High carb diets are bad news for the blood vessels

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Cardiovascular conditions such as heart disease and stroke are a major cause of disability and death. For years we’ve been led to believe that fat is the major dietary offender here, but in reality the science shows that naturally occurring fats are not the issue. In fact, some fats (principally so-called omega-3 fats) actually help protect against these conditions.

The other side of this, of course, is that when it comes to cardiovascular disease, carbohydrates are far from blameless. Certain carbohydrates, principally those that are most disruptive for sugar and insulin levels, are strongly associated with an increased risk of cardiovascular disease. See here and here for more about this.

One way to measure the impact of diet on risk of cardiovascular disease risk is to see what effect it has on blood ‘markers’ such as blood fat, glucose and insulin levels. Another way is to assess the impact diet has on the function of the arteries in the body. The standard way to measure this is assess what is known as ‘flow mediated dilatation’ (FMD) in the brachial artery (a major artery in the arm). This measurement essentially tests the ability of arteries to relax, which is something that is good for blood flow in the artery, and is associated with a reduced risk of cardiovascular disease risk. Reduced FMD is a sign of reduced health in the artery, and is a potential risk factor for heart attacks and stroke.

In a recent study, 40 overweight and obese adults were put on one or two diets for a period of 12 weeks. One of the diets was low fat, the other was carbohydrate restricted. Both diets provided the same number of total calories (1500 cals per day). FMD was measured at the beginning and end of the study.

In the low-fat diet eating group, FMD decreased over the course of the study.

In the carb-restricted group, it increased.

In the carb-restricted groups, levels of insulin unhealthy blood fats called triglycerides also fell significantly.

This study is reminiscent of a study I reported on here [2]. In this study, overweight and obese men were fed with one of four test meals, after which FMD was assessed 2 hours later [2]. The four text meals were water (placebo), glucose (high GI), cornflakes (high GI) and bran flakes (low GI). Compared to the placebo, FMD was reduced after all of these meals, but only to a statistically significant degree after the two high GI meals.

Taken together, the results of these two studies suggest that a carbohydrate-rich diet, particularly if those carbohydrates are disruptive to blood sugar levels, is bad news for the health of the blood vessels.


1. Volek JS, et al. Effects of dietary carbohydrate restriction vs low-fat diet on flow-mediated dilation. Metabolism. 2009 Jul 24. [Epub ahead of print]

2. Lavi T, et al. The Acute Effect of Various Glycemic Index Dietary Carbohydrates on Endothelial Function in Nondiabetic Overweight and Obese Subjects J Am Coll Cardiol 2009;53:2283-2287

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