The main dietary macronutrients are fat, protein and carbohydrate. These macronutrients can all be used as a source of energy. Ethanol too is a macronutrient, and it too can be used as a source of energy. The use of macronutrients for energy is interesting because they all have slightly different absorption routes and are used in different ways under different circumstances by the body. Understanding these differences is important if health is to be maintained. The fact that fibre contributes significantly to the daily energy needs of humans is often misunderstood.
Carbohydrates contain roughly 3.75 kcal per gram and include sugars and polysaccharides. The carbohydrates are absorbed to the circulation where they are converted to either glucose for energy production or stored as glycogen in the liver and muscle. The belief that low fat foods do not make you fat because they contain no fat is absurd because glucose is also readily converted to fat and stored in adipose tissue in a process called de novo lipogenesis. Refining carbohydrates by removing their bran and germ layers increases their digestion rates and along with sugar refined carbohydrate is a primary driver of Western lifestyle disease and obesity.
Fat is a hugely diverse group of macronutrients that contain roughly 9 kcal per gram. The belief that fat makes you fat is not based on any real science but is a central meme of the media, diet industry and medical establishment. Long chain fatty acids are absorbed through the lymph system and processed in the liver for transport around the body on proteins. Medium and short chain fatty acids however are absorbed in the blood and processed directly in the liver to produce energy, and in this way resemble carbohydrates. Essential fatty acids cause weight loss because they are not stored, but used for energy or the manufacture of hormones.
Proteins contain about 4.1 kcal per gram, and are generally not used as a source of energy in the body unless intakes are high. Proteins contains amino acids, and the body uses these for important functions like making structural proteins and enzymes. Proteins are also nitrogenous, and the amino acids are used to construct and rebuild muscle tissue. Muscle tissue is actually a store of energy, and during times of starvation this tissue is broken down and converted to glucose for energy. High protein intakes increase satiety and reduce overall energy intake without the need to forcibly restrict calories and also preserve lean mass.
Alcohol, chemically called ethanol, provides roughly 7 kcal per gram. Alcohol is hugely misunderstood nutritionally, with most people considering it a cause of weight gain. However, the nutritional literature clearly shows a weight loss effect with regular alcohol consumption. Alcohol can cause weight loss because it upregulates oxidation of fatty acids in the liver. Alcohol also decreases intake of refined carbohydrates and sugar, both of which are drivers of weight gain. People who drink alcohol regularly eat more calories than those who don't, but they have lower body weights and fewer heart attacks.
Most people think fiber contains no energy, but this is not true. Fibre is just carbohydrate that is not digestible to human enzymes. In this respect most people assume it passes straight through the human gut providing bulk to the food and nothing more. However, fibre is metabolised in the same way in humans as in ruminant animals, in that our gut bacteria ferments to fibre to produce short chain fatty acids. These are absorbed into the circulation and used as a source of energy. In this way fibre can contribute significantly to the daily energy needs of humans.