New evidence links high-carb diets with increased risk of diabetes and other ills

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.

And of course, a carb-controlled diet will tend to be better for individual who have been diagnosed with 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.

References:

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

7 Responses to New evidence links high-carb diets with increased risk of diabetes and other ills

  1. Peter Silverman 13 March 2008 at 11:38 am #

    What about GL and cardio-vascular disease: higher or lower than GI?

  2. Peter Silverman 13 March 2008 at 11:44 am #

    I don’t care about spelling, but if you do I would go for wreak over reek, unless you’re talking about garlic.

  3. Dr John Briffa 13 March 2008 at 11:56 am #

    Peter
    Ta for spelling alert. This study found no statistically significant association between GL and either heart disease or stroke.

  4. Pete A 14 March 2008 at 11:24 am #

    I would have to agree that i am also puzzled by the fact that GI seems to be a better guide to risk than GL. My only thought is that maybe the spikes in blood glucose may be more damaging than just the elevated levels in blood glucose?

    Whatever is said however, a diet rich in Highly refined carbohydrates shows a massive increase of desease that a diet very low in refined carbohydrates does not. ANd the food standards agency is still going on about fat and ignoring carbohydrates.

  5. Bill Cockerill 14 March 2008 at 8:05 pm #

    Perhaps the difference found between GI and GL and the risk of disease is not caused by carbs but through another confounding factor? That is, this study might hint at something other than carbs that increases risk for these diseases and it occurs more often with high GI than high GL diets? Did the paper control for confounding explanations? e.g. fat intake, exercise, age, smoking and so on… Even if they did, perhaps there is another unmeasured reason that explains the difference between GI and GL?

  6. Neil 15 March 2008 at 12:21 pm #

    Pete A said
    “maybe the spikes in blood glucose may be more damaging than just the elevated levels in blood glucose?”

    My guess would be that mealtime spikes BG are more normal or physiological than extended periods of elevated BG (eg through snacking or nibbling)

  7. Pete A 17 March 2008 at 11:11 pm #

    On reflection I like Neil’s idea that ‘ extended periods of elevated BG (eg through snacking or nibbling)’ could well be the cause.
    Although the people who eat a large amount of High GI foods may well get larger spikes than those of us who only eat low Gi ‘good food?’

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