Strong evidence
Carbohydrate availability is a determinant of performance in prolonged and high-intensity training.
In practice
If sessions are falling apart late on, being under-fuelled is a likelier explanation than being under-trained. Eat properly before hard or long sessions.
Source
- Thomas DT, Erdman KA, Burke LM (2016). Nutrition and athletic performance. Medicine & Science in Sports & Exercise, 48(3):543–568. doi.org/10.1249/MSS.0000000000000852