Effects of timing of pre-exercise ingestion of carbohydrate on subsequent metabolism and cycling performance.
Level 2 - randomized trial
Individual randomized crossover trial
PubMed 12527977 · doi:10.1007/s00421-002-0728-8
What was done
Eight trained subjects (mean age 28 years, VO2max 63.1 ml·kg⁻¹·min⁻¹) completed three experimental trials. Participants ingested 75 g of glucose dissolved in 500 ml of water and rested for 15, 45, or 75 minutes (15-Pre, 45-Pre, and 75-Pre) before performing 20 minutes of cycling at 65% maximal power output followed by a 685 kJ time trial. Plasma glucose, insulin, perceived exertion, and time-trial power output were measured.
What was found
Pre-exercise glucose was significantly higher in 15-Pre [6.6 (0.6) mmol·l⁻¹; P < 0.05] than 45-Pre [4.5 (0.2) mmol·l⁻¹] and 75-Pre [3.7 (0.2) mmol·l⁻¹]. Pre-exercise insulin was higher (P < 0.05) in 15-Pre [72.6 (10.4) µU·ml⁻¹] than 45-Pre [50.8 (9.9) µU·ml⁻¹], which was higher (P < 0.05) than 75-Pre [33.9 (5.5) µU·ml⁻¹]. These differences disappeared within 10 min of exercise. Hypoglycemia (<3.5 mmol·l⁻¹) occurred in 2 subjects (15-Pre), 3 subjects (45-Pre), and 5 subjects (75-Pre). Mean time-trial power was 268 (10) W, 269 (7) W, and 276 (12) W for 15-Pre, 45-Pre, and 75-Pre, respectively, with no significant differences in performance or perceived exertion.
Why it matters
Consuming carbohydrate 15 to 75 minutes before exercise can cause transient rebound hypoglycemia in early exercise, but this metabolic response does not impair subsequent endurance performance.
Limits
The sample size was very small (n = 8) and sex distribution was not stated in the abstract. The abstract reports no fasted or placebo control group, and the findings in trained individuals during a fixed-work cycling time trial may not generalize to other athletic contexts.
Cited by
- supports Consuming rapidly digestible carbohydrates immediately before starting exercise can induce a transient dip in blood glucose because insulin release and exercise-induced muscle glucose uptake simultaneously lower circulating glucose levels.