By: Miranda Tomenson, MSc
It is well known that carbohydrate loading is effective at improving performance by increasing muscle glycogen stores. It is also known that consuming carbohydrates during prolonged exercise (> 90 minutes) can increase performance time to exhaustion (meaning you can exercise for longer at a given intensity) by maintaining blood glucose levels. This is very relevant for endurance events like triathlon, cycling and running. While there is plenty of published information on the topic, most of it does not appear to be gender specific. In fact, there is some evidence that females respond differently than males to carbohydrate loading (6). Also, there is some research that shows that the effect of carbohydrate loading can be different in different phases of the menstrual cycle (glycogen storage is enhanced in the luteal phase compared to follicular phase)(2,7) . Thus, I decided to look further into this subject.
The most recent study on the subject that I could find was published in the Journal of Applied Physiology by Andrews, et al. (1). This study examined the effect of carbohydrate loading and supplementation in female runners. Trained female runners (aged 20 – 40 years old) performed three 24km treadmill runs. In the 4 days leading up to each run, the athletes followed the same workout routine and a prescribed diet. The diet was either mixed carbohydrate (50% of their total calories from carbohydrate) or high carbohydrate (72% of calories from carbohydrate). Both diets were a similar caloric value. During the run, the athletes were either given a solution containing 6% carbohydrates (0.47g carbohydrates/bodyweight in kg/hour) or a placebo with no carbohydrates.
The three trials were as follows:
#1 – No carbohydrate (CHO) loading & No CHO supplementation
#2 – No CHO loading & CHO supplementation
#3 – CHO loading & CHO supplementation
Results:
The results (Table I) showed that there was a trend towards an improved run time with supplementation alone and carbohydrate loading with supplementation, but the results were not considered statistically different. Additional results looked at the respiratory exchange rate (RER, which indicates percent of energy coming from carbohydrates), lactate concentration (another measure of carbohydrate metabolism), blood glucose levels, and blood glycerol levels (a product of fat metabolism). Results showed RER, lactate and blood glucose values were similar near the end of the run, both with supplementation alone (trial#2) and carbohydrate loading with supplementation (trial#3), which was significantly higher compared to none (trial#1). Glycerol was similar at the end of the run with both supplementation alone (trial#2) and carbohydrate loading with supplementation (trial#3), and significantly lower compared to none (trial#1).
Taken together, this indicated that both supplementation alone and carbohydrate loading with supplementation resulted in #1) carbohydrates as the preferred source of fuel during prolonged exercise and #2) higher blood glucose levels at the end of exercise. Both of which may aid in the ability to maintain a given exercise intensity for endurance exercise. It is interesting that this study showed that carbohydrate loading with supplementation and supplementation alone gave very similar results. This would suggest that carbohydrate loading did not have much of an effect.
In order to explain why carbohydrate loading didn’t appear to have any significant effect, the authors of the study suggest that the amount of carbohydrates given during the 4 days (72% CHO, 5.5g CHO/kg/day) leading up to the run test may not have been sufficient. This is different from men who are able to improve their run performance by ingesting a diet of 75% CHO. The results in this study are corroborated by a study by Tarnopolsky et al. (6) that showed that women who consumed a similar amount of carbohydrate (6.4g CHO/kg/day) were not able to increase glycogen stores. However, a study by Brewer et al. (3) showed that when both men and women consumed a higher CHO diet of 7-7.5g CHO/kg/day for 3 days, they increased their run time to exhaustion significantly (pooled data from men and women). Additionally, Walker et al. (7) showed that when six trained women consumed an average of 8g CHO/kg/day they were able to increase their glycogen stores by 13% and improve performance in endurance cycling (cycling time to exhaustion at the same intensity was increased). Thus, it’s possible that if the subjects had consumed more than 7g/kg/day in this study, they might have been able to improve carbohydrate utilization and their run performance. In order to do this, it would likely mean they would have had to also increase their daily calorie consumption, which is not necessarily the case for men.
Even though the performance time with CHO supplementation during exercise didn’t significantly improve performance, it is important to note that
75% of the subjects had a faster run time when supplementing carbohydrates than those ingesting water only. Also, supplementation was able to increase carbohydrate utilization and maintain higher blood glucose levels. Similar studies as this one have shown similar benefits from ingesting 26g CHO per hour in a 6 – 8% CHO solution (5).
SO…what does that mean? How much carbohydrate should female endurance athletes ingest prior to and during an endurance event?
Before: Based on this study, my recommendations for females are that they need to consume more than 6.5g CHO/kg/day for the 3-4 days leading up to any competition that lasts longer than 90 minutes. I would suggest that, because 8g CHO/kg/day has been found to increase muscle glycogen and performance in cyclists (7), this amount should also be used for carbohydrate loading by runners and triathletes. So, for example, if you weigh 60kg then you should consume about 480g of carbohydrates per day. Keep in mind that is the equivalent to about 1920 calories, which means taking in at least 2500 calories per day. For females, it is important that you are not afraid of taking in a few extra calories in the few days prior to the race. Your weight will increase a bit, but that is mostly due to water retention (1g of carbohydrates retains about 3g of water). You would have to take in an extra 3300 calories to gain a pound. This regimen would probably only result in an excess of 1500 – 2000 calories which will most likely be burned off during the race.
During: My recommendations are similar to what the study showed, but may vary in extreme temperature situations. I would recommend ingesting a 6 – 8% solution of carbohydrate during exercise. That’s equivalent to 1 scoop of eLoad (28g) per 500mL water (28g/500mL = 0.06 = 6%). In fact, 12% CHO solution is considered maximum, as any higher starts to impair gastric emptying and may cause discomfort (5). In terms of how much fluid is needed, this study recommended ~ 580mL/hr, but I would also use the rule that you need to replace about 50% of water lost with fluid ingested.
Happy carb loading!
References:
1. Andrews JL,Sedlock DA,Flynn MG,Navalta JW ,Ji H. Carbohydrate loading and supplementation
in endurance-trained women runners. J Appl Physiol 95: 584-590, 2003.
2. Bailey SP, Zacher CM, and Mittleman KD. Effect of menstrual cycle phase on carbohydrate supplementation during prolonged exercise to fatigue. J Appl Physiol 88: 690 – 697, 2000.
3. Brewer J, Williams C, and Patton A. The influence of high carbohydrate diets on endurance running performance. Eur J Appl Physiol 57: 698 – 706, 1988.
4. Hargreaves M. Metabolic responses to carbohydrate ingestion: effects on exercise performance. In: Perspectives in Exercise Science, edited by Lamb DR and Murray R. Carmel, IN: Cooper, 1999, p. 93 – 119.
5. Mitchell JB, Costill DL, Houmard JA, Fink WJ, Pascoe DD, and Pearson DR. Influence of CHO dosage on exercise performance and glycogen metabolism. J Appl Physiol 67: 1843 – 1849, 1989.
6. Tarnopolsky MA, Atkinson SA, Phillips SM, and MacDougall JD. Carbohydrate loading and metabolism during exercise in men and women. J Appl Physiol 78: 1360 – 1368, 1995.
7. Walker JL, Heigenhauser GF, Hultman E, and Spriet LL. Dietary carbohydrate, muscle glycogen content, and endurance performance in well-trained women. J Appl Physiol 88: 2151 –
2158, 2000.
