What you eat really does have an impact on how effectively and efficiently you can provide energy to your working muscles. The body converts food into adenosine triphosphate (ATP) for fuel through several different energy pathways. Understanding these systems can help you train and eat more effectively, and boost your overall sports performance. Because the body cannot easily store ATP (and what is stored gets used up within a few seconds), it is necessary to continually create ATP during exercise. In general, the two major ways the body converts nutrients to energy are:
These two pathways can be further divided into three main energy systems (listed below). Most often it's a combination of energy systems that supply the fuel needed for exercise. The intensity and duration of the exercise determine which method gets used when. The ATP-CP energy pathway (sometimes called the phosphagen system) is an anaerobic pathway because it doesn't require any oxygen to create ATP. The "CP" stands for creatine phosphate, a naturally occurring compound that enables short bursts of energy.
The ATP-CP pathway supplies about 10 seconds worth of energy and is used for short bursts of exercise, such as a 100-meter sprint. This pathway first uses up any ATP stored in the muscle (about 2 to 3 seconds worth). Then it uses creatine phosphate (CP) to recycle ATP until the CP runs out (another 6 to 8 seconds). After the ATP and CP are used, the body will move on to either aerobic or anaerobic metabolism (glycolysis) to continue to create ATP to fuel exercise. Glycolysis is both an anaerobic and anaerobic system which creates ATP exclusively from carbohydrates, with lactic acid being a byproduct. Anaerobic glycolysis provides energy by the (partial) breakdown of glucose without the need for oxygen.
Glycolosis is considered both an aerobic and anaerobic pathway. This process produces energy for short, high-intensity bursts of activity lasting no more than several minutes. After several minutes, the lactic acid build-up reaches a threshold known as the lactate threshold (LT). When you reach this threshold, you experience muscle pain, burning, and fatigue, making it difficult to keep exercising at this intensity. However, training can increase the threshold. Aerobic metabolism fuels most of the energy needed for long duration activity. It uses oxygen to convert macronutrients (carbohydrates, fats, and protein) to ATP. This system is a bit slower than the anaerobic systems because it relies on the circulatory system to transport oxygen to the working muscles before it creates ATP.
Aerobic metabolism is used primarily during endurance exercise, which is generally less intense and can continue for long periods of time. During exercise, an athlete will move through these metabolic pathways. As exercise begins, ATP is produced via anaerobic metabolism. With an increase in breathing and heart rate, there is more oxygen available and aerobic metabolism begins and continues until the lactate threshold is reached and anaerobic metabolism kicks in again. Sports nutrition is built upon an understanding of how macronutrients, such as carbohydrates, fat, and protein, contribute to the fuel supply needed by the body to perform. Macronutrients contribute to the process in different ways.
Each macronutrient has unique properties that determine how it gets converted to ATP.
Because your body uses different pathways to create energy, and each pathway relies on different macronutrients, it's important to consume fat, carbohydrates, and protein in your diet.
Nutrients get converted to ATP based on the intensity and duration of activity, with carbohydrate as the main nutrient fueling exercise of a moderate to high intensity, and fat providing energy during exercise that occurs at a lower intensity. Fat is a great fuel for endurance events, but it is simply not adequate for high-intensity exercises such as sprints or intervals. If exercising at low intensity (or below 50% of max heart rate), you have enough stored fat to fuel activity for hours or even days, as long as there is sufficient oxygen to allow fat metabolism to occur. As exercise intensity increases, carbohydrate metabolism takes over. It is more efficient than fat metabolism but has limited storage capacity. Stored carbohydrate (glycogen) can fuel about two hours of moderate to high-level exercise. After that, glycogen depletion occurs (stored carbohydrates are used up). If that fuel isn't replaced, athletes may hit the wall or "bonk." An athlete can continue moderate to high-intensity exercise for longer by simply replenishing carbohydrate stores during exercise. This is why it is critical to eat easily digestible carbohydrates during moderate exercise that lasts more than a few hours. If you don't take in enough carbohydrates, you will be forced to reduce your intensity and tap back into fat metabolism to fuel activity. In fact, carbohydrates can produce nearly 20 times more energy (in the form of ATP) per gram when metabolized in the presence of adequate oxygen than when generated in the oxygen-starved, anaerobic environment that occurs during intense efforts (sprinting). The three main energy systems the body uses to create ATP are: the ATP-CP energy pathway (or phosphagen system), glycolysis, and aerobic metabolism. The ATP-CP energy system powers very short bursts of exercise, and supplies up to 10 seconds of power and energy to your body. The ATP-CP energy system works by using ATP and creatine phosphate (CP) to give your body fuel. While ATP provides about 2 to 3 seconds of energy, the CP provides 6 to 8 seconds. Together, they can provide enough energy for a quick 10-second sprint. The ATP-CP, or phosphagen, system is the first energy pathway that is used during exercise. This energy pathway is quickly depleted and allows for a quick burst of fuel to lift heavy weights or perform a short sprint. Energy pathways in the body can adapt as you increase your fitness. With appropriate training, these energy systems become more efficient and allow you to exercise at a higher intensity for longer periods of time.
Fat is an important component of a diet designed to fuel exercise. One gram of dietary fat equals nine calories, and one pound of stored fat provides approximately 3,600 calories of energy. This calorie density (the highest of all nutrients), along with our seemingly unlimited storage capacity for fat, makes it our largest reserve of energy. These calories are less accessible to athletes performing quick, intense efforts like sprinting or weight lifting. But fat is essential for longer, slower, lower intensity and endurance exercise, such as cycling and walking. Everything we eat is made up of macronutrients (protein, carbohydrates, and fat) and micronutrients (vitamins and minerals). These are converted to energy inside the body, helping to fuel all of our bodily functions. Dietary fat has been blamed for many health problems, but it is actually an essential nutrient for optimal health. Adipose tissue (stored fat) provides cushion and insulation to internal organs, protects nerves, moves certain vitamins (A, D, E, and K) throughout the body, and is the largest reserve of stored energy available for activity. Stored body fat is different from dietary fat. Body fat is only stored in the body when we consume more calories than we use, from any and all foods we eat, not just from dietary fats. There is an optimal level of body fat for health and for athletic activity. Fat provides the main fuel source for long-duration, low- to moderate-intensity exercise (think endurance sports, such as marathons). Even during high-intensity exercise, where carbohydrate is the main fuel source, the body needs fat to help access the stored carbohydrate (glycogen). Using fat to fuel exercise, however, is not a simple process. It has three key parts:
So athletes need to carefully time when and how much fat they eat. In general, it’s not a great idea to eat foods high in fat immediately before or during intense exercise. First, the workout will be done before the fat is available as usable energy. And second, doing so can cause uncomfortable gastrointestinal symptoms, such as nausea, vomiting, and diarrhea. Popular low-carbohydrate and high-fat diets, such as the ketogenic diet and Paleo diet, all work on the same premise. The theory is that lower carbohydrate intake, coupled with high fat and moderate to high protein intake, leads to burning body fat as the main fuel source while exercising. There is, in fact, some scientific evidence that long-term low-carb/high-fat diets are safe and may help improve metabolic risk factors for chronic disease. In studies, these diets have shown to be beneficial for performance in ultra-endurance sports—but at least several months of adaptation to a low-carb/high-fat diet are required for metabolic changes to occur.
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