Relative to the normal weight category (BMI 18.5 to
25), obesity (BMI 30) was associated with 111 909 excess deaths (95% confidence interval [CI], 53 754- 170 064) and underweight with 33 746 excess deaths (95% CI, 15 726-51 766). Overweight was not associated with excess mortality (−86 094 deaths; 95% CI, −161 223 to −10 966).

Underweight and obesity, particularly higher levels of obesity, were associated with increased mortality relative to the normal weight category. The impact of obesity on mortality may have decreased over time, perhaps because of improvements in public health and medical care. These findings are consistent with the increases in life expectancy in the United States and the declining mortality rates from ischemic heart disease.

Overweight and obesity have reached epidemic proportions in the USA and most of the rest of the world. Particularly concerning is the very high prevalence of class III obesity (BMI ≥40 kg/m2), which has reached ~3% in the USA. In the past few years, controversy has surrounded the idea that some individuals with obesity can be considered healthy with regards to their metabolic and cardiorespiratory fitness, which has been termed the 'obesity paradox'. These controversies are reviewed in detail here, including discussion of the very favourable prognosis in patients with obesity who have no notable metabolic abnormalities and who have preserved fitness. The article also discusses the suggestion that greater emphasis should be placed on improving fitness rather than weight loss per se in the primary and secondary prevention of cardiovascular diseases, at least in patients with overweight and class I obesity (BMI 30–35 kg/m2).

Relative to normal weight, both obesity (all grades) and grades 2 and 3 obesity were associated with significantly higher all-cause mortality. Grade 1 obesity overall was not associated with higher mortality, and overweight was associated with significantly lower all-cause mortality. The use of predefined standard BMI groupings can facilitate between-study comparisons.

Mortality increased with increasing BMI in adults without diabetes, but decreased with increasing BMI among their counterparts with diabetes. Future studies need to be better designed to answer the question of whether normal weight adults with diabetes have a higher risk of mortality, by minimizing the possibility of reverse causation. Future studies should also account for prevalent diabetes in all investigations of the BMI-mortality relationship.