just stick to [X]

Very easy to say, not so easy to do. Food is a particularly tough issue, as there are strong countervailing motivations, in effect all through the day.

Isn't it kinda the first thing you would want to apply your rationality to?

Health in general, yes. Weight is a significant aspect of that.

because it seems to me that additional years to life will bring you more utility than any other activity you could spend that money on.

Additional years of health are probably the most bang for the buck. Yeah.