Review details why many carbs aren’t ‘harmless for the heart’

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I don’t make any secret of the fact that I think the low-fat/high-carb paradigm that has seemingly shaped much nutritional advice for the last few decades is not founded in good science. And variously, on this site, I’ve presented the evidence that supports this stance. Some of this research has examined the distinct lack of evidence that fats found naturally in the diet (including saturated fat) contribute to the our burden with regard to obesity and disease. Other evidence presented here, however, has highlighted that the over-consumption of carbohydrates poses very real hazards for the body and our health.

One particular condition for which the low fat/high carb principle has been vigorously applied is heart disease. The image of a fatty diet leading to fat (e.g. cholesterol) in the bloodstream which then ends up dumping itself on the inside of the arteries is one which most of us will have had in our minds at some time. For some of us, this neat little story will be quite first entrenched in our psyche, a bit like the World being round. Yet, the science does not support a strong link between supposedly artery-clogging saturated fat and heart disease, and eating less of it has been shown to be a spectacular failure in terms of reducing disease risk. The only rational conclusion one can draw from the science is that saturated fat, at worst, has a pretty benign effect on health.

Today’s blog is not so much about why low-fat is necessarily good for us: it’s about why high-carb diets can be bad, including for the heart. This post is based on some information I read yesterday to be found in a paper published last year in the journal Current Atherosclerosis Reports. This paper is essentially a summary of the science which shows that carbohydrate, particularly those releasing sugar quite rapidly into the bloodstream, can cause cardiovascular disease (CVD – essentially heart disease and stroke).

The paper starts with a discussion of the science showing that individuals who do not handle sugar all that well (for instance, those who have relatively high sugar levels after being ‘challenged’ with a standard dose of sugar) have generally higher risk of CVD over time. They also site the evidence linking poor blood sugar control with increased risk of narrowing in the arteries.

The authors of this study list a number of mechanisms through which ‘spikes’ of blood sugar might increase the risk of arterial damage and CVD. These include:

Increased ‘oxidative stress’ (free radical damage)

Increased inflammation

Protein glycation (glucose ‘bonding’ to proteins in the body and damaging them)

Increased coagulation (essentially, making the blood ‘stickier’ and more likely to clot)

They also describe some of the evidence which links diets of high glycaemic index and/or glycaemic load (see here for some discussion of these terms) and other risk factors for CVD including raised levels of bloods known as triglycerides and an inflammatory substance known as C-reactive protein.

The authors also present some evidence linking high GI/GL diets with increased CVD risk. Overall, these appear to show that:

High GI/GL diet (compared to low GI/GL diets) are associated with an increased risk of CVD of somewhere between 20 and 100 per cent.

The paper also includes a summary of the clinical research on low GI/GL diets, and in particular, their ability to induce weight loss as well as improvements in biochemical risk factors for CVD. One study cited, which I was not aware of before yesterday, concerned a substance called acarbose. Acarbose basically slows the digestion of starch, and in so doing will reduce the GI/GL of foods eaten with it. In this study, individuals were randomly assigned to take acarbose or placebo over a 3-year period [2]. During the course of the study, those taking acarbose had half the risk of developing high blood pressure or having a cardiovascular event (e.g. heart attack or stoke).

There’s a little addendum to this blog I’d like to add. I was planning to read this study on an early morning flight. However, the paper caught the eye of the two travellers to my left. Both were diabetic, and both were health professionals too. We ended up spending the whole of the 2½ flight talking about diet and health, including the nutritional management of diabetes. One of these ladies described the dietary support she’d had as ‘crap’. Having an enquiring mind, she questioned the high-carb advice given to her initially, had educated herself regarding the GI, and had subsequently stabilised her condition. Listening to this lady talk was, honestly, music to my ears.


1. Brand-Miller, et al. The Glycemic Index and Cardiovascular Disease Risk. Current Atherosclerosis Reports 2007;9:479-485

2. Chiasson J, et al. Acarbose treatment and the risk of cardiovascular disease and hypertension in patients with impaired glucose tolerance: the STOP-NIDDM trial. 2003;290:486-494

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