The glycemic index (GI) significantly affects carbohydrate choices in a fitness diet by indicating how quickly a carbohydrate-containing food raises blood glucose levels. Foods with a low GI, like whole grains and legumes, provide slow-releasing energy, making them preferable for sustained energy during physical activities. Conversely, high-GI foods can lead to rapid spikes and subsequent crashes in energy, adversely affecting performance. Athletes should incorporate low- to moderate-GI foods into their pre- and post-workout meals to maintain stable energy levels, enhance performance, and support optimal recovery strategies.