TL;DR: Chess can burn calories

The Oxford English Dictionary defines sport as “An activity involving physical exertion and skill […] one regulated by set rules or customs in which an individual or team competes against another or others”. Many arguments against chess as a sport are based upon the principle that chess is not physically exerting, as it is played sitting down and chess pieces are lightweight. However, studies have shown that grandmaster chess players show significant signs of physical exertion during important tournaments, with increased heart rates, breathing rates, blood pressure, and muscle contractions.

In 2018, a company called Polar monitored the heart rates of chess players during a tournament. On average, humans burn 120 to 260 calories through sitting down for 2 hours, but Polar found that the Russian grandmaster Mikhail Antipov had burned 560 calories in 2 hours of the chess game. Robert Sapolsky estimates that a grandmaster chess player can burn up to 6,000 calories a day whilst competing in a tournament. As a result of this physical exertion, there are multiple reports of chess players losing substantial amounts of weight during tournaments.

Scientists theorise that these signs of physical exertion are caused by the mental stress that high standard chess players endure. Games can last hours and tournaments weeks, which require extreme levels of concentration and mental dexterity. This mental exertion translates into physical effects, such as muscle tension, perspiration, rapid breathing, and an increased heart rate; effects which also happen to be caused by physical exercise.

Chess is a competitive game which requires great mental skill, is regulated by complex rules, and can be physically exerting, according to some scientific studies. Chess accedes to the Oxford English Dictionary’s definition of sport, and can therefore be considered as one.

TL;DR - Burning calories does not mean that an activity is a sport

[Studies] (https://www.healthline.com/nutrition/6-unusual-ways-to-burn-calories#TOC_TITLE_HDR_5) have shown that activities so simple as drinking cold water, fidgeting, chewing gum and writing. Even typing can burn between [41 and 61 calories per half hour] (https://www.plansponsor.com/trivial-pursuits-how-many-calories-are-burned-typing-on-a-computer/?layout=print). This means that we, simply by conducting a day's work, burn calories.

Sport's, conversely, involve severe physical exertion. Consider a five set tennis match, most notably the [11 hour marathon match at Wimbledon in 2010] (https://olympics.com/en/featured-news/longest-tennis-match-history-grand-slam-record). Although this might be an outlier in terms of length, the number of calories burned will have been astronomical. Lewis Hamilton also revealed [he lost 10 pounds during a single F1 race] (https://www.essentiallysports.com/lewis-hamilton-opens-up-on-the-bizarre-weight-loss-and-physicality-of-racing-in-f1/).

Chess can be physically exerting, but is not necessarily exhausting. The activities that easily come under the definition of sport result in extreme physical exertion. Chess and sports may share mental stress but nor is this unique to sport. An interview or a deadline induce mental stress but that does not make them sports.

In sum, chess shares certain similarities with established sports but that does not mean it is one.