Observational study suggests low-carb diets are associated with higher coronary calcium scores
A new observational study published in Arteriosclerosis, Thrombosis, and Vascular Biology concludes that low-carb diets and animal-based foods are associated with greater progression of coronary calcium scores.
On the surface, this sounds very concerning for those following low-carb diets. However, rest assured that this study does not use an accurate definition of low carb, so it is flawed from the start.
The study authors inaccurately define low carb as less than 40% of calories from carbohydrate.
As we have written before, 40% of calories from carbs equates to 200 grams of daily carbs for a 2,000-calorie diet. That’s more carbs than most low-carb eaters have in an entire week!
I hope the authors can understand the difference between someone who eats 20 grams or even 50 grams of carbs per day and someone who eats 200 grams. But by reading the paper, it is hard to be certain that they do.
There are many other issues with this study. Since it is a nutritional observational study, it can only show an association, not a cause-and-effect relationship. It is also subject to the usual inaccuracies introduced by using food frequency questionnaires, in this case collected at only two time points. In addition, like most observational analyses, this study suffers from healthy user bias.
The paper shows that 54% of the group labeled as “low carb” were active or former smokers compared to just 27% in the group defined as “balanced carb.” Since smoking is the most significant risk factor for heart disease, it seems like a much more likely contributing factor than reducing carbohydrate or eating animal foods.
Low-carb eaters were also more likely to be male and have less education, two other factors that are associated with greater risk for heart disease.
Lastly, the group labeled “low carb” averaged 600 more calories per day than the high-carb group. Low-carb eaters consuming more calories directly conflicts with studies that show people following a properly designed low-carb diet naturally decrease caloric intake. In fact, it is often argued that low-carb diets work primarily by lowering calorie intake.
It would be inaccurate to interpret this study as one more study in a large body of evidence showing that low-carb diets are dangerous. In truth, it is one more study showing that people who eat a mixed high-carb and high-fat diet tend to be less healthy than their counterparts.
It says nothing about the benefits or risks of an appropriately designed low-carb diet.
As long as researchers continue to define low-carb diets inaccurately, we will continue to see misleading headlines. Perhaps the best we can do is to continue to point this out.
That said, I hope that sooner or later researchers begin to recognize that low-carb diets deserve to be studied — and that future research will be designed to help us understand where they are most useful for improving health.
Thanks for reading,
Bret Scher, MD FACC