Last week I wrote a piece challenging prevailing nutritional wisdom about the role of dietary fat in the burgeoning rates of fatness seen in many countries [click here]. Of course, obesity is not the only condition usually put down to fat in the diet – another is heart disease. In fact, health professionals often trot out their concerns about the artery-clogging effects of fat when offering opinion on ‘low carb’ programmes such as the Atkins and South Beach diets. The typically high levels of fat consumed on these plans, we are told, is one sure way to speed demise through heart attack.

However, as I pointed out in another blog last week, the role of fat, including saturated fat, in so-called ‘cardiovascular’ conditions such heart disease has been seriously overstated [click here]. Plus, we know that eating a lot of carbohydrate can boost levels of a hormone called insulin in the body – excesses of which are linked with an increased risk of obesity, type 2 diabetes and heart disease. Bearing these facts in mind, there is an obvious argument for challenging the widely-held notion that a heart-healthy diet is one that is low in fat and high in carb.

Researchers from the Harvard School of Public Health in the USA recently took a fresh look at this subject in the form of a study published recently in the New England Journal of Medicine (NEJM). This research assessed the relationship between low carbohydrate dieting and heart disease risk in more than 80,000 women assessed for some 20 years [1]. This study found that, contrary to conventional wisdom, low-carb eating was not associated with an increased risk of heart disease.

However, these researchers went further by assessing the relationship between heart disease risk and something known as the ‘glycaemic load’ of the diet. To understand this measure, we need to know that it is based on a food’s so-called ‘glycaemic index’ (GI). Basically, GI gives a measure of the speed and extent of sugar release from a food. Generally, the higher the GI, the more insulin will be secreted, and the greater the risks for health.

However, the GI is a little hampered as a measure of the likely health effects of a food because it does not take into account how much of that food is eaten. Obviously, a food of relatively high GI eaten in quantity is going to be more disruptive to blood sugar and insulin levels than one with a similar GI that is generally eaten in small quantity. To account for this, the GI of a food can be multiplied by the amount of carbohydrate found in a typical portion of food. This figure is then divided by 100. The result, termed the ‘glycaemic load’ (GL) is generally believed to give a better measure of the effects of a food on blood sugar and insulin in the real World. GL of less than 10 are generally regarded as healthy.

Perhaps the most striking finding of the recent NEJM study is that women eating diets of the highest GL were TWICE as likely to suffer from heart disease compared to those eating diets of the lowest GL.

Once the maths are done, many of the foods that have medium or high GI, turn out to have low GLs. Examples include kiwi fruit (GI 53 GL 6), pineapple (GI 59 GL 7), watermelon (GI 72 GL 4), cooked carrots (GI 58 GL 3) and beetroot (GI 64 GL 5). Beans and lentils also tend to have low low GI and GLs. On the other had, many of the grain-based foods that we are encouraged to base our diet on turn out to have not just high GIs, but elevated GLs too. Examples include bagel (GI 72 GL 25), white rice (GI 65 GL 23), rice cakes (GI 78 GL 17), cornflakes (GI 81 GL 21), baked potato (GI 85 GL 26), pasta (GI 44 GL 21).

Personally, I do not have an issue with carbohydrates per se. Many carbohydrate-rich foods are, broadly speaking, nutrient-dense foods that have generally low glycaemic loads. Examples of such foods include fruits, vegetables (other than the potato), beans and lentils. However, these foods are very distinct from the starchy staples we are encouraged to base our diet on which have relatively high glycaemic loads, and can therefore be very disruptive to the body’s chemistry and pose hazards for health. Another problem with foods such as bread, potatoes and breakfast cereals is that they tend to be low in nutritional value compared with other carbs such as fruit and veg [2].

Eating a diet based on biochemically-disruptive carbs that do not offer much from a nutritional perspective does not sound like a recipe for good health, does it? Could this, I wonder, help to explain the burgeoning rates of obesity and diabetes that have come at the same time as increasingly strident advice we’ve been given to eat a low-fat, high-carb diet in the last few decades. And could it possibly be that the establishment’s appetite for such as diet might have been partly driven by a food industry keen to sell us branded, heavily marketed, highly profitable carb-rich fodder such as bread and breakfast cereals?

References:

1. Halton TL, et al. Low-carbohydrate-diet score and risk of coronary heart disease in women. New England Journal of Medicine. 2006 355:1991-2002

2. Drewnowski A. Concept of a nutritious food: toward a nutrient density score. Am J Clin Nutr 2005 82:721-732

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