Canadian professor Tom Wolever has provided a spirited argument for the adoption of glycaemic index as a measure of carbohydrate quality, lining it up against wholegrains. Which approach should be preferred?
Those who have been following the carbohydrate quality debate will enjoy a recent feisty review by Professor Tom Wolever from the University of Toronto published in the European Journal of Clinical Nutrition. Wolever is the co-inventor of the glycaemic index concept and is obviously a little peeved that other measures of carbohydrate quality such as wholegrain have gained acceptance in dietary guidelines whereas GI has not.
Wholegrains versus GI
Here is how Wolever weighs up the relative merits of wholegrains and GI as measures of carbohydrate quality (minimally edited).
The evidence for the health benefits of wholegrains comes from epidemiological studies showing strong associations between high intake of wholegrains and reduced risk for cardiovascular disease, diabetes and obesity. Recent large clinical trials in subjects without diabetes showed that increased wholegrain consumption reduced systolic blood pressure, but none showed any significant effect of wholegrains on blood glucose, insulin sensitivity and/or insulin secretion, inflammatory markers or body weight (Tighe 2010, Brownlee 2010, Kristensen 2012). One study found that wholegrains reduced LDL-cholesterol, but another found the opposite.
On the other hand, trials of low-GI diets of similar magnitude in subjects without diabetes showed significant beneficial effects of a low-GI diet on blood lipids,(Jebb 2010, McMillan-Price 2006) inflammatory markers (Gogebakan 2011) body weight (Larsen 2010) and possibly insulin sensitivity. When the results of the four low-GI studies are combined, there is a significant reduction in LDL-cholesterol and C-reactive protein and a strong trend towards weight reduction.
Low-GI diets have at least as many, if not more, statistically significant effects than wholegrain-enriched diets; this does not support the hypothesis that wholegrain is a better marker of carbohydrate quality than GI.
I confess I have struggled to understand how wholegrains could lower risk for diabetes and coronary heart disease for the reasons that Wolever summarises. A small effect on systolic blood pressure really isn’t good enough. There is always the possibility that wholegrains may be operating via a yet-to-be-discovered mechanism, but this appears unlikely. Wholegrains have no effect on the inflammatory marker CRP suggesting no effect on underlying disease processes.
So how can one explain nutritionists’ recent embrace of wholegrains as a measure of carbohydrate quality? Perhaps the decision makers like the message – simple and easy to understand. But then again, if the message has shortcomings should we be broadcasting it? ‘Eat less fat’ was a simplification of ‘eat less saturated fat’ and look where that took us.
Alternatively, perhaps there is a philosophical dimension to it. Nutritionists like the idea of wholegrains because it fits nicely with our philosophy about nutritional health being a natural consequence of eating simple, whole foods. But we are not philosophers; nutritionists are meant to be scientists and to make recommendations based on the facts.
The facts about GI?
The experts are yet to agree on what the facts relating to GI actually are. If you want to understand the GI debate and the case for and against, Wolever’s review is for you. He lists the criticisms of GI made by its detractors, such as imprecise measurement, lack of reproducibility, inaccurate tables and the notion that GI does not apply to mixed meals. He then addresses the issues one by one and challenges his critics to prove him wrong.
And the politics
As you may have guessed, there is a little politics involved here. In North America several influential nutritionists took a stand against GI early on and have defended their position ever since. They now concede that glycaemic ‘response’ is relevant to health but have reservations about the relevance of the GIs of individual foods. Wolever won’t have an easy task convincing them. Sometimes the acceptance of new approaches in science proceeds slowly, funeral by funeral.
In Europe leading nutritionists have been more accepting of the GI concept, especially following the results of the Diogenes study, which showed beneficial effects on weight management. The European Union is now funding a large new trial, with an Australian arm, into whether a high protein, low GI diet is superior to a conventional healthy diet for the prevention of type 2 diabetes.
It is noteworthy that Wolever doesn’t see GI as an alternative to wholegrains but as a complement to it. They are both measures of carbohydrate quality. I would add the caveat that some wholegrain foods and some low GI foods are nutrient-poor, again highlighting that carbohydrate quality is a complex, multi-faceted concept.