🍽️ A 30-year study reveals why these diets have opposite effectiveness depending on the person

For decades, the usefulness of a low-carbohydrate diet or a low-fat diet has fueled discussions. However, a persistent observation remains: while some followers of these diets see an improvement in their health, others do not. Why?

A study published in JACC, the journal of the American College of Cardiology, analyzed the dietary habits of nearly 200,000 American adults over more than thirty years. Its results indicate that healthy versions of low-carbohydrate or low-fat diets, focused on whole and plant-based foods, are associated with a lower risk of coronary heart disease. In contrast, these same diets but based on processed or meat products increase the risk.


The researchers used regular dietary questionnaires to assess the quality of the diets. They developed scores distinguishing healthy versions, rich in vegetables, whole grains, and unsaturated fats, from less healthy versions, but still with an overall low content of carbohydrates or fats. This method confirmed that the benefits for the heart depend more on food choices than on the overall proportion of carbohydrates or fats.

Metabolomic analyses, which examine molecules in the body, support these conclusions. They show that people opting for qualitative foods have more favorable biological markers.

The study's limitations include the fact that the participants are healthcare professionals, potentially more health-conscious. Furthermore, self-reported diets can introduce errors. However, the identified biological mechanisms are likely applicable to a broader public.

This research therefore invites a rethinking of nutritional advice, focusing on the diversity and quality of foods rather than simply reading the label.

Macronutrient composition versus food quality

Macronutrient composition refers to the proportions of carbohydrates, lipids, and proteins in a diet, often expressed in percentages or grams. Historically, nutritional recommendations have focused on these ratios, thinking they determined health. However, this neglects the source and processing of foods, which greatly influence their effects on the body.

Food quality, on the other hand, considers aspects such as fiber content, vitamins, antioxidants, and the degree of processing. For example, carbohydrates from vegetables or whole grains have a different impact than those from processed sugars. Similarly, fats from avocados or nuts are more beneficial than those from meats.

This distinction explains why diets with similar macronutrient compositions can have opposite health outcomes.