God gave us each a metabolic rate. It varies from one person to the next. He will not penalize us for what He gave us. However, if we deliberately overeat or eat unhealthily, it doesn't matter if we're thin or heavy, we ought to do better. It isn't Heaven or Hell, though. It's healthy or unhealthy.
Telling someone that they need to diet because they are overweight, while you gorge because you don't see yourself as unhealthy is like telling someone they can't smoke because they have some signs of lung disease while you smoke 10 packs a day because you haven't yet been diagnosed.
1 Cor 3:17 isn't about what goes into a man--even smoke or drugs or alcohol or too much food. It's about division in the church. It doesn't say "If you destroy your temple" but if anyone does. Division is destructive. Smoking, alcohol, and orders of one pound bacon cheeseburgers with extra mayo, super-sized fries, and big gulp sodas are all unhealthy and inadvisable in large quantity, but none are Heaven/Hell issues, and none is sinful in the first taste.