Could grapefruit juice protect against diabetes?
“Grapefruit juice 'could be the key to weight loss’,'' is the misleading headline in The Daily Telegraph.
It reports on a study in which mice fed a combination of a high-fat diet and grapefruit juice still put on weight – albeit at a lower rate than mice fed a sugary drink. Their blood sugar levels and insulin sensitivity were also better regulated than mice that did not drink grapefruit juice.
The mice were given either a high-fat diet or a low-fat diet in a range of experiments.
Mice fed a high-fat diet and grapefruit juice had an 18% reduced rate of weight gain compared with mice given sugary water with the same number of calories as the grapefruit juice. They also had 13% lower fasting blood sugar levels. There was no effect on weight gain in mice fed a low-fat diet.
Drinking grapefruit juice improved insulin sensitivity in mice, regardless of their diet (in people, reduced insulin sensitivity can be a sign of impending diabetes).
Grapefruit juice lowered blood sugar as effectively as metformin, a drug widely used to treat people with type 2 diabetes. However, none of the mice actually had diabetes, so this research has little immediate relevance to humans with the condition.
For the time being, people with diabetes should not swap their metformin for grapefruit juice on the basis of this study.
Where did the story come from?
The study was carried out by researchers from the University of California and was funded by the California Grapefruit Growers Cooperative, although it had no role in the study design, data collection, analysis or decision to publish.
The study was published in the peer-reviewed science journal PLOS ONE. This is an open-access journal, so the study is freely available to all.
Both the Mail Online and The Daily Telegraph’s headlines incorrectly state that grapefruit juice can help people lose weight. Leaving aside the fact that this study involved mice, rather than humans, none of the mice actually lost any weight – they just differed in the rate they put on weight.
The Daily Express’ headline was also irresponsible, as it suggests that grapefruits “tackle diabetes as well as a leading drug”, with an accompanying picture of a smiling woman (not a mouse) tucking into a grapefruit. None of the reports seemed to mention that the work was funded by the California Grapefruit Growers Cooperative. This doesn’t mean that the study results aren’t correct, but it's worth mentioning so people can make their own conclusions.
The Mail Online does, however, include a balancing comment from the British Dietetic Association, which said that until further trials are carried out in humans, it's too early for people to try grapefruit diets.
What kind of research was this?
This was a set of animal experiments on mice that aimed to look at the effect of grapefruit juice on weight, blood sugar levels and insulin sensitivity. Previous research had suggested that grapefruit juice would have a positive effect on all three, so the researchers wanted to perform controlled studies to investigate this. As the study was conducted on mice in a laboratory, the researchers had complete control of their diet and fluid intake – something that would be very difficult to achieve in humans.
Animal experiments can give some indication of what might happen in humans.
However, biological differences between species mean that we can’t be certain that an “intervention” (in this case, grapefruit juice) will have exactly the same effect in humans.
What did the research involve?
Four-week-old mice were fed a low-fat diet (10% fat) or a high-fat diet (60% fat) for 100 days, and had access to either:
- 50% grapefruit juice with water and 0.15% saccharin (artificial sweetener)
- water with 4% glucose and 0.15% saccharin (to match the number of calories in the grapefruit juice)
The researchers monitored the weight, fasting blood glucose and fasting insulin of the mice.
To investigate if grapefruit juice has an effect on diet-induced obesity, mice were fed a high-fat diet for 10 weeks. The researchers then continued to feed the mice the high-fat diet with either the grapefruit juice or sweetened water described above.
The researchers then used a second group of mice to compare the effects of grapefruit juice, naringin (a flavonoid antioxidant present in grapefruits) and metformin (a drug that lowers blood sugar for people with diabetes). Mice were fed a high-fat diet for 106 days and given one of the following:
- water with 4% glucose and 0.15% saccharin
- 50% grapefruit juice with water and 0.15% saccharin
- 0.72mg naringin in water with 4% glucose and 0.15% saccharin
- 7.5mg metformin in water with 4% glucose and 0.15% saccharin
Lastly, the researchers compared the combination of metformin plus grapefruit juice against each on their own, or sweetened water.
What were the basic results?
Mice fed a high-fat diet ate and drank similar quantities, but those that drank grapefruit juice weighed 18.4% less than those that drank sweetened water after 100 days. Fasting blood glucose levels were 13% lower and fasting insulin levels were 72% lower in the grapefruit juice group compared to the sweetened water group. This showed that the mice drinking the grapefruit juice had an improvement in insulin sensitivity (reduced insulin sensitivity in humans can lead to diabetes).
Mice fed a low-fat diet consumed the same amount of food regardless of whether they had access to grapefruit juice or sweetened water. The mice drank slightly more water than grapefruit juice, and consumed a few more calories. There was no difference in weight or fasting blood glucose levels between the two groups after 100 days. Fasting insulin levels were two times lower in the grapefruit group.
Obese mice ate the same amount, regardless of whether they had access to grapefruit juice or water. After 55 days, the grapefruit juice group weighed 8% less.
Mice drinking naringin, metformin or grapefruit juice had 20% lower blood glucose than those drinking sweetened water after 106 days.
Metformin plus grapefruit juice was as effective at lowering blood glucose as metformin on its own or grapefruit juice, with all of these groups having 11% to 14% lower blood glucose than those having sweetened water.
How did the researchers interpret the results?
The researchers concluded that they have provided new evidence for potential health-promoting properties of grapefruit juice in rodent-based high-fat diet-driven obese and non-obese models. They say that “these results justify additional studies in animal models and humans to assess the mechanisms and scope of GFJ [grapefruit juice] action”.
This intriguing study has found that grapefruit juice reduced weight gain and improved blood sugar levels in mice fed a high-fat diet compared to mice drinking the same number of calories through sugary water. It should be noted that the mice still gained weight on the unhealthy 60%-fat diet, even if they drank the grapefruit juice.
Grapefruit juice caused improved insulin sensitivity in mice fed high- or low-fat diets. Other than that, grapefruit juice did not have an effect on weight or blood sugars for mice on a low-fat diet.
These studies showed that grapefruit juice lowered blood sugar as effectively as metformin, a drug widely used for diabetes. However, none of the mice actually had diabetes, so this isn’t a massively useful finding.
Also, as there are biological differences between humans and mice, we can’t be certain what effect, if any, grapefruit juice would have for people with diabetes. So if you are diabetic and on metformin, you should not stop taking your metformin and switch to grapefruit juice on the basis of this study.
Grapefruit juice should not be consumed if you are taking certain medications, as it increases their level in the blood. They include statins, amiodarone (for irregular heartbeats), Viagra, sertraline, diazepam and calcium channel blockers.
If you do have diabetes or have been told that you are at risk of developing it in the future, then you should avoid eating a high-fat diet, even if you are drinking grapefruit juice. Weight gain is a significant risk factor for type 2 diabetes.
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