We know that calories are our energy source.  

The main factor in weight gain or loss is calories in vs calories out (it’s really that simple).

If you want to maintain your body weight you need to eat around the same number of calories that you are burning.  

If you want to gain muscle you need to be eating more calories than you are burning.

If you want to lose fat, you need to be eating less calories than you are burning.

Even though we know all of these things we still may not know how much and what to eat!