Obesity and risk: measuring it in years of life

https://www.thelancet.com/journals/landia/article/PIIS2213-8587(18)30288-2/fulltext

"Causes of death with weak or no biological link to excess bodyweight, such as mental health, behavioural, neurological and accidental causes, and suicide, were not associated with increased BMI, but with underweight (<18 kg="" m="" p="">
"In this study, the J-shaped association between BMI and all-cause mortality was partly driven by the associations between lower BMI or underweight and increased mortality from mental and behavioural, neurological, and external causes. There is a high likelihood that these associations reflect methodological issues such as reverse causation (ie, low BMI is the consequence rather than the cause of these conditions), but further research is needed to disentangle complex associations between these mental health and neurological conditions and bodyweight."

"This study also quantified the population attributable fraction (PAF), an estimate of the contribution of overweight and obesity to total mortality, or the potential population-level reduction in mortality given the hypothetical scenario in which everyone had been of optimal bodyweight. Because this metric accounts for the prevalence of exposure in the population, it arguably better reflects the disproportionate burden for underweight versus overweight and obese categories than the J-shaped association. As such, the authors estimated that, assuming causality, overweight and obesity (BMI ≥25 kg/m²) contributed to 5·5% of total deaths, whereas underweight contributed to 0·7%, in their UK-based study population. The estimate for excess bodyweight with mortality is similar to that of the 2015 Global Burden of Disease Study,3 which estimated that, globally, overweight and obesity contributed to approximately 7·1% (95% uncertainty interval 4·9–9·6) of total deaths."

"Although the debate might persist as to the precise point at which the association between continuous BMI with excess mortality becomes statistically significant, it is important to note that most people will gain weight throughout midlife, which is associated with increased subsequent risk of chronic diseases and mortality."

"Many individuals with a BMI in the range of overweight (25·0–29·9 kg/m²) are already on a trajectory of gaining more weight that will transition them into the BMI range of obesity (≥30·0 kg/m²). Therefore, although the excess mortality associated with overweight is relatively small, it is important for overweight individuals to prevent further weight gain. Even among older populations, a plateau or decline in bodyweight often masks a trajectory of fat mass gain, offset by losses in lean body mass (ie, decreases in muscle tissue and bone density).6 Therefore, it is important for older individuals to prevent an increase in waist size, a marker of abdominal obesity, while minimising loss of muscle mass. Additional studies might shed more light on the role of body fat distributions and different fat depots in chronic disease morbidity and mortality. Meanwhile, current efforts need to be intensified to identify more effective and impactful strategies for prevention of weight gain and obesity-related comorbidities. "
Excellent editorial piece by Tobias and Hu
https://www.thelancet.com/journals/landia/article/PIIS2213-8587(18)30309-7/fulltext