Picking up arbitrarily trends - obesity in this case, even though plausible, still doesn't feel right, ie. you could also choose average global temperature or %

Picking up arbitrarily trends - obesity in this case, even though plausible, still doesn't feel right, ie. you could also choose average global temperature or %-age of CO2 and it would probably match the trend as well. Also they're not addressing more complex implications - ie. if %-age of women (who are slower) increases and directly it doesn't account for total slowdown (as they've shown) maybe it has indirect impact on other runners (males) who are running _together_ with more slower participants (women). I think assumption that people tend to run at the paste of people around them is not completely out of place compared to assumption that obesity slows down marathons.