1. tell them drinking puts your muscles in a state of catabolism and if you drink youll destroy all the hard work youve been doing
2. tell them you have heartburn(nobody wants to drink with heartburn unless your an alcoholic)
3. tell them if you have a drink you might punch someone in the face
4.your on probation/parole/pre-sentencing investigation/etc.
one of those has to work, but honestly, you dont need to give a reason to anyone else, when they ask why, tell them you dont need a ****ing reason. If the pressure you any further, then **** them, they dont care about you anyway.