Running utilizes both sugar and fat for energy, depending on the context and the demands placed on the muscles. The body always uses a blend of fuel sources, constantly adjusting the ratio. “Sugar” refers primarily to glucose, which is stored as glycogen in the muscles and liver. The body’s energy system aims to efficiently generate adenosine triphosphate (ATP), the universal energy currency for muscle contraction.
Defining the Body’s Fuel Sources
The body relies on carbohydrates and fats to power running and sustained activities. Carbohydrates are stored as glycogen, mainly in the skeletal muscles and the liver. Muscle glycogen is a direct, readily available fuel source for contracting cells. Liver glycogen primarily maintains stable blood glucose levels for the rest of the body, especially the brain.
Fat is stored as triglycerides in adipose tissue and within muscle fibers. Fat has a significantly higher energy density than carbohydrates, providing nine calories per gram compared to four. However, breaking down triglycerides into usable fatty acids (beta-oxidation) is slower and requires more oxygen than carbohydrate metabolism. Glycogen is the quick, short-term energy reserve, while fat is the vast, long-term reserve.
How Running Intensity Affects Sugar Burn
Running intensity is the most immediate factor determining the fuel mix. At rest, the body uses a high proportion of fat because demand is low, allowing time for the slower fat oxidation process. As intensity increases, the body’s need for ATP rises quickly. To meet this demand, the body relies more on carbohydrates because glycolysis, which breaks down glucose, is faster than fat metabolism.
During high-intensity running, such as sprints, the body relies heavily on muscle glycogen and blood glucose. At maximum effort, energy production can become anaerobic (without adequate oxygen), making carbohydrate the only fuel usable at that rate. Conversely, a low-intensity jog allows the aerobic system to efficiently utilize fatty acids, deriving a greater percentage of energy from fat. Even during low-intensity efforts, some carbohydrate is necessary for the complete breakdown of fat.
The Impact of Run Duration on Fuel Switching
While intensity dictates the rate of fuel use, duration determines the necessity of switching fuel sources. As a run continues at a sustained pace, the limited stores of muscle and liver glycogen are progressively depleted. At moderate intensity (60-70% of maximum heart rate), muscle glycogen stores can be significantly lowered within 60 to 90 minutes.
When carbohydrate reserves dwindle, the body begins a metabolic shift to increase its reliance on fat reserves. This physiological shift is a survival mechanism to spare remaining glucose for the central nervous system, which primarily runs on sugar. As the body increasingly turns to fat, the pace becomes harder to maintain, a phenomenon distance runners call “hitting the wall” or “bonking.” This reliance on the slower fat-burning pathway results in decreased power output and reduced ability to sustain the original running speed.