What’s the science on carb loading (aka – eating that plate of pasta)?
The concept of carb loading aims to increase the stored muscle glycogen in an effort to prolong endurance and/or improve performance. Low glycogen training has been shown to negatively impact exercise intensity in both anaerobic (doesn’t need oxygen to produce energy, e.g. resistance training) and aerobic (needs oxygen to produce energy, e.g. endurance training) exercise. Maximising glycogen stores can reduce fatigue during both anaerobic and aerobic exercise. However, most of the promising research shows results in aerobic, endurance-based activities.
Carb loading for short-duration activity
Very few studies have investigated the impact of carb loading on short-duration anaerobic performance. A study in male basketball players found no difference in peak power after seven days of carb loading following a four-week low carbohydrate diet. While carbohydrate loading didn’t produce higher peak power, the results from the study have implications for the importance of adequate glucose availability. After following the four-week low carbohydrate diet, the players’ performance decreased. However, after seven days of carbohydrate loading and replenishing glycogen storage, peak power returned to baseline. This means carbohydrate loading following a four-week low carb diet is effective at recovering baseline anaerobic power, emphasising the importance of glucose availability for optimal performance. Another research study looked at the impact of carb loading on jump squat power and found no improvement. So, we can conclude carb loading is not necessary for anaerobic, short-duration exercise but the amount of glucose available at the start of an activity is an important factor.
Carb loading for long-duration activity
The effects of carbohydrate loading on long-duration endurance exercise have garnered much more attention in the research world. However, the results differ depending on the event distance/duration and population studied. It’s been found that after an 80-minute rugby game, glycogen stores are no different in players who carb-load for 36-hours before the game to players who don’t. In other research based on individual running race times, carbohydrate loading failed to improve times for 10km and 25km treadmill runs. However, carbohydrate loading prior to a 30km cross-country run and a 30km treadmill run limited fatigue in well-trained athletes. Therefore, carbohydrate loading extends the time to glycogen depletion in running events longer than 30 km, ultimately allowing athletes to maintain race speed for a longer duration and improve race times. It appears endurance activities greater than 90 minutes support the practice of carbohydrate loading as an effective strategy to improve performance and exercise capacity.
There appears to be some discrepancies between women and men using glycogen stores during activity. Women tend to have lower resting muscle glycogen concentrations, which can impact the benefits of carbohydrate loading. Research has shown large differences in these benefits when comparing males and females. When both sexes followed a high carbohydrate diet (~75% of calories from carbohydrate foods), cycling performance increased by 45% in males and only 5% in females. Further, males managed to increase their glycogen stores by 41%, while there were no changes among females.
Knowing this, it may be beneficial for women to increase total calorie and carbohydrate ingestion during the loading phase to maximise glycogen stores.