Given the countless calculations and analyzes a gamer must do, his brain can burn as many calories as a professional sportsman. It can be physically stressful to sit for hours and hours during the day and always have to be in full concentration. Those who engage in long, multi-day competitions usually prepare and work out at the gym to be able to physically face the challenge.
It's obvious that poker involves math, skill and competition. Each player aims to defeat their opponents and use different strategies to dominate the game. The sporting component in these competitions is very large. Surely the players themselves, who invest a long time in improving their technique to perform professionally, would more easily agree that poker is indeed a sport.