Mixed
For deaths related to mental/behavioral disorders, neurological conditions, and non-transport accidents, lower BMI (up to 24-27 kg/m²) is associated with increased mortality risk, showing an inverse linear association rather than a J-shape.
Don't assume being thin is always safer. For neurological and mental health-related causes of death, this study found that lower BMI (down to 24-27) was associated with higher risk. Maintaining a healthy weight (not underweight) is important for protecting against these specific outcomes.
For mental and behavioural, neurological, and accidental (non-transport-related) causes, BMI was inversely associated with mortality up to 24–27 kg/m², with little association at higher BMIs; for deaths from self-harm or interpersonal violence, an inverse linear association was observed.
Why this rating
Large cohort size and rigorous exclusion of smokers and early follow-up reduce confounding, making the inverse association for these specific causes highly reliable.
Source
Association of BMI with overall and cause-specific mortality: a population-based cohort study of 3·6 million adults in the UK
Krishnan Bhaskaran et al. · The Lancet Diabetes & Endocrinology · 2018
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