Causal criteria in nutritional epidemiology

Making nutrition recommendations involves complex judgments about the balan ce between benefits and risks associated with a nutrient or food. Causal cr iteria are central features of such judgments but are not sufficient. Other scientific considerations include study designs, statistical tests, bias, confounding, and measurement issues. At a minimum, the set of criteria incl udes consistency, strength of association, dose response, plausibility, and temporality. The current practice, methods, and theory of causal inference permit flexibility in the choice of criteria, their relative priority, and the rules of inference assigned to them. Our approach is as follows. Consi stency across study designs is compelling when the studies are of high qual ity and are not subject to biases. A statistically significant risk estimat e with a >20% increase or decrease in risk is considered a positive finding . A statistically significant linear or otherwise regularly increasing tren d reinforces the judgment in favor of a recommendation. A plausible hypothe sis likewise reinforces a recommendation, although the rules of inference f or biological evidence are highly variable and depend on the situation. Tem porality is, for nutrition recommendations, more a consideration of the ext ent to which a dietary factor affects disease onset or progression. Evidenc e supporting these criteria provides a strong basis for making a nutrition recommendation, given due consideration of the balance between presumed ben efits and presumed harms. Recommendations should make clear their breadth o f application; a narrow recommendation involves a single disease or conditi on whereas a broad recommendation involves all relevant diseases or conditi ons.