Can Jason and Lucia change fitness or body shape?
Yes. Training and food can affect stats and physique.
The demonstration described strength, cardio, fitness, stamina, food buffs, and usable gym equipment.
The evidence
Visible in official footage; wider behavior may remain unconfirmed.
FamitsuFamitsu interview with Rockstar North co-head Rob NelsonEL PAÍSEL PAÍS visits Rockstar North for a GTA VI demonstrationRockstar GamesGrand Theft Auto VI official siteRead the full supporting guideWhat would change this answer?
Final balancing details or a revised Rockstar systems description.