How much does obesity shorten your life?
At age 40, the difference between the best and worst answer here is about 10.6 years of life expectancy — 11.0 for men and 10.2 for women. That is the gap between “lean” and “severely obese”, holding everything else constant.
Body weight is the second-largest lever in the model, and the shape of the curve matters more than the direction. Mortality risk is not a straight line that climbs with every kilogram — it is a shallow bowl, flat between roughly BMI 20 and 25, rising slowly either side, and steeply only at the extremes.
What each answer costs, at 40
| Answer | Hazard ratio | Years vs. best |
|---|---|---|
| Lean(BMI 18.5–22.5) | 0.96 | best |
| Healthy(BMI 22.5–25) | 0.98 | −0.2 |
| Overweight(BMI 25–27.5) | 1.05 | −0.9 |
| Overweight(BMI 27.5–30) | 1.18 | −2.1 |
| Underweight(BMI under 18.5) | 1.42 | −4.0 |
| Obese(BMI 30–35) | 1.42 | −4.0 |
| Obese(BMI 35–40) | 1.90 | −7.0 |
| Severely obese(BMI 40+) | 2.70 | −10.7 |
Computed from the SSA 2023 period life table with this factor’s hazard ratio applied and every other factor held neutral. Source: Global BMI Mortality Collaboration, Lancet 2016.
Does it matter less as you get older?
Yes, and by a lot. A hazard ratio applies to the years you have left, so the same answer costs less in absolute terms the later you are asked it. That is an argument for changing it early, not for concluding it no longer matters.
| If you are now | Men | Women |
|---|---|---|
| 30 years old | 12.1 yrs | 10.8 yrs |
| 40 years old | 11.0 yrs | 10.2 yrs |
| 50 years old | 9.7 yrs | 9.3 yrs |
| 60 years old | 8.2 yrs | 8.1 yrs |
| 70 years old | 6.2 yrs | 6.5 yrs |
What the evidence actually shows
The Global BMI Mortality Collaboration pooled 239 studies covering more than ten million adults across four continents. Above BMI 25, each additional five units carried roughly 30% higher all-cause mortality. At BMI 40–45 the cost is eight to ten years. The same analysis found elevated risk below 18.5 as well, which is why the model does not simply reward being as light as possible.
What to do about it
Mortality risk is flattest between BMI 20 and 25. Moving one band toward that range is worth more than any supplement on the market.
Where this number is weak
BMI cannot distinguish muscle from fat, and it systematically misreads very muscular people as overweight. It also performs differently across ancestries — the risk thresholds for South and East Asian populations sit several units lower. Treat the band as a rough position on the curve, not a verdict.
More generally: these are observational cohorts, which show association rather than proof of cause. The full methodology sets out every simplification the model makes.
What is it costing you?
The calculator weighs this against your other nine factors and ranks them.
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