Carbohydrates provide your body with energy. The simplest are sugars, like glucose, fructose, sucrose and lactose. Complex carbohydrates, like starch, are made up of lots of sugar molecules joined together. The 'identity tags' (antigens) on the surface of all cells are made from carbohydrates joined to proteins.
http://www.sciencemuseum.org.uk/WhoAmI/FindOutMore/Yourbody/Whatdoyourcellsdo/Whatisacellmadeof/Whatdocarbohydratesdoforyou
The amount of energy you'll get from carbohydrate, protein and fat is measured in calories per gram. Fats have the most energy and proteins have the same amount as carbohydrates, but their value as a source of energy is determined by more than the calories gained from one gram. Other factors, such as your activity level and diet, impact which macronutrient is used for energy.
http://healthyeating.sfgate.com/gives-energy-per-gram-fat-protein-carbohydrates-8319.html
The capacity of your body to store muscle and liver glycogen, however, is limited to approximately 1,800 to 2,000 calories worth of energy, or enough fuel for 90 to 120 minutes of continuous, vigorous activity. If you’ve ever hit the wall while exercising, you know what muscle glycogen depletion feels like.
http://www.humankinetics.com/excerpts/excerpts/the-bodyrsquos-fuel-sources
Like an automobile only runs on gasoline, the human body runs on only one kind of energy: chemical energy. More specifically, the body can use only one specific form of chemical energy, or fuel, to do biological work – adenosine triphosphate (ATP).
http://ironman.memorialhermann.org/performance-improvement/sports-science/nutrition/how-the-body-uses-energy/
Energy System 1: Ready fuel for immediate energy. The Immediate Energy system, or ATP-PC, is the system the body uses to generate immediate energy. The energy source, phosphocreatine (PC), is stored within the tissues of the body. When exercise is done and energy is expended, PC is used to replenish ATP.
http://ironman.memorialhermann.org/performance-improvement/sports-science/nutrition/how-the-body-uses-energy/
Carbohydrates that are not easily absorbed, such as fiber, or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids contribute 10 kJ/g (2.4 kcal/g) and 13 kJ/g (3.1 kcal/g) respectively.
https://en.wikipedia.org/wiki/Food_energy
Fat = 9 calories per gram. The calorie content of food depends. on the amount of carbohydrate, pro-. tein, fat, and alcohol it contains. As. you can see, fat is the most concen-. trated source of energy and yields. more than twice as many calories per. unit weight as carbohydrate and pro-.
http://www.dining.ucla.edu/housing_site/dining/SNAC_pdf/CaloriesCount.pdf
Carbohydrates have four calories per gram. Sugars and starches get digested to produce glucose, which is the form of energy preferred by every cell in your body. They also have the advantage of being converted into energy faster than fats and protein.
http://healthyeating.sfgate.com/gives-energy-per-gram-fat-protein-carbohydrates-8319.html
In many countries, only one of the units is displayed; in the US and Canada labels spell out the unit as calorie or as Calorie. Fats and ethanol have the greatest amount of food energy per gram, 37 and 29 kJ/g (8.8 and 6.9 kcal/g), respectively. Proteins and most carbohydrates have about 17 kJ/g (4 kcal/g).
https://en.wikipedia.org/wiki/Food_energy
Fat is the body’s most concentrated source of energy, providing more than twice as much potential energy as carbohydrate or protein (9 calories per gram versus 4 calories each per gram). During exercise, stored fat in the body (in the form of triglycerides in adipose or fat tissue) is broken down into fatty acids.
http://www.humankinetics.com/excerpts/excerpts/the-bodyrsquos-fuel-sources
how much energy does a carbohydrate supply the body?
A carbohydrate supplies about 17 kJ/g or 4 kcal/g of energy to the body. A carbohydrate supplies about 17 kJ/g that is 4 calories per gram.