First and foremost, I'd like to remind you that fat and weight are not synonymous, but they are related. It is possible to gain weight and lose fat, but it is also difficult.
I'm missing critical detail about your trainer, such as who he has trained and the effect he has had on those clients. If your trainer is a reputable person who is specifying your diet and training regimen, I highly recommend you just go with the program. If it needs adjusting they will adjust it on the fly. If not, then you may be right to have concerns.
Second, understand your TDEE (Total Daily Energy Expenditure). This is the amount of energy required to sustain your body's natural processes and your activities. Eating above your TDEE will cause you to gain mass, and eating below your TDEE will cause you to lose mass. Also remember that mass includes both muscle and fat.
Lastly, understand that your exercise regimen dictates just how much of your mass changes affect your fat. This is where the rubber meets the road so to speak.
- If you are eating well above your TDEE, you will gain fat along with muscle no matter how much you exercise
- If you are eating well below your TDEE, you will lost muscle along with fat no matter how much you exercise
- When seeking to change your body composition, eating slightly above or below TDEE is the best approach in order to preserve or build muscle while burning fat.
The problem is that TDEE is nearly impossible to be 100% accurate with. You'll find that the amount of energy needed to maintain a certain mass is a lot more than you expect. The calculators you may find online are reasonably useful to help you get in the ballpark. After that you have to take consistent measurements to determine the effect the changes in diet are making to your body.
- Routinely weigh yourself at the same time of day. Keep your weight in a spreadsheet or app, and average your weight for the week. This gives a more consistent view of your weight changes over time while accounting for daily fluctuations in water weight.
- Routinely measure yourself with a tape measure to make sure your body composition is changing the way you want. If your weight isn't changing the way you want it to, then this can give you an indication of what it is you are gaining.
- Re-educate yourself. If your clothes are fitting better and you like what you see in the mirror, and the scale is telling you that you are gaining weight, then chances are you may have less fat and a bit more muscle.
Finally, just a note that one day of eating a lot of calories or fasting isn't going to make a permanent change to your body composition. It takes several days/weeks to see any real change in body composition.