I never study Biology. So I am not sure my following theory is right or wrong.

At beginning, I though "digest food" is the same as "absorb nutrition from food".  But after much thinking, if I am slim people, my digest food system is fast, meaning I can absorb food for my body very fast, so my body should be very fat what?? So, I start thinking it is wrong theory.

Now I am thinking "digest food" is not "absorb nutrition from food". The faster digest the food, the lesser nutrition of food that i get, because my body doesn't have enough time to absorb all the nutrition from the food, because the food is too fast to be digested to become shit. For example, my body need 5 hours to absorb nutrition from the food, so if my digest system is fast, it digest the food in 2 hours, meaning at the 3rd hour, there is no food on my stomach.

So, conclusion:
People slim is because body digest food is faster than body absorb nutrition from food. So, if you want to be fat, you must:
1) Increase the speed of absorb nutrition from food, or
2) Slow down the speed of digesting food, or
3) Eat another meal immediately once the food is digested. (This mean you need to eat 6 or 7 times per day and your shit will be a lot.... haha)