Let's first define "bad advice" and "good advice."
"good advice" - Advice that actually works, and makes the person like you more
"bad advice" - Advice that doesn't help the person and makes the person like you less.
"Eat less food" would fall in the latter category, pretty obviously.
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"Everybody should have infinite self control" is an obscenely naive starting point that erases the whole practical problem. The practical problem is "What habits require the minimum amount of self-control/unpleasantness to get the results I desire"?