Passage Summary: Doctors want people to eat less to live longer based on rat studies, but a researcher argues that those rats were just overfed to begin with, so the diet just fixed a problem rather than providing a special longevity boost.

Conclusion: The idea that reduced-calorie diets will extend the life spans of North Americans is not supported by the evidence.

Reasoning: Laboratory animals live longer on restricted diets only because they normally overeat; the diet simply returns them to a natural, healthy caloric intake rather than extending life beyond normal limits.

Analysis: The researcher is attempting to dismiss the doctors' recommendation by providing an alternative explanation for the study's results. To weaken this, we should look for an answer that suggests North Americans are actually in the same boat as the lab animals. If the average North American also eats much more than is 'natural' or 'optimal,' then restricting their calories would, by the researcher's own logic, help them reach their 'normal' (and thus longer) life span. It’s a classic case of a researcher accidentally providing a reason for the very thing they are trying to debunk—turns out, we might just be lab rats with better outfits.