Carbohydrates (think glucose):
Here's a quick summary of how you use energy during endurance events. For muscle contraction, you need ATP. To generate ATP, endurance athletes primarily use aerobic metabolism. Glycogen is broken down into glucose and fats are broken down into fatty acids and glycerol all of which are oxidized in your cells to make ATP. Fats provide more energy per gram than carbohydrates (9 Cal/g of fat versus 4 Cal/g of carbs), but your body must use more oxygen to metabolize the fat. For performance athletes, it is more efficient to use carbohydrates for as long as glycogen stores are available.
There have been lots of studies published about the link between carbohydrate ingestion and increased exercise performance. One reason is that taking in carbohydrates allows you to make ATP while retaining muscle and liver glycogen stores for a longer period of time.
Glucose is absorbed in your small intestine using a sodium-dependent transporter called SGLT1. SGLT1 becomes saturated at a glucose intake rate of 60g/hr. However, fructose and other types of carbohydrates are absorbed by different transporters in the gut, so when a mixed carbohydrate beverage is consumed, you can absorb carbohydrates at a rate of about 90g/hr. If you take in more than 90g of carbs per hour, your body can't use it to make ATP. It appears that absorption of carbohydrates is largely independent of body mass.
On a side note, I actually have a funny story about SGLT1. When I was in Boulder last year for spring break, I visited the Skratch labs headquarters. I just walked in one afternoon and told someone there that I was a cyclist and wanted to learn more about the Skratch labs drink mix. I got to talk to Allen Lim, a Ph.D. and founder of the company, for over an hour about biking and hydration. He found out that I was a med student and starting quizzing me about chemistry and electrolytes. Then he told me never forget SGLT1. To absorb glucose, you need sodium. So there you go. I have remembered SGLT1.
So what is the optimal carbohydrate ingestion rate for cycling performance? One of the most convincing studies I found involved 51 cyclists and triathletes who completed a 2 hour ride at moderate-high intensity and then immediately afterwards did a 20km time trial as quickly as possible. 12 different carbohydrate beverages were tested in a double blind manner, giving athletes a range of carbohydrates per hour: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, and 120 g/hr of carbohydrates in a 1:1:1 glucose-fructose-maltodextrin ratio. All subjects also consumed a noncaloric placebo on one test occasion. Results showed an increase in time trial performance as the ingestion of carbohyrates increased with a peak effect reached at 78g/hr. As athletes consumed more than that, performance began to decrease. According to this study then, the optimal carbohydrate ingestion rate for cycling is 78g of carbs/hr.
One interesting theory out there is that the gut is trainable. Individuals who have a high daily carbohydrate intake may also have an increased capacity to absorb and utilize carbs during exercise. Intestinal carbohydrate transporters are increased when the body is regularly exposed to lots of carbohydrates, and some scientists think that gut training and high carbohydrate intake during racing may be a prerequisite for the first person to break the 2-h marathon barrier. I guess we will have to wait and see if this proves true!
Electrolytes and Fluids:
Fluids and electrolytes are lost through the skin, through breathing and through sweat, urine, and poop. During endurance sports, the amount of water lost through sweat is significantly increased from baseline. Sweat also contains sodium (Na+) and smaller amounts of potassium, chloride calcium, and magnesium. The level of sodium in sweat is typically about 35mEq/L (but ranges from 10-70 mEq/L), while other electrolyte contents in sweat are less (potassium 3-15 mEq/L, chloride 5-60 mEq/L, calcium 0.3-2 mEq/L, magnesium 0.1-1.5 mEq/L). Fluid and electrolyte losses vary greatly between individuals, race distances, course profiles, and weather conditions. Sweat rates can vary from extremely low values to more than 3L/h. The best way to determine fluid losses is to weigh yourself during training and racing and limit weight losses to 2–3%.
Intake of sodium during endurance events is important for the following reasons. First, if you just drink water, you can dilute the amount of sodium in your body and put yourself at risk for hyponatremia (low sodium levels, less than 135 mOsm/L). This can cause symptoms such as weakness, dizziness, vomiting, and even coma if sodium levels get too low. Secondly, as I mentioned above, sodium is needed for glucose uptake via SGLT1. So if you want to absorb glucose, you need some sodium around. Finally, water likes to follow sodium and glucose. So as you absorb sodium, water absorption follows, making it easier to hydrated during endurance events.
There are no clear recommendations about the amount of fluid and the correct electrolyte concentrations to drink due to the fact that individuals sweat at different rates and lose different amounts of electrolytes in their sweat. After sifting through a number of papers, I think the consensus in the literature is to drink enough fluids to replace fluid losses. When this becomes too difficult (due to stomach fullness or time spent getting water), avoid losses amounting to greater than 2% of your body weight as this is associated with a decrease in performance and physical well-being. Definitely avoid drinking too much water. Body water and weight should not increase during exercise. In one study of 488 Boston Marathon runners, 13% had hyponatremia at the end of the race, and 71% of those with low sodium levels actually gained weight during the race, suggesting that over-hydration is a major risk factor for developing hyponatremia.
Some studies recommended starting with a fluid intake of 400–800 ml per hour and a sodium intake of 30 to 50 mmol/L (1.7 to 2.9 g/L NaCl). These recommendations are more of starting points though. Use pre- and post-exercise body weights to determine individual sweat rates and to customize fluid replacement programs.
Caffeine actually used to be on the banned substance list, but the WADA removed it in 2004, so it appears to be one sports-enhancing drug that is allowed in cycling. Caffeine is absorbed in about 30–90 min and acts as a central nervous system stimulant as well as an ergogenic aid by increasing free fatty acid oxidation resulting in glycogen sparing.
One interesting study I found about caffeine use on cycling performance had a similar design to the one described before. 20 cyclists completed a 2h steady state ride immediately followed by a high-intensity time trial (TT). Cyclists were given either caffeine or placebo before, during or late in the stages of the test. Authors found that caffeine intake of 6 mg/kg ingested 1 hr before exercise improved TT performance by 3.4% or on average about 60 seconds (statistically significant). When caffeine was given in 6 x 1 mg/kg doses every 20 min throughout the test, TT performance increased by 3.1% or on average about 54 seconds (not statistically significant in this paper). Interestingly, the authors also investigated whether the ingestion of 3x5 ml/kg coke-a-cola during the last 40 minutes of the test improved TT performance. They found that late ingestion of coke enhanced TT performance by 3.3% (statistically significant). The important finding here is that a relatively small dose of caffeine (about 1.5 mg/kg) taken during the last 40 min of prolonged exercise is just as effective at improving performance compared to taking larger 6 mg/kg doses before or throughout long races.
Fats and Proteins:
I lumped these supplements together at the end because there is really no data out there supporting their use in endurance racing. Some studies found a performance benefit with supplementation, but most the studies I read about did not.
Fat is ingested as either long-chain or medium-chain triglycerides (TGs). Long chain TGs take 2 hours to be absorbed and even longer to be digested. Medium chain TGs are absorbed more rapidly and it was hypothesized that the body could use medium chain TGs as an energy source, which would spare glycogen stores. The most recent study I found on this topic concluded that intake of medium chain TGs does not increase cycling performance. This study had 8 endurance trained athletes complete a 100km TT while consuming a 6% carbohydrate solution, a 6% carbohydrate plus 4.3% medium chain TG solution and a placebo. As we've seen before, authors found that ingesting carbohydrates improved TT performance by 7% compared to placebo (average time to complete the TT improved from 178 mins with placebo to 166 mins with the carbs), but the addition of medium chain TGs did not provide any further performance enhancement (avg time to complete the TT with TGs added was 169 mins). Furthermore, the addition of medium chain TGs was associated with an increased rate of GI problems.
As for protein supplementation, I found a study that addressed this issue using the same 2hr steady state cycling followed by a TT set-up. In a double-blinded way, 12 endurance-trained athletes consumed either 65g/h of carbohydrates or 65g/h of carbs plus 19g/h of protein. Average power output sustained during time trial was similar for both drinks with no differences in time to complete the TT (60:13 ± 1:33 for the carb drink vs. and 60:51 ± 2:40 mins for the carbs+protein drink). Authors of this study also looked at muscle breakdown parameters and strength 24hrs after the test to see if consuming protein during an event might help with recovery afterwards. They found no differences between the groups, suggesting that protein consumption during a race does not enhance race performance or post-race recovery.
Finally, I wanted to address the issue of branched chain amino acids (BCAA) as these are marketed in a number of products including the Roctane GUs and drink mixes that a number of my teammates use. Branched chain amino acids are valine, leucine and isoleucine. These are called essential amino acids because your body does not make them so you have to get them from your diet. They are also different from other amino acids because they can be metabolized directly by your muscles. It has been hypothesized that BCAAs might act as a source of energy and spare muscle glycogen stores. However, studies designed to show a performance benefit with BCAA supplementation failed to demonstrate any enhancement. But, there is evidence to suggest that supplementation with BCAAs after exercise can decrease rates of muscle damage, improve muscle soreness, and play a role in regulating the immune system and preventing infections. So it appears that consuming BCAAs during racing does not offer any advantage, but they may have some beneficial effects in recovery drinks.
Well, that's all I have for now. Long post and lots of reading. My take-away message is that I need to stop drinking water and start drinking something that has mixed carbs and sodium in it along with a bit of caffeine at the start and towards the end. And maybe I should work on training my gut so that I can utilize high doses of carbs during races upwards of 78g/hr!
- Almond, CS et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med. 2005; 352(15): 1550-6.
- Angus, DJ et al. Effect of carbohydrate or carbohydrate plus medium-chain triglyceride ingestion on cycling time trial performance. Journal of Applied Physiology. 2000; 88(1): 113-119.
- Breen L, et al. No Effect of Carbohydrate-Protein on Cycling Performance and Indices of Recovery. Medicine & Science in Sports & Exercise. 2010; 42(6): 1140-48.
- Cox et al. Effect of different protocols of caffeine intake on metabolism and endurance performance. Journal of applied physiology 2002; 93(3): 990-9.
- Coyl, Edward. Fluid and fuel intake during exercise. Journal of Sports Sciences. 2004; 22: 39–55.
- Jeukendrup, Asker. Nutrition for endurance sports: Marathon, triathlon, and road cycling. Journal of sports sciences. 2011; 29(1): S91-S99.
- Negro, M et al. Branched-chain amino acid supplementation does not enhance athletic performance but affects muscle recovery and the immune system. Journal of Sports Medicine and Physical Fitness.2008; 48(3): 347-51.
- Sawka, Michael. American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and science in sports and exercise. 2007; 39(2): 377-90.
- Smith et al. Curvilinear dose-response relationship of carbohydrate (0-120 g·h(-1)) and performance. Med Sci Sports Exerc. 2013; 45(2): 336-41.
- Rehrer, Nancy. Fluid and Electrolyte Balance in Ultra-Endurance Sport. Sports Med. 2001; 31(10): 701-715.