Underweight subjects (BMI <18.5 kg/m2) are a subgroup of high mortality. Therefore, the comparison of the obese should not be with the underweight, or the heterogeneous group of subjects with a BMI <25 kg/m2, but with normoweight (18.5 kg/m2> BMI <25 kg/m2) . The high mortality of the underweight has been associated with increased age, comorbidity, severity of acute ischemia and a worse nutritional status and increased cellular catabolism. Increased susceptibility to the adverse effects of drugs could be another mechanism involved, as inferred from the ASSENT-3 Study. Also other pharmacokinetic hypotheses regarding body mass have been raised. The CVR of the underweight increases in secondary and tertiary prevention, and in these contexts, it could even be the anthropometric category of higher risk. In the TIMI Risk Score study, a body weight <67 kg was one of the 8 variables most strongly predictive of mortality at 30 days of acute myocardial infarction. Meanwhile, Reeves et al reported that in coronary bypass surgery, the underweight patients had a higher risk of complications and death than the normoweight. Therefore, the guidelines should alert about the risk of the underweight, which is taken into account in the proposed ergo-anthropometric assessment, specifically the benefits of being thin are seen only in those that keep fitness.