Breakfast consumption does not rapidly increase liver or muscle glycogen content in well-trained cyclists

A carbohydrate-rich breakfast is commonly consumed by cyclists to compensate for an overnight decline in liver glycogen content and, as such, to maximize liver glycogen stores in the hours before exercise. However, the extent to which liver glycogen content increases in response to the intake of a carbohydrate-rich breakfast in well-trained cyclists remains unexplored. Twelve well-trained male cyclists (age: 25 ± 5 yr; V̇o_2peak: 67 ± 5 mL·min^-1·kg^-1; W_max: 5.8 ± 0.7 W·kg^-1) participated in this trial. Carbon-13 magnetic resonance spectroscopy (¹³C-MRS) at 7 T and magnetic resonance imaging (MRI) at 3 T were applied to assess muscle and liver glycogen concentrations and volume, respectively, before and 3 h after ingesting a carbohydrate-rich breakfast providing 3 g carbohydrates per kg body mass. Following breakfast ingestion, muscle glycogen concentrations, muscle volumes, and total muscle glycogen content did not change (P > 0.05). Liver glycogen concentrations increased by ∼10% (from 164 ± 30 to 180 ± 33 mmol/L; P = 0.036), whereas liver volumes decreased by ∼6% (from 1.96 ± 0.28 to 1.84 ± 0.27 L; P < 0.001) in the 3 h following breakfast ingestion. Consequently, no net change in overall liver glycogen content was observed following breakfast ingestion (from 53 ± 15 to 54 ± 13 g; P = 0.516). Ingesting a carbohydrate-rich breakfast (providing 3 g carbohydrates per kg body mass) does not elevate liver or muscle glycogen content during the subsequent 3-h postprandial period.NEW & NOTEWORTHY This is the first study to simultaneously assess both muscle and liver glycogen content following ingestion of a practical carbohydrate-rich breakfast in well-trained cyclists. No changes were observed in muscle glycogen concentrations or content. Liver glycogen concentrations increased postprandially, but liver glycogen content remained unchanged due to a concurrent decline in liver volume. These findings highlight the importance of accounting for liver volume changes when interpreting postprandial liver glycogen storage responses.