An athlete is used to performing high-intensity (90% VO2max) aerobic exercise; however, she will now be changing to longer-duration, submaximal exercise on a regular basis. This change in training strategy will typically result in what changes concerning energy supply?

transition from almost exclusively carbohydrate to mostly fat

Explanation:

When an athlete is performing high-intensity aerobic exercise, this is for strength and hence exercise for strength needs an athlete to take more of food with carbohydrates and not that of fats, as what is needed is energy. However, when the reason for the exercise is now for endurance, longer-duration submaximal exercise is performed and then the best food for such exercise is fat, as the exercise turns most of the fats into muscle.

tell their parents and make sure she gets from a counselor or therapist. be supportive and be there for her. be a kind and good friend. spend time with her to make her feel like she's not alone. i'm not a professional but i really hope they stays strong.