Published on 20 October 2014

Recent reports in the press that grapefruit juice may be helpful for people trying to lose weight and in turn prevent type 2 diabetes have been labelled “misleading” and “irresponsible” by the NHS.

A number of UK newspapers reported on the study by researchers at the University of California, published in PLOS One.

The Daily Telegraph reported that the study on mice found a lower rate of weight for those on a combination of a high-fat diet and grapefruit juice, compared to mice fed a sugary drink. Their blood sugar levels and insulin sensitivity were found to be better regulated than mice that did not drink grapefruit juice.

In humans, reduced insulin sensitivity can be a sign of impending diabetes, however the tests on mice found that drinking grapefruit juice improved insulin sensitivity in mice, regardless of their diet.

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 type 2 diabetes, so the research had little immediate relevance to humans with the condition.

People with type 2 diabetes and other medical conditions should be careful when considering adding grapefruit juice to their diet, and should first read the patient information leaflet of any medication being taken.

Grapefruit juice has been known to increase the levels of medication in the blood, which can be potentially dangerous. Some of the medicines affected are statins, amiodarone (for irregular heartbeats), Viagra (sildenafil), sertraline, diazepam and calcium channel blockers.

NHS Behind The Headlines advised that for the time being, people with diabetes should not swap their metformin for grapefruit juice on the basis of this study. It also added that the study was funded by the California Grapefruit Growers Cooperative, although it had no role in the study design, data collection, analysis or decision to publish.

Grapefruit juice and a glass.

The NHS report into the study concluded: “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.”

Pam Dyson, Specialist Diabetes Dietitian, Oxford Centre for Diabetes, Endocrinology and Metabolism (OCDEM) and DRWF leaflet author, said: “Grapefruit is often quoted as a ‘superfood’ and many claims have been made for its properties as an aid to weight-loss. The 1930s were the first time that the grapefruit diet was recommended and the claim was made that grapefruit contained an enzyme that helped to burn off fat. Unfortunately, this was untrue and the main reason that the grapefruit diet worked was because it reduced energy (calorie) intake significantly, sometimes to as low as 800 kcal/day. It is true that grapefruit juice is relatively rich in vitamins and minerals, and has fewer calories than other juices, but its role in weight loss is still open to question.

“Scientists and researchers are always on the look-out for new ways of treating diabetes and obesity, and although it is tempting to think that grapefruit juice would be a useful treatment, there is insufficient evidence to support this at present. The article that created all this fuss was from a study done in non-diabetic mice, so its relevance to humans, and especially people with diabetes, is limited. If we look at the few studies that have been done in human subjects, none of them have involved people with diabetes, and they have produced contradictory results, meaning that we cannot yet draw any conclusions for the role of grapefruit juice.

“On a practical level, the mice in this recently reported study were fed grapefruit juice as their only source of liquids. To apply this on a human scale would mean drinking 1½ - 2 litres of grapefruit juice a day, with no water, tea, coffee or soft drinks! In addition, most people with type 2 diabetes are prescribed statins and blood pressure medication that are affected by grapefruit juice and are routinely advised to avoid it, significantly limiting the potential benefits. In summary, there is insufficient evidence to recommend grapefruit juice as a treatment for either diabetes or obesity at present, and it may well prove dangerous for some people taking certain medications.”

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