When I ask athletes why they don’t eat anything during training, the most common reply is simply, ‘I just don’t feel like eating.’ But with evidence showing that consuming calories during training can improve performance, why does working out make us lose our appetite?
Appetite is regulated by circulating hormones, some released by the stomach and fat cells. Certain hormones stimulate appetite, while others increase our sense of satiety, or fullness. Eating and fasting can affect these appetite-regulating hormones, but exercise has also shown to have an effect.
A multitude of physiological responses are seen during exercise, including blood being redistributed away from the gut as well as sympathetic nervous-system activity. These processes result in the release of hormones into the blood stream, some of which contribute to the regulation of appetite, leading to a reduction in hunger signals along with an increase in satiety signals.
Greater exercise intensity also causes a greater reduction in appetite. This makes sense as greater intensities cause more blood flow redistribution away from the gut. So essentially your body is trying to fuel the muscles, heart and lungs while dissipating the heat you’re generating while exercising; adding the stress of digesting food is unlikely to be a priority.