This week a The Lancet Public Health observational study caused a controversy claiming an U-shaped effect of low carbohydrate diet on all-cause mortality. This controversy gives us the opportunity to promote a more general scientific discussion: "the fallacies of all-cause mortality and U-shaped phenomena" for suggesting causation in observational studies.
All-cause mortality implies an infinite number of causes, making this outcome specially vulnerable to the typical confounding of observational studies. Characteristics of low-carb or high-carb individuals may influence deaths independent of cardiovascular events.
During summer, people tend to eat more ice-cream with lots of sugar. At the same time, it is during summer that most drowning deaths take place. Then, by not knowing the specific cause of death and by suffering from confirmation bias, one may conclude that sugar in summer diets increases cardiovascular death. Thus, it is imperative to show cause-specific mortality in observational studies.
Second, dose-response relationship is an important criteria for causation. Thus, its opposite, the U-shaped curve, should be an argument against causation and in favor of confounding effect.
Unless the interventional is prone to become poison in high doses (the case for certain drugs), an U-shaped phenomenon indicates bias due to the observational nature of the study design. A carbo-restricted subject is different from a moderate-carb individual, who is different from high-carb person. The extremes tend to be associated with more atypical behavior, unpredictable influences, specially in all-cause mortality.
Finally, observational studies should be avoided as tools to explore efficacy concepts. It only causes confusion and unnecessary controversies. Observational approaches should be valued as means to test real world effectiveness of concepts first proved by randomized clinical trails (efficacy).
Regarding dieting (or anything else), both sides of the aisle should avoid weak observational arguments. It should be more about good science and less about specific dieting. By this approaching, together and slowly, with no sensationalism, we will get the right concepts to guide individual decisions.