How Does The Body Use Energy

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This is an excerpt from Endurance Sports Diet-3rd Edition by Suzanne Girard Eberle.

The Body'due south Fuel Sources

Our ability to run, bicycle, ski, swim, and row hinges on the capacity of the body to excerpt energy from ingested food. Every bit potential fuel sources, the sugar, fat, and protein in the foods that you eat follow different metabolic paths in the body, only they all ultimately yield water, carbon dioxide, and a chemical energy called adenosine triphosphate (ATP). Think of ATP molecules equally loftier-energy compounds or batteries that store free energy. Anytime you need energy—to breathe, to tie your shoes, or to wheel 100 miles (160 km)—your body uses ATP molecules. ATP, in fact, is the only molecule able to provide free energy to muscle fibers to ability musculus contractions. Creatine phosphate (CP), like ATP, is also stored in small amounts within cells. It'south another high-energy compound that can be rapidly mobilized to help fuel short, explosive efforts. To sustain physical activity, however, cells must constantly replenish both CP and ATP.

Our daily nutrient choices resupply the potential energy, or fuel, that the torso requires to continue to function normally. This energy takes three forms: carbohydrate, fat, and poly peptide. (Run into table 2.i, Estimated Energy Stores in Humans.) The body can store some of these fuels in a grade that offers muscles an immediate source of energy. Carbohydrates, such equally saccharide and starch, for example, are readily broken downwards into glucose, the trunk's principal energy source. Glucose can be used immediately as fuel, or can be sent to the liver and muscles and stored as glycogen. During exercise, musculus glycogen is converted back into glucose, which only the musculus fibers tin can utilise as fuel. The liver converts its glycogen back into glucose, likewise; all the same, it's released directly into the bloodstream to maintain your blood sugar (blood glucose) level. During exercise, your muscles option up some of this glucose and utilize it in add-on to their own private glycogen stores. Blood glucose also serves as the most significant source of free energy for the brain, both at residual and during exercise. The body constantly uses and replenishes its glycogen stores. The sugar content of your diet and the type and amount of preparation that you undertake influence the size of your glycogen stores.

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The capacity of your body to store muscle and liver glycogen, even so, is limited to approximately 1,800 to 2,000 calories worth of free energy, or plenty fuel for 90 to 120 minutes of continuous, vigorous activity. If you've ever hit the wall while exercising, yous know what musculus glycogen depletion feels like. Every bit we exercise, our musculus glycogen reserves continually decease, and blood glucose plays an increasingly greater role in coming together the torso's energy demands. To keep up with this greatly elevated need for glucose, liver glycogen stores become apace depleted. When the liver is out of glycogen, you lot'll "bonk" as your blood glucose level dips too depression, and the resulting hypoglycemia (low blood saccharide) will further tedious you down. Foods that you eat or drink during exercise that supply carbohydrate can help delay the depletion of muscle glycogen and prevent hypoglycemia.

Fatty is the body's most concentrated source of energy, providing more than twice as much potential energy as carbohydrate or poly peptide (ix calories per gram versus 4 calories each per gram). During do, stored fat in the body (in the form of triglycerides in adipose or fat tissue) is broken down into fatty acids. These fatty acids are transported through the blood to muscles for fuel. This procedure occurs relatively slowly as compared with the mobilization of carbohydrate for fuel. Fat is also stored within muscle fibers, where it tin can be more easily accessed during exercise. Different your glycogen stores, which are limited, body fat is a virtually unlimited source of energy for athletes. Even those who are lean and mean have enough fat stored in muscle fibers and fat cells to supply upward to 100,000 calories—plenty for over 100 hours of marathon running!

Fatty is a more than efficient fuel per unit of weight than carbohydrate. Sugar must be stored forth with water. Our weight would double if nosotros stored the same amount of energy as glycogen (plus the water that glycogen holds) that we store every bit trunk fat. Most of us take sufficient free energy stores of fat (adipose tissue or body fat), plus the body readily converts and stores excess calories from any source (fat, carbohydrate, or protein) as body fat. In guild for fatty to fuel exercise, however, sufficient oxygen must be simultaneously consumed. The second role of this chapter briefly explains how pace or intensity, as well as the length of fourth dimension that you practise, affects the torso'due south ability to use fat as fuel.

As for protein, our bodies don't maintain official reserves for use every bit fuel. Rather, protein is used to build, maintain, and repair body tissues, as well as to synthesize of import enzymes and hormones. Under ordinary circumstances, protein meets only 5 percent of the torso's free energy needs. In some situations, nevertheless, such as when nosotros eat too few calories daily or not enough saccharide, equally well every bit during latter stages of endurance do, when glycogen reserves are depleted, skeletal muscle is cleaved down and used every bit fuel. This cede is necessary to access sure amino acids (the edifice blocks of poly peptide) that can be converted into glucose. Call up, your encephalon too needs a constant, steady supply of glucose to function optimally.

Fuel Metabolism and Endurance Exercise

Carbohydrate, protein, and fat each play singled-out roles in fueling exercise.

Carbohydrate

Provides a highly efficient source of fuel—Considering the torso requires less oxygen to fire carbohydrate as compared to protein or fat, carbohydrate is considered the body's most efficient fuel source. Saccharide is increasingly vital during high-intensity exercise when the body cannot process enough oxygen to come across its needs.
Keeps the brain and nervous system functioning—When blood glucose runs depression, yous become irritable, disoriented, and lethargic, and you may be incapable of concentrating or performing fifty-fifty simple tasks.
Aids the metabolism of fat—To burn fat effectively, your body must intermission down a certain amount of sugar. Because carbohydrate stores are limited compared to the body's fatty reserves, consuming a diet inadequate in carbohydrate essentially limits fat metabolism.
Preserves lean protein (muscle) mass—Consuming acceptable sugar spares the trunk from using protein (from muscles, internal organs, or i's nutrition) as an energy source. Dietary protein is much better utilized to build, maintain, and repair body tissues, as well equally to synthesize hormones, enzymes, and neurotransmitters.

Fatty

Provides a concentrated source of free energy—Fat provides more than twice the potential energy that poly peptide and carbohydrate do (9 calories per gram of fat versus 4 calories per gram of carbohydrate or protein).
Helps fuel low- to moderate-intensity activity—At rest and during practise performed at or below 65 percent of aerobic chapters, fat contributes 50 percent or more of the fuel that muscles need.
Aids endurance by sparing glycogen reserves—Generally, every bit the duration or time spent exercising increases, intensity decreases (and more oxygen is bachelor to cells), and fat is the more than important fuel source. Stored carbohydrate (muscle and liver glycogen) are afterward used at a slower rate, thereby delaying the onset of fatigue and prolonging the action.

Poly peptide

Provides free energy in late stages of prolonged exercise—When musculus glycogen stores fall, as normally occurs in the latter stages of endurance activities, the body breaks down amino acids found in skeletal muscle protein into glucose to supply up to 15 percent of the energy needed.
Provides energy when daily diet is inadequate in full calories or carbohydrate—In this situation, the body is forced to rely on poly peptide to meet its free energy needs, leading to the breakdown of lean musculus mass.

Larn more about Endurance Sports Nutrition, Tertiary Edition.