amount of exercise is a better predictor of lifespan than weight
First, there is no reason for you to care about ranking ("better"), you should only care whether something is a good predictor of lifespan. Predictors are not exclusive.
Second, weight effect on lifespan is nonlinear. As far as I remember it's basically a U-shaped curve.
I think it's only U-shaped if you're plotting mortality rather than lifespan on the y-axis...
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