\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\) Show Learning Objectives
You learned in earlier chapters that the macronutrients (carbohydrate, fat and protein) are the only nutrients that give us energy in the form of the calorie. The vast majority of the energy for exercise is derived from carbohydrate and fat. If you remember the protein chapter, you will recall that protein is used for many other important roles in the body. In fact it is considered a waste of good protein/amino acids to use it for energy, especially when carbohydrate and fat do this so well! Next up we discuss how your body actually uses these energy-yielding nutrients. It is important to note, as you move forward, that the direct source of energy for cellular work, including muscle contraction, is a compound called adenosine triphosphate (ATP). Unfortunately very little ATP is stored by the body, so you must continually replenish ATP stores. To do this, we harness the energy from the macronutrients, again mostly carbs and fats, and an additional source known as Creatine Phosphate (CP). What Fuels Our Activities?Adenosine triphosphate (ATP; Figure \(\PageIndex{1}\)) – the energy carrying molecule in the body - provides the direct source of energy for our activities. ATP includes adenosine and 3 phosphate groups. When a phosphate is broken away/off, energy is released. The body regenerates ATP by adding a phosphate back. ATP must be generated continuously since muscles store only enough ATP for 1 – 3 seconds of activity. Figure \(\PageIndex{1}\): Structure of adenosine triphosphate (ATP). (CC BY 3.0; OpenStax College via Wikimedia Commons)After depleting ATP stores, muscles turn to other sources. Initially, creatine phosphate (CP) is used. CP stores some energy that can be used to make ATP. Creatine phosphate stores enough energy for 3 to 15 seconds of maximal physical effort. When CP is broken down into a molecule of creatine and an independent phosphate molecule, energy is released. This energy, along with the independent phosphate molecule, can then be used to regenerate ATP. After creatine phosphate, carbohydrates are the next source of energy for the production of ATP as shown in Figure \(\PageIndex{2}\). Glucose is the primary carbohydrate used to generate ATP in a process called glycolysis (breakdown of glucose). For each molecule of glucose, glycolysis yields 2 ATP molecules and 2 molecules of pyruvate. Pyruvate, which is the primary end product of glycolysis, is further metabolized in one of two ways:
Triglycerides (fatty acids) can also be metabolized to generate ATP. Triglyceride metabolism for energy is a relatively slow process, but is used for low intensity exercise and exercise of long duration. The amount of energy provided depends on the chain length of the fatty acid. For most daily activities, including exercise, we use a mixture of carbohydrate and fat for energy (see Figure \(\PageIndex{3}\)). At rest and lower exercise intensities, we rely more on fat as a fuel source. As exercise intensity increases, we rely more on carbohydrate for energy. Proteins (amino acids) are used minimally as a fuel source for exercise (about 3-6%, if needed). Figure \(\PageIndex{3}\): Fuel Sources Used During Various Exercise Intensities. (CC BY 4.0; Allison Calabrese via Human Nutrition 2020 Ed [Hawaii])For a review of fuel use during exercise, watch this short video: "Nutrition Notes: Fuel Sources for Exercise" by Gatorade Sports Science Institute How Does Physical Activity Affect Nutrient Needs?Energy needs may be higher for athletes, it depends on body size as well as the type, intensity, and duration of physical activity. The recommended diet includes:
Regular exercise increases our need for fluids. During exercise, heat is dissipated through sweating. Heat illnesses occur because our muscles and skin compete for blood flow, so maintaining water balance is critical for physically active people. Hydration recommendations include:
The requirements for some vitamins and minerals may be altered in athletes. Adequate intake of these nutrients can be met with a healthy diet and should not require supplementation.
Key Takeaways
11.4: Fuel and Nutrient Needs for Physical Activity is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by LibreTexts. What is the major source of fuel for the body during rest in light activity?The energy used in post-prandial state during rest and physical activity is derived predominantly from the oxidation of carbohydrate (CHO) and fat.
What is the main source of fuel for the body?Carbohydrates are the main energy source of the human diet. The metabolic disposal of dietary carbohydrates is direct oxidation in various tissues, glycogen synthesis (in liver and muscles), and hepatic de novo lipogenesis.
What is the major fuel in resting muscles?Carbohydrate is the main fuel for resting skeletal muscle in the fed situation, whereas fat oxidation accounts for two thirds or more of oxygen consumption in the postabsorptive and fasted situation.
What is the preferred source of fuel at rest?Our skeletal muscle cells that are responsible for all human movement prefer to use fats as they main source of fuel at rest. These cells only use glucose when insulin levels rise and the increased blood sugar needs to neutralised and stored as glycogen or fat.
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