Carbohydrates help to spare what during exercise

Carbohydrate and fat are the two primary fuel sources oxidized by skeletal muscle tissue during prolonged (endurance-type) exercise. The relative contribution of these fuel sources largely depends on the exercise intensity and duration, with a greater contribution from carbohydrate as exercise intensity is increased. Consequently, endurance performance and endurance capacity are largely dictated by endogenous carbohydrate availability. As such, improving carbohydrate availability during prolonged exercise through carbohydrate ingestion has dominated the field of sports nutrition research. As a result, it has been well-established that carbohydrate ingestion during prolonged (>2 h) moderate-to-high intensity exercise can significantly improve endurance performance. Although the precise mechanism(s) responsible for the ergogenic effects are still unclear, they are likely related to the sparing of skeletal muscle glycogen, prevention of liver glycogen depletion and subsequent development of hypoglycemia, and/or allowing high rates of carbohydrate oxidation. Currently, for prolonged exercise lasting 2-3 h, athletes are advised to ingest carbohydrates at a rate of 60 g·h⁻¹ (~1.0-1.1 g·min⁻¹) to allow for maximal exogenous glucose oxidation rates. However, well-trained endurance athletes competing longer than 2.5 h can metabolize carbohydrate up to 90 g·h⁻¹ (~1.5-1.8 g·min⁻¹) provided that multiple transportable carbohydrates are ingested (e.g. 1.2 g·min⁻¹ glucose plus 0.6 g·min⁻¹ of fructose). Surprisingly, small amounts of carbohydrate ingestion during exercise may also enhance the performance of shorter (45-60 min), more intense (>75 % peak oxygen uptake; VO(₂peak)) exercise bouts, despite the fact that endogenous carbohydrate stores are unlikely to be limiting. The mechanism(s) responsible for such ergogenic properties of carbohydrate ingestion during short, more intense exercise bouts has been suggested to reside in the central nervous system. Carbohydrate ingestion during exercise also benefits athletes involved in intermittent/team sports. These athletes are advised to follow similar carbohydrate feeding strategies as the endurance athletes, but need to modify exogenous carbohydrate intake based upon the intensity and duration of the game and the available endogenous carbohydrate stores. Ample carbohydrate intake is also important for those athletes who need to compete twice within 24 h, when rapid repletion of endogenous glycogen stores is required to prevent a decline in performance. To support rapid post-exercise glycogen repletion, large amounts of exogenous carbohydrate (1.2 g·kg⁻¹·h⁻¹) should be provided during the acute recovery phase from exhaustive exercise. For those athletes with a lower gastrointestinal threshold for carbohydrate ingestion immediately post-exercise, and/or to support muscle re-conditioning, co-ingesting a small amount of protein (0.2-0.4 g·kg⁻¹·h⁻¹) with less carbohydrate (0.8 g·kg⁻¹·h⁻¹) may provide a feasible option to achieve similar muscle glycogen repletion rates.

Carbohydrates, fats and proteins undergo a synergistic process that provides energy to keep your engines running. The amount of each nutrient used to fuel cardiovascular exercise varies from person to person and the type of exercise performed. Whether you want to reduce body fat or improve your running time for the 400-meter sprint, your performance depends on your ability to metabolize various nutrients efficiently.

Carbohydrates are your body's first fuel source to jump-start your workout; carbs are stored in your muscles as glycogen. Your liver also stores glucose -- a simple carbohydrate -- and releases it into your bloodstream during exercise. A 1999 study published in the "Journal of Applied Physiology" showed that cyclists who consumed a low-carbohydrate diet increased their performance time by almost 100 percent in cold temperature when they were tested after consuming a high-carb diet. When the same test was done in a hot environment, the same cyclists increased their performance time by about 22 percent. The amount of carbohydrates used depends on the exercise intensity. At 25 percent of your maximum heart rate -- or MHR -- no muscle glycogen is used and a small percentage of blood glucose is used as fuel. As exercise intensity increases toward 65 to 85 percent of your MHR, the demand for carbs increases. Because carbohydrate metabolism requires no oxygen to produce energy and the process is faster than fat metabolism, your body prefers to use carbs as fuel at very-high-intensity exercise.

While carbs are the kindling to stoke the energy furnace, fat is like the slow-burning log your body uses for prolonged cardiovascular exercise. Unlike carbs, fat metabolism needs oxygen to convert into energy. The amount of fat used for fuel, which is in the usable form of triglycerides, depends on duration and exercise intensity. At low-intensity exercise, fat is the primary fuel source. As exercise intensity increases to 65 to 85 percent of your MHR, the amount of fat used also increases, but the percentage of fat use drops as the body relies more on carbohydrates. Between one to three hours of cardiovascular exercise, your body increases fat use from 75 percent to 85 percent of your energy expenditure. However, as your carbohydrate level in your muscles and liver decreases, fatigue settles in and performance diminishes, even if fat is still present.

Although protein can be used for energy by converting its structure to glucose, it's not advisable to rely on it for energy. Proteins provide structure for all organ systems as well as your immune system. However, your body will use protein as an alternative energy source if carbohydrate level is low. Protein is still used at a very low percentage for fuel during cardiovascular exercise, even when there are sufficient carbohydrates present. Even so, new proteins are also formed during exercise to counter the rate of protein breakdown. To maintain lean muscle, exercise physiologist Len Kravitz of the University of New Mexico suggests you consume a meal consisting of carbohydrates and protein within 45 minutes after your workout. The carbohydrates spare proteins from being used as energy.

Oftentimes, you don't need to do more than an hour of cardio to improve your fat-burning potential. Your body continues to expend calories at a higher rate than at a resting state for many hours after high-intensity exercise. This condition is called EPOC -- excess post-exercise oxygen consumption -- where your body continues to rely on fats and carbohydrates to fuel the body as it repairs damaged muscle tissues, balances hormone levels and cools body temperature. In a 2011 study published in "Medicine and Science in Sports and Exercise," a 45-minute bout of high-intensity exercise can elevate metabolism for 14 hours in which subjects were burning an average of an extra 190 calories at the 14th hour.