Healthy low carb & low fat diets

Executive Summary

* A study about mortality and healthy and unhealthy, low-carb and low-fat, diets received much coverage.

* The study was large and long – 45% of participants died during the study, so there was much mortality to review.

* The headline claims were that the overall low-carb diet and the unhealthy low-carb diet were associated with higher mortality, while the healthy low-carb diet was associated with lower mortality. All low-fat diets (overall, healthy and unhealthy) were associated with lower mortality.

* There were many issues with the study:

- The usual flaws of Food Frequency Questionnaires applied. This study conducted one questionnaire back in the mid-1990s, while people were followed up for an average of 23.5 years.

- Low carb was taken as up to 42% of calorie intake, which is not low carb.

- As usual, judgements were made about what was considered healthy vs unhealthy.

- There were large differences between people in different intake groups. Exercise, smoking and alcohol intake were vastly different in some cases. These were adjusted for, but a completely different person can't be adjusted for.

* The paper offered no plausible explanation for the findings.

* The paper noted that this was a population study and therefore can only suggest associations, not causation. Nevertheless, language slipped into inferring causation in several passages in the paper.

* People will take away from this paper the idea that low-fat diets are good – even unhealthy ones – and low-carb diets are bad (unless plant-based). There were so many issues, rendering the findings invalid.