I think you two may be arguing over different interpretations of the word "fat".
sadeianlinguist says that he knows people who are very athletic, healthy and who are also, say, "large people", and hence they are 'fat'. This disproves the claim that fat people are necessarily unhealthy.
Toaster126 seems to say that if someone is athletic and healthy, then they should not be considered fat, rather that they are 'built' or 'buff'. Fat people are unhealthy by definition: a person is fat if and only if the amount of fat in their body so high that it is causing them to have health problems (or is likely to do so in the future) .
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