Fruit juice and blood sugar control

It’s a myth that drinking a glass of fruit juice ‘spikes’ your blood glucose yet this information is being peddled on some social media accounts. We fact check these claims using published scientific evidence.

Glycaemic index (GI) is a measure of post-meal blood glucose concentration in relation to a reference, normally a glucose drink or white bread. It is well accepted that low GI foods are digested and absorbed slowly while high GI foods are rapidly digested and absorbed. GI can only be calculated for carbohydrate-containing foods and drinks.

When scoring foods and drinks, 0-55 counts as low, while medium is 56-69, and high extends from 70-100. An apple, for instance, has a GI of 44, while a baked potato scores 75. Several factors reduce the GI score, for example fibre, fat, protein and certain plant bioactives such as polyphenols.

High GI diets have been linked to an increased risk of chronic diseases, such as type 2 diabetes, cardiovascular disease and some cancers. So, classifying foods according to GI can help people to make healthier dietary choices.

The most recent GI tables were published in 2021 and include data from more than 4000 foods and drinks. These report an average low GI of 47 for fruit and vegetable juices [2]. Not only this, but previous evidence from the same research group shows that fruit and fruit juices have similar blood glucose curves [2].

GI has become a regular feature in the posts of self-appointed nutrition experts, whose unfounded claims include:

• Smoothies and 100% fruit juices produce similar glucose responses to cola and have similar effects on health

• Drinking fruit juices or smoothies increases the risk of type 2 diabetes

• Orange juice is ultra-processed.

There are several problems with these claims, which are typically based on opinion or individual results from continuous glucose monitors (CGMs) rather than scientific research.

A CGM is a tiny sensor implanted under the skin, which sends real-time glucose data to a receiver or smartphone. It is a vital resource for people with diabetes when used under medical supervision.

However, CGMs have become a fashion accessory and their use by people without diabetes can encourage fear about foods which ‘spike’ blood glucose levels. In fact, hour by hour variations in blood glucose are normal and are well controlled by insulin, a hormone produced by the pancreas.

Reporting personal data from CGMs is problematic because they are based on one person whereas GI studies recruit 10-15 participants. Ten is the minimum number of people needed to reduce the margin of error when testing a food.

Another major reason for inaccuracy is that CGMs measure interstitial fluid glucose (in the spaces around cells) rather than arterial or capillary blood glucose. GI studies tend to use finger prick methods to take blood.

Other issues with using CGMs at home include a lack of standardisation for overnight fasting, smoking, food portion sizes, other foods in the meal, and exercise.

Worse still, social media influencers have drawn conclusions about the GI of foods using data from people with diabetes without making this clear. Individuals with and without diabetes produce different glucose curves for the same food so these data are not interchangeable.

While we know that lower GI diets are better for health, scientists are unclear about the relevance of so-called glucose ‘spikes’ and whether they mean anything for health in individuals without diabetes. Fruit juice has a role in healthy diets because it is low GI – like fruit – and provides a source of vitamin C, potassium, folate and polyphenols.

References

1. Atkinson FS et al. (2021) https://pubmed.ncbi.nlm.nih.gov/34258626/

2. Brand-Miller JC et al. (2009) https://www.sciencedirect.com/science/article/pii/S0002916523239131