Variously on this site I, and quite a few commentators, have expressed concern for the conventional vogue of advising a diet rich in carbohydrate. Excesses of carbohydrate can cause gluts of the hormone insulin which in turn can predispose us to all sorts of problems including weight gain, type 2 diabetes, metabolic syndrome and cardiovascular disease.
However, not all carbs are created equal: some disrupt blood sugar levels more than others. The extent to which a food raises blood sugar can be measured and is expressed as its glycaemic index or ‘GI’. However, the extent to which a food disrupts blood sugar and insulin will depend not just on its GI, but how much we eat of it. One way to get an idea of the overall effect of a food is to take its GI and multiply it by the amount of carbohydrate found in a standard portion of food. Divide this by 100 and we have a measurement known as the ‘glycaemic load’.
I’m going over this again in case it’s new to some people. And also because the study that I am writing about today looked specifically at the GIs and GLs of people’s diets and their relationship to health and disease.
The study, published in this month’s edition of the American Journal of Clinical Nutrition, amassed the available evidence assessing dietary GI and/or GL and risk of a variety of conditions including type 2 diabetes, heart disease, stroke, breast cancer and gallstones. The studies were what is known as ‘prospective’ studies: where individuals were followed over time to see what, if any, association exists between the GI or GL or their diet and their risk of disease.
The researchers assessed the relative risk of each condition in individuals eating a high GI diet, compared to those eating a low GI diet. They did the same, but this time focusing on GL rather than GI.
There significant findings from this study were:
A high GI diet was associated with a 40 per cent increased risk of type 2 diabetes
A high GL diet was associated with a 27 per cent increased risk of type 2 diabetes
A high GI diet was associated with a 25 per cent increased risk of heart disease
A high GI diet was associated with a 26 per cent increased risk of gallbladder disease
A high GL diet was associated with a 41 per cent increased risk of gallbladder disease
A high GI diet was associated with an 8 per cent increased risk of breast cancer
No significant associations were found between either dietary GI or GL and risk of stroke, eye disease, and cancer of the colon, pancreas, stomach and womb.
The authors of the study also assessed the relationship between GI/GL and all diseases. This analysis revealed:
A high GI diet was associated with a 14 per cent increased risk of all diseases
A high GL diet was associated with a 9 per cent increased risk of all diseases
Taken as a whole, these results suggest quite strong links between generally raised blood sugar levels and certain chronic diseases. Of particular relevance here seems to be the relationship between dietary GI and GL and risk of type 2 diabetes. Another recent study also found a link between high-carb consumption and enhanced risk of diabetes.
Despite all this evidence, some would still have us believe that a diet for the prevention and treatment of diabetes should be the standard ‘healthy’ high-carb/low-fat diet the ‘virtues’ of which we’ve been hearing about for some decades now.
We should not be surprised that high GI and GL diets have links with increased disease risk, and type 2 diabetes in particular. What I think it a little more surprising about the results of this study is that it found generally stronger links between the GI and disease risk than those for dietary GL. My belief, certainly before this study, was that the GL would be a better guide to the overall biochemical disruption a food is likely to wreak, and therefore the health issues associated with its eating.
The authors of this recent study seem to attempt to explain this by saying: The reason that low-GI diets may offer greater protection than low-GL diets may be due to the fact that low-GL diets are more heterogeneous and can include either low-GI, high-carbohydrate foods or low-carbohydrate foods (eg, meat and cheese) (59). Although both diets will reduce postprandial glycemia, it is likely that the 2 dietary patterns will have very different metabolic effects��
In other words, as I read it, a low GI diet contains, generally speaking, a lot of low GI foods (obviously).
However, a low GL diet may be made up of a low, say, low GI, high-carb foods (e.g. apple), or low-carb food (e.g. meat, cheese). I’m with the authors to this point, but what I’m failing to understand is why this cannot be true for a low GI diet too.
At the risk of sounding quite ignorant, I don’t understand the authors’ explanation for why GI seems to be a more significant than GL with regard to disease risk, and I’d be very appreciative if someone would care to shed some light.
What this study does do, however, is lend further support to the notion that getting control of blood sugar balance is an important aspect of optimal health. And what that means, for some of us at least, is a diet lower in carbs, particularly foods rich in refined sugar and grains.
Barclay AW, et al. Glycemic index, glycemic load, and chronic disease risk-a meta-analysis of observational studies. Am J Clin Nutr. 2008;87(3):627-37