Alcohol is sometimes called 'the fourth macro-nutrient' because it is not fat, not protein, not carbohydrate, but it does contain significant amounts of calories. In fact, alcohol contains seven calories per gram which makes it more calorie dense than carbohydrate or protein and less dense than fat.
Alcohol is not required for human nutrition and, in high doses, acts as a poison causing serious harm to organs including the liver and brain. Alcohol is also a potentially addictive drug and, in susceptible people causes alcoholism, a progressive and often fatal addictive disease.
In small doses alcohol appears to provide some benefits to human health including reduced risk for hypercholesterolemia and heart disease. Furthermore, alcohol clearly improves insulin sensitivity (the opposite of insulin resistance) and may, therefore, mitigate against some of the effects of metabolic syndrome. Finally, there is limited evidence that alcohol may actually help women lose weight whereas it appears to cause weight gain men.
Alcohol is not found in most living green plants and animals, but is usually produced by the fermentation of sugars by yeasts (single-celled fungi).
Alcoholism is the disease associated with addiction to alcohol.