By Jennifer Vierling, Tailwind Nutrition
We’ve all had great days on the bike when everything clicks as well as bad ones when your gut is in knots and nothing seems to work. What’s the difference? Training, adequate rest, and mental preparation are all key, but nutrition is just as important. Nutrition that works for a few hours can leave you feeling sick by hour 3, let alone 6, 12 or 24. You can feel great in cool weather and fall apart on a hot day. Sometimes it seems random, and the sports fuel industry contributes to the confusion with a myriad of claims and buzzwords. So what works for cycling and what doesn’t?
There’s no single answer since people vary in so many ways, but to stack the odds in your favor, it’s helpful to understand the function and interdependencies between 3 key elements of nutrition: hydration, electrolytes, and fuel.
Cyclists tend to under hydrate, particularly on hot, dry, or windy days when sweat evaporates more quickly. Dehydration is dangerous, as is over hydrating with water alone, which can lead to hyponatremia and even death. Beyond serious consequences, adequate hydration is critical to processing fuel and maintaining electrolyte balance. Under hydrating limits the absorption rate of carbohydrates (energy) and electrolytes, so drinking enough water is the basis of good nutrition. How much fluid is enough? This varies from person to person, by temperature, and with exercise intensity, but 24 oz (one large water bottle) per hour is a good starting point, and more if it’s hot.
Most people think of electrolytes as preventing cramps, but that’s an oversimplification of their role in the body (and of cramping). Electrolytes are salts and minerals used by the body to carry on normal functions. Sodium in particular is essential to life and tightly regulated by the body. Drinking too much plain water can dilute sodium in blood to dangerous levels (hyponatremia), and conversely, under hydrating concentrates sodium levels by reducing the water content in blood.
Electrolytes are lost through sweat. When electrolyte levels are out of whack, normal body function is compromised, including one’s ability to keep turning the pedals. The goal then of electrolyte nutrition is to maintain electrolyte balance through adequate hydration and by replacing sweat loss. The presence of sodium in water accelerates the absorption of both from the small intestine, so combining electrolytes with water is the most efficient way to replace both.
The composition of sweat varies from person to person (and even day to day), but on average contains sodium at 900mg/L, potassium at 200mg/L, calcium at 15mg/L, and magnesium at 13mg/L, with additional trace elements. Keep these concentration guidelines in mind when evaluating the electrolyte content of drinks and fuels. Unfortunately, many sports drinks contain only a fraction of the electrolytes found in sweat or tout other ingredients as “electrolytes” that aren’t found in sweat. When in doubt, look at the ingredients and nutrition information to determine the quantities of electrolytes and compare them to sweat.
Finally, cyclists need fuel to avoid running out of energy within a few hours. The liver stores glycogen and meters out glucose to power muscles during exercise until stores run low and the body shuts off non-essential consumption (like pedaling). You’ve probably experienced this as bonking. Moderate and higher intensity workouts burn 500+ calories per hour, so you might be tempted to try to replace these calories, but humans are limited to processing 250-300 calories per hour. Eating more fills your stomach, but it doesn’t get processed. Instead, excess food waits its turn in the digestive tract where it can cause GI distress. We also metabolize fat to help close the gap, but fat metabolism isn’t fast enough to keep up with moderate to higher burn rates. Eventually you will run out of energy and have to stop. So the goal of fueling during exercise is to maximize absorption and utilization of fuel to extend the life of glycogen stores without upsetting the digestive system.
Fueling well begins with taste, and taste is highly personal. Many sports nutrition products are designed to taste good initially or at rest, but become difficult to take after a few hours on the bike. Sweet flavors and thick or sticky consistencies in particular often lead to consuming less as the day goes on, which is exactly the opposite of what you need.
The next stop is the stomach where you’d like fuel to slosh around as little as possible. The factors affecting gastric emptying include the caloric content of fuel, temperature, volume of liquid in the stomach, intensity of exercise, the body’s need for fuel, osmolality (a measure of the concentration of molecules in liquid), and other variables. Solid fuels are the slowest to empty since they have to be broken down and mixed with water to pass. Carbohydrate liquids at concentrations up to 10% empty the fastest, at a rate similar to water when consumed at regular intervals during prolonged exercise. Gels and chews fall somewhere in the middle since they need to be dissolved into liquid. Here again, you can see the importance of mixing fuel and with water to promote gastric emptying.
From the stomach, fuel empties into the small intestine where it is processed and absorbed into the bloodstream. Glucose (often found as Dextrose) is easily absorbed because it’s already in a usable form, and recent research has found combining glucose with another carbohydrate source boosts the maximum rate of absorption. Additionally, the combination of glucose and sodium with water accelerates the rate of water absorption, taking carbohydrates and sodium along with it. Long chain fuels such as Maltodextrin (a chain of Dextrose molecules) must be broken apart, which slows absorption and taxes your digestive system. Complex molecules like protein make heavy demands on the digestive system, which is why protein is one of the leading causes of GI distress amongst endurance athletes.
Everyone’s digestive system is different, and there’s more to the nutrition story than space permits, but the fundamentals of nutrition during exercise are becoming clearer as exercise science advances: adequate hydration is key to maintaining electrolyte balance and absorbing fuels; replace the electrolytes you’re sweating out; and choose a fuel you can consume at a regular rate (250-300 calories per hour) throughout exercise that maximizes calorie absorption and minimizes demands on the digestive tract. Putting these 3 basic concepts together can help you have more of those good days than bad, and who doesn’t want that? Happy pedaling!
_______________________________________ See Gisolfi, CV et al: Effect of sodium concentration in a carbohydrate-electrolyte solution on intestinal absorption. Med Sci Sprots Exerc, 27:1414, 1995  http://en.wikipedia.org/wiki/Perspiration  The effects of consuming carbohydrate-electrolyte beverages on gastric emptying and fluid absorption during and following exercise. By Murray R., Sports Medicine (Auckland, N.Z.) [1987, 4(5):322-51]  See Sport Nutrition: An Introduction to Energy Production and Performance by Asker E. Jeukendrup, Michael Gleeson  See Joe Friel’s post for an interesting discussion of recent research on protein during exercise: http://www.joefrielsblog.com/2011/09/should-you-use-carbohydrate-protein-sports-drinks.html