There is a common belief that foods that are high in fibre do not raise blood glucose levels as much as those that are low in fibre, and therefore “high fibre” can be used as a surrogate marker for “low GI” when shopping for foods. Is this belief valid?
Before I can answer this question, it’s important to consider what information about carbohydrates can be found on most food labels around the globe.
Carbohydrates and food labelling
As discussed in previous editions of GI News, currently, in most parts of the world, only very basic information about carbohydrate is included on food labels. Unfortunately, Nutrition Facts / Information panels are not mandatory in many parts of the world, and in places where they are, only total (available) carbohydrate (i.e., all maltodextrins, starches and sugars) and total sugars (e.g., all naturally occurring and added sugars) are generally included.
Dietary fibre, which is a kind of carbohydrate, is not a mandatory component of the Nutrition Facts / Information panel in most countries at present, with the exceptions being Canada and the United States of America (USA). In some nations, it must be included under certain circumstances, like when a fibre claim is made, and in many countries, it can also be included as voluntary information.
Unfortunately, the GI is not a mandatory component of food labels in any part of the world at present. However, voluntary GI labelling scheme’s do exist in many countries, including Australia, China, India, New Zealand, Singapore, South Africa and the USA.
Total carbohydrate and blood glucose levels
People with diabetes – in particular those that use insulin – can use the information about the total amount of available carbohydrate on food labels to help manage their blood glucose levels. Carbohydrate counting – whether using gram (g) amounts, 10 g portions or 15 g exchanges – has been proven in randomised controlled trials to significantly lower glycated haemoglobin (HbA1c), the 3-month average of blood glucose levels, by 0.35% – 0.64% points in people with type 1 diabetes. While it may sound small, lowering HbA1c by 0.5% points may reduce the risk of deaths related to diabetes by around 10%, heart attack by 7%, and microvascular complications (e.g., retinopathy, neuropathy, nephropathy, etc…) by around 18%.
GI and blood glucose levels
There is good evidence from randomised controlled trials that choosing mostly low GI foods and meals instead of high GI alternatives lowers HbA1c by 0.5% points in people with diabetes.
Sugars and blood glucose levels
Surprising to most, the total amount of sugars in a food or meal is not useful from a blood glucose management perspective, for several reasons:
- The GI of commonly consumed sugars (naturally occurring and added) ranges from a low of around 20 for the sugar fructose to a high of 105 for the sugar maltose.
- Commonly added sugars like sucrose, high-fructose corn syrups and most honeys, have GI values in the 60’s and therefore are medium GI.
- Sugars are not the only kind of carbohydrate that effect blood glucose levels – maltodextrins and starches also raise them, and they are all included as a part of total carbohydrate in Nutrition Facts / Information panels.
This is why, for people with diabetes, total carbohydrate is a more useful component of Nutrition Facts / Information panels than sugars.
Dietary fibre and blood glucose levels
Jason Bao and colleagues recently investigated the effect of 121 different foods and 13 mixed-meals in 1000 kJ (240 Calorie) or 2000 kJ (480 Calorie) portions, respectively, on blood glucose and insulin levels. The foods and meals all had widely differing amounts of carbohydrate, fat, protein and dietary fibre, reflecting what we commonly consume in our regular diets. Consistent with the evidence from clinical trials in people with diabetes that investigated the effect of either total available carbohydrate or GI on blood glucose levels, they found that the foods glycemic load, which is the product of available carbohydrate and GI (GL = GI x available carbohydrate per serve ÷ 100), was the strongest predictor of a foods and meals effect on blood glucose and insulin levels.
Perhaps surprisingly, for individual foods containing between 0 and 24 grams of fibre and meals containing between 0 and 21 g of fibre, total fibre content was not a predictor of either blood glucose or insulin response for the foods or meals.
While this may sound surprising, its important to remember that there are a large number of factors that determine a foods GI, including (but not limited to):
- type and amount of sugars
- type and amount of starch (i.e., amylose or amylopectin)
- starch gelatinisation
- acidity (pH)
- polyphenol content
- type of dietary fibre
There is evidence that viscous dietary fibres like agar, alginate, β-glucan, guar gum, konjac, pectin, psyllium and xanthan gum do help lower the glycemic index of foods or meals. However, other types of dietary fibre may not have the same effect, particularly when processed. Unfortunately, the type of dietary fibre is not included in mandatory Nutrition Facts / Information panels anywhere in the world at present.
Therefore, total dietary fibre is not a useful surrogate for GI (or GL) with respect to the effects of individual foods or meals on blood glucose levels. Just because a food or meal is high in fibre does not mean it will have a low GI.
The GI and GL are the best predictors of a food or meals effect on blood glucose levels
Therefore, if you want to know how a food will affect your blood glucose levels, look at the amount of total available carbohydrate per serve (based on the amount you will be eating – which is not necessarily the same as what is printed on the pack) and the foods GI. Sydney University’s GI Research Service lists the GI values for hundreds of foods on its website www.glycemicindex.com which can be accessed for free on your PC, tablet or smart phone when shopping.
- Food and Agriculture Organisation of the United Nations. Food Labelling.
- Bell and colleagues. Efficacy of carbohydrate counting in type 1 diabetes: a systematic review and meta-analysis.
- Thomas and Elliott. Low glycaemic index, or low glycaemic load, diets for diabetes mellitus.
- Stratton and colleagues. Association of glycaemia with macrovascular and microvascular complications of type 2 diabetes (UKPDS 35): prospective observational study.
- Bao and colleagues. Prediction of postprandial glycemia and insulinemia in lean, young, healthy adults: glycemic load compared with carbohydrate content alone.
- Jenkins and colleagues. Dietary fibres, fibre analogues, and glucose tolerance: Importance of viscosity.
- Sydney University’s free on-line GI database.
Alan Barclay, PhD is a consultant dietitian and chef (Cert III). He worked for Diabetes Australia (NSW) from 1998–2014 . He is author/co-author of more than 30 scientific publications, and author/co-author of The good Carbs Cookbook (Murdoch Books), Reversing Diabetes (Murdoch Books), The Low GI Diet: Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment, New York).
Contact: You can follow him on Twitter, LinkedIn or check out his website.