The battle of the carbs


For many years, doctors have recommended a diet based in low carbohydrate intake. Clinical trials have shown that it reverses diabetes, leads to weight loss, and improves most heart disease risk factors, all of which should logically lead to a longer life. However, a recent publication in Lancet Public Health, based on data from the Atherosclerosis Risk in Communities (ARIC) study made headlines around the world when it declared that a low-carb diet will shorten life.

The ARIC study is a project of the National Institutes of Health in the US that, since 1987, followed about 15,500 middle-aged men and women in four US communities. It showed that middle-aged Americans who consumed the least amount of carbohydrates were more likely to die from heart disease or cancer in the subsequent decade than their peers who had the highest carbohydrate intake.

In a second part of the study, the findings were replicated in a meta-analysis (a study pooling together other studies) of seven studies with people from Greece, Japan, Sweden, and the United States, that included people followed for almost two decades.

Compared with the highest intake of carbohydrates, participants with the lowest intake (39% of calories coming from carbohydrates) had a significant 32% increase in total mortality, 35% increase in cancer mortality and 51% and 50% increases in coronary artery disease and stroke mortality respectively.

The results have been generating a lively discussion, with several objections pointed. One of the points raised is that 37 or 39% of calories from carbohydrates is not really “low-carb” according to the latest standards of practice.

Evidence from the past 5 years shows better health when carbohydrates are kept below 30% of calories, with the greatest benefits, including reversal of type 2 diabetes, occuring with a very low-carb diet, where carbohydrates normally total between 5 and 20%. Another limitation of the ARIC study is that it is an observational study, and therefore can only show association,  not causation. The poorer outcomes seen in the ARIC study could be due to outside factors or other unhealthy behaviors that the researchers did not or could not measure. 

The gold standards of science are what are known as randomized controlled clinical trials, because they can demonstrate cause and effect. The authors of the ARIC study ignore, or pass breezily by more than 70 low-carb clinical trials with at least 7000 people. The moderate carb, low-fat diet, on the other hand, has been part of the official dietary guidelines for Americans for decades, and is part of the food pyramid. This diet has been tested in rigorous clinical trials funded by the National Institute of Health, involving more than 50,000 people.

The results point out that such a diet does not fight any kind of disease, and does not reduce mortality. To be fair, the authors of the ARIC study concede that “probably for a short time”, 6 or 12 months, low-carb diets might be useful for weight loss and good for parameters such as blood glucose and lipids.

They also note that different types of low-carbohydrate diets matter. Whereas low-carb diets that rely heavily on animal-based proteins and fats are linked to a higher mortality, low-carb diets that consist primarily of plant-based proteins and fats are associated with a longer life. Nevertheless, the ARIC study shows that something unusual might be happening, and, as is common in science, more research is needed to find the answer.


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