[JabavuAdams]

How much less happy? And will you still agree when you're 49?

What if you change your habits sufficiently that after an initial period of (slight) unhappiness, eating less no longer makes you unhappy?

[cstejerean]

If you want to increase your lifespan dramatically (like a change from 50 to 100) you need to eat a lot less. You end up eating so little that you literally don't have enough energy to do anything (think < 1000 calories per day).