By Gabe Mirkin, M.D.
Endurance athletes know that they need to take some form of sugar during competition in sports that last more than 70 minutes of all-out effort. Researchers at the University of Bath in England have just shown that cyclists exercise with less effort and have less gut irritation from drinks made with sucrose (regular table sugar, found in fruit juices), rather than glucose, which is found in many sports drinks (American Journal of Physiology–Endocrinology and Metabolism, December 2015).
- Sucrose is a double sugar of glucose bound to fructose. It occurs naturally in many plants and is found in virtually all fruit juices. Sucrose is often extracted and refined from either sugar cane or sugar beets to make common table sugar.
- Glucose is a single sugar that is a building block for all carbohydrates. It is found in all plants, but is rarely found as a single molecule without some other form of sugar.
Best Drinks for Prolonged Exercise
Scientists have shown that compared to drinks that contain only one sugar (glucose), drinks that contain two sugars (glucoseand fructose) help you race faster in competitions lasting more than two hours (Applied Physiology, Nutrition, and Metabolism, April 2012). Drinks that contain both glucose and fructose include fruit juices, soft drinks made with high fructose corn syrup (HFCS) and any drink made with sucrose, which is regular table sugar (Curr Opin Clin Nutr Metab Care, July 2010;13(4):452-7).
Sports drinks that contain only maltodextrin (made from starches extracted from rice, corn, potato and whole grains) are not as effective for racing because maltodextrin contains just the single sugar called glucose. However, drinks that contain maltodextrin plus fructose improve race times better than those that have just maltodextrin which contains only glucose.
Why Racers Need Both Glucose and Fructose
The limiting factor to how fast you can race in sports requiring great endurance is the amount of sugar that you can get into your muscles while you race. How much sugar you can get into your muscles is limited by how fast you can absorb the sugar from your intestines into your bloodstream and then from your bloodstream into your muscles.
Sugar is transported through the intestines into the bloodstream and then to your muscles by special carrier proteins called carbohydrate transporters. Each carbohydrate transporter is specific for each type of sugar, so if you eat two different sugars, you use two different carbohydrate transporters and therefore can take in more sugar. Sugar absorption into your bloodstream is increased by as much as 65 percent by taking glucose and fructose, compared to glucose alone.
According to one report, “The increased carbohydrate oxidation with multiple transportable carbohydrates was accompanied by increased fluid delivery and improved oxidation efficiency, and thus the likelihood of gastrointestinal distress may be diminished. Studies also demonstrated reduced fatigue and improved exercise performance with multiple transportable carbohydrates compared with a single carbohydrate.” (Curr Opin Clin Nutr Metab Care, July 2010;13(4):452-7).
Definition of Carbohydrates, Fiber
Carbohydrates are single, double, triple and more sugar molecules bound together in a row. They are found in all plants and all foods made from plants, such as bakery products and pastas. Thousands of sugars bound together are called starch, and millions of sugars bound together so tightly that you cannot break them down are called fiber. Only single sugars can pass from your intestines into your bloodstream.
When you eat starch, enzymes in your intestines knock off the end sugars, one at a time, and you absorb each sugar as it is freed from the chain. Because you lack enzymes to break down fiber, fiber cannot be absorbed from your upper intestines. However, when fiber reaches your colon, bacteria there use their enzymes to split off sugar molecules from these long chains of sugars, called soluble fiber, and you can absorb sugar and short chain fatty acids through your colon.
During athletic competitions, you can’t wait for foods to reach your colon, so you need to take single and double sugars or starches. Taking two different types of sugars, glucose and fructose together, provides more sugar than taking just one type.
You Go Faster When Muscles Burn More Sugar
Muscles use mostly carbohydrates and fats, and small amounts of proteins, to supply energy for exercise. You have an almost infinite amount of fat stored in your body, but only a very limited amount of sugar stored in your blood, liver and muscles. The faster and more intensely you exercise, the greater the percentage of sugar your muscles use, and the greater percentage of sugar that your muscles use, the faster you can go.
Running Out of Muscle Sugar
When your muscles run out of their stored sugar called glycogen, you “hit the wall” and have to slow down. Your muscles hurt and you haveto work harder and you move more slowly. Many marathoners start to run out of muscle sugar at 18 to 20 miles. All endurance athletes learn, sooner or later, that they can keep going as long as they have sugar stored in their muscles, so they go faster when they take sugar during competitions lasting more than 70 minutes.
Running Out of Liver Sugar
Your brain gets almost all of its fuel from sugar in your bloodstream. When your blood sugar level drops, your brain cannot get enough fuel to function properly, you feel tired and confused and can pass out. Cyclists call this “bonking.” There is only enough sugar in your bloodstream to last three minutes. To keep your blood sugar level from dropping, your liver must constantly release sugar from its cells into your bloodstream, but there is only enough sugar in your liver to last 12 hours at rest, and far less than that when you exercise. During intense exercise, your muscles draw sugar from your bloodstream at a rapid rate. Your liver can run out of its stored sugar, causing your blood sugar level to drop – and you bonk.
Bonking is common in bicycle races when a rider does not eat frequently, but is rare in long distance running races. When you run, your leg muscles are damaged from the constant pounding on the roads and you must slow down. However, you pedal in a smooth, rotary motion which does not damage your muscles very much, so you can continue to pedal at a rapid cadence for many hours.
In Any Hard Exercise Lasting More than 70 Minutes, Take Some Source of Sugar
When you move intensely for more than 70 minutes, you run low on your muscle and liver stores of sugar. In hot weather you can run out of sugar earlier than in cold weather. Almost 80 percent of the energy used to power your muscles is lost as heat. In hot weather, your heart has to work much harder to pump this extra heat in your blood from your muscles to your skin where the heat can be dissipated, so you use more sugar.
Avoid or Restrict Sugar When You Are Not Exercising
Taking sugared drinks can cause very high rises in blood sugar, particularly if you do not exercise or are overweight or diabetic. Every cell in your body is like a balloon full of fluid. When blood sugar levels rise too high, sugar sticks to the outer membranes of cells. Once there, sugar can never get off. Eventually sugar is converted by a series of chemical reactions to sorbitol, which destroys the cells. This cell destruction causes all of the side effects of diabetes: blindness, deafness, heart attacks, strokes, kidney failure and so forth.
Resting muscles are inactive. They need insulin to remove sugar from your bloodstream to protect you from cell damage. However, contracting muscles actively remove sugar from the bloodstream without even needing insulin. So during vigorous exercise, contracting muscles protect you from a high rise in blood sugar. The effect is maximal during vigorous exercise and for up to an hour after you finish. This benefit usually disappears completely in less than 17 hours.
Foods and Drinks to Take During Prolonged Exercise
The following foods are good sources of separate molecules of glucose and fructose, or of sucrose, which is glucose and fructose bound together:
- Fruit juices
- Most carbonated soft drinks with sugar
- All fruits
- Sweet vegetables such as peas, corn, carrots, beets and sweet potatoes
- Nuts and seeds, such as walnuts, Brazil nuts, pecans, cashews, almonds, sunflower seeds
- Foods made from flour such as cake, cookies, crackers, chips, cereals and breads.
Gabe Mirkin, M.D., is a sports medicine doctor and fitness guru. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin has run more than 40 marathons and is now a serious tandem bike rider with his wife, Diana. His website is http://drmirkin.com/. Click to read Gabe’s full bio.
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