contribute to weight loss and other health benefits? Today, we'll highlight some of the recent clinical data regarding weight loss as well as discuss the potential dangers of grapefruit consumption.

Weight loss, cholesterol, and diabetes
A recent randomized controlled trial[1] investigated whether grapefruit consumption resulted in weight loss. In this study, 74 overweight adults were put on a 6-week diet of either a placebo control diet (no grapefruit) or a diet which included one half fresh grapefruit with each meal (3 meals per day). All participants were encouraged to limit the intake of other fruits and vegetables with high polyphenol and carotenoid such as berries, spinach, sweet potatoes, and tomatoes in order to assess the impact of grapefruit consumption. There was no significant difference in terms of calorie intake between the two groups.

The results showed there was no significant loss of body weight in the grapefruit-eating group. However, there was a slight (but statistically significant) reduction in waist circumference as well as a significant reduction in systolic blood pressure, blood lipids, and cholesterol.

Not surprisingly, this study indicates that simply eating grapefruit likely won’t result in weight loss. The study didn’t look at groups that underwent calorie restriction or specific exercise plans, so it’s not to say that grapefruit won’t HELP lose weight, the study simply shows that you shouldn’t expect to lose weight if the only thing you change in your lifestyle is consuming more grapefruit. Just as important, eating grapefruit likely can help reduce blood pressure and cholesterol.

An older study (published 2006) looked at the effects of grapefruit and grapefruit supplement consumption before meals. In this study, 91 obese patients were randomly split into groups that received either placebo pills and apple juice, grapefruit capsules and apple juice, placebo pills with grapefruit juice, or placebo pills with half a grapefruit, and these treatments were consumed before each meal (3 meals per day).

After 12 weeks, the groups that consumed the grapefruit products lost on average at least 1 kg of body weight, but the group that consumed the fresh grapefruit lost an average of 1.6 kg body weight. Importantly, there was a significant reduction in the insulin levels 2-hours post-glucose for the group that consumed grapefruit. The researchers concluded that consuming half a grapefruit before meals could significantly improve overall weight loss.[2]

However, a study in 2011 from Vanderbilt University also investigated the effects of preloading meals with grapefruit and grapefruit juice and reached a very different conclusion. The idea here was to fill up on high-density, low calorie foods prior to meals in order to restrict overall calories without feeling starved. In this study, 85 obese adults preloaded each meal with solid grapefruit, grapefruit juice, or water for 12 weeks.

Interestingly, the total amount of food consumed (in terms of mass) did not change over time, but the average caloric intake decreased more than 20%. As a result, the participants lost an average of 7% overall weight. The differences in weight loss and waist circumference were not statistically significant among the groups meaning that preloading meals with water can be just as effective as preloading meals with fruit. However, it is important to note that the groups that consumed grapefruit or grapefruit juice significantly improved their cholesterol levels.[3]

Despite the lack of clinical evidence regarding weight loss from grapefruit consumption, animal models continue to show promise. A recent study showed that overweight rats who consumed grapefruit extract every day for 5 weeks showed reduced weight gain, decreased oxidative stress, and improvement to insulin resistance. However, the rats in this study were given 1 gram of grapefruit extract per kg body weight which would be the human equivalent of dozens of grapefruit extract pills which is not practical or recommended.[4]

Another recent study showed that diabetic rats given a moderate amount of grapefruit juice significantly improved glucose intolerance, and the researchers determined this was achieved by suppressing hepatic gluconeogenesis.[5]

Dangers

Ever notice how some medications specifically tell you not to take with grapefruit juice? This is due to particular components in grapefruit (primarily furanocoumarins) which inhibit metabolic enzymes called cytochrome P450 CYP enzymes, and these enzymes are critical for the metabolism of most drugs.

So what this means is that when you take a drug at the recommended dose, your body will be exposed to a therapeutic concentration of the drug for a particular period of time. However, when your CYP enzymes are inhibited, the drug that you took is no longer being properly metabolized, and therefore your body is being exposed to a much higher concentration of the drug for longer periods of time, and this can cause severe side effects and can sometimes be fatal.

So although grapefruit can provide numerous health benefits, it can be a very dangerous fruit to consume especially if you take certain medications that are known to interfere with grapefruit consumption. Always consult with your doctor or pharmacist if you’re a grapefruit eater and are prescribed a new medication or if you take daily medication and are looking to introduce grapefruit into your diet.

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[1] Dow et al. Metabolism. 2012 Jul;61(7):1026-35.

[2] Fujioka et al. J Med Food. 2006 Spring;9(1):49-54.

[3] Silver et al. Nutr Metab (Lond). 2011 Feb 2;8(1):8.

[4] de la Garza et al. J Med Food. 2015 Jan 19. [Epub ahead of print]

[5] Hayanga et al. Eur J Nutr. 2015 Mar 20. [Epub ahead of print]