The claim that both overweight and underweight wreck your endurance doesn't hold up in the research.

Other research actually contradicts both parts of that claim. For the excess fat part, a study on people with COPD found that overweight patients could walk farther and had stronger muscles than normal-weight patients—the opposite of what the post says. For the underweight part, studies on kids found that being underweight didn't make them do worse on fitness tests, and another found that underweight kids performed about the same as normal-weight kids, with both doing better than overweight kids. So the evidence suggests the relationship between weight and stamina isn't as simple as the post makes it sound, and at least in some groups, being overweight might even be protective.