This study investigated 3 Japanese medalists in judo at the Rio Olympics to clarify factors affecting changes in their eating behavior. The subjects underwent semi-structured interviews, and the data obtained were analyzed using a modified grounded theory approach. This revealed the following points: 1) Individual reflection and introspection had the effect of strengthening intrinsic motivation, leading to acquisition of eating behavior indicators that had high autonomy. 2) Nutritionist guidance and involvement were present as external factors, and the autonomy support of nutritionists affected changes in athlete eating behavior. 3) Meals prepared by the athletes’ mothers while in elementary and junior high schools had some effect on subsequent changes in eating behavior. The above points suggest that a guidance environment enabling autonomy support is needed, even in the area of eating behavior.