As a personal trainer you're able to perform three types of treatments.
Weight loss session
Will lose weight after the conversation. The amount of weight is random. Higher weight loss is achieved when the treatment is your specialty.
Weight gain session
Will gain weight in the upcoming days. The total weight increase is random, and hidden. Higher values are achieved when the treatment is your specialty.
The daily change in weight is visible on the NPC's
Will lose weight slightly. If all conditions are met, the athletic physique trait