October 9, 2017

expert reaction to study looking at blood glucose levels after drinking nutrient extractor fruit juices compared with whole fruit

New research published in Nutrition & Diabetes reports that fruit blended into a juice via a commercially available nutrient extractor results in blood glucose levels the same or lower than seen with whole fruit.

Prof. Susan Jebb, Professor of Diet and Population Health, University of Oxford, said:

“This is a small study aiming to measure the glycaemic response to fruit consumed intact or after blending.  It involved very few people, 19 for the first study and just 9 for the second. All were young and healthy, they were not obese and had no signs of diabetes. In the first experiment a mix of fruit was used and the glycaemic index (GI) – a measure of the change in blood glucose levels – appeared to be lower after blending. In the second, with only mango, there was no difference in the GI between the intact fruit or after blending. The study used a standard method to measure the GI of a food but it takes no account of the impact of incorporating blended or intact fruit into the diet in everyday life, for example by measuring glucose levels over a 24 hour period.

“The effect of food on blood glucose is notoriously variable depending on the exact details of the food. It is quite possible that the GI of the fruit used in blended version differed from the intact fruit, especially since the first experiment included banana where GI depends heavily on the ripeness of the fruit. Given the blending seems to have been done in a single batch, any differences would be systematically observed. In both cases the fruit was frozen and thawed which is not typical for usual consumption and may well affect the GI, so results may not be transferable to usual eating practices.

“In the absence of a convincing mechanism by which blending would lower the GI of some fruits and the failure to replicate the finding in the second experiment, it is premature to draw any conclusions about the effect of blending fruit on the glycaemic response. Since the study did not involve any people with an impaired ability to handle glucose such as those with diabetes or pre-diabetes, it is irresponsible to suggest that these results “may have important clinical implications” for people with type 2 diabetes.”

Prof. Pete Wilde, Food and Health Programme, Quadram Institute Bioscience, said:

“To verify that this is a real effect we’d need further mechanistic studies to understand how the nutrient extract samples show this remarkable drop in glycemic index (GI) compared to whole fruit in the mixed fruit arm, and why there wasn’t the same drop in the mango arm.

“It is a simple study with no attempt to investigate the causal mechanisms.

“The authors suggest that by chewing the whole fruit, it is exposed to salivary amylase for longer than for the nutrient extract, which is presumably drunk more quickly.  So their suggestion is that less starch is turned to glucose in the nutrient extract, which may lead to slower starch digestion in the duodenum – but this would require further experiments to clarify.

“Another factor at play could be that freezing the samples may have had a different effect on starch structure in the nutrient extract compared to the whole fruit, where the starch is still encapsulated in the cells.

“However, none of this would explain why the mango didn’t show any significant effect, so we can’t be sure – we need more research.

“In summary, this is potentially an interesting finding, but requires further studies to confirm the effects on pure fruit samples of different forms, together with mechanistic studies on the structural changes taking place in the starch, and its influence on the digestibility of the starch.”

* ‘Nutrient-extraction blender preparation reduces postprandial glucose responses from fruit juice consumption’ by KM Redfern et al. published in Nutrition & Diabetes on Monday 9 October 2017.

Declared interests

Prof. Susan Jebb: “Prof Susan Jebb is employed by the University of Oxford and receives no personal funding from the food industry.  Susan Jebb is conducting research into the treatment of obesity, some of which include support from WeightWatchers, Slimming World and the Cambridge Weight Plan.  Susan was the independent Chair of the Public Health Responsibility Deal Food Network and was a science advisor to the Foresight obesity report.  From 2007-10 she was the principal investigator for a research study funded by the food industry to investigate the potential for a functional beverage to help weight loss. The results of this work have been published: http://www.ncbi.nlm.nih.gov/pubmed/23920353.”

None others received.