Generally regarded as the most pleasant taste, sweetness is almost always caused by a type of simple sugar such as glucose or fructose, or disaccharides such as sucrose, a molecule combining glucose and fructose.
Carbohydrates are compounds made up of types of sugars. Carbohydrates are classified by their number of sugar units: monosaccharides (such as glucose and fructose), disaccharides (such as sucrose and lactose), oligosaccharides, and polysaccharides (such as starch, glycogen, and cellulose).
Proteins are organic compounds that consist of amino acids joined by peptide bonds. The body cannot manufacture some of the amino acids (termed essential amino acids); the diet must supply them. Proteins, in nutrition, are broken down through digestion by proteases back into free amino acids.
Fats consist of a glycerin molecule with three fatty acids attached. Fatty acids are unbranched hydrocarbon chains,1 connected by single bonds alone (saturated fatty acids) or by both double and single bonds (unsaturated fatty acids). Fats are needed to keep cell membranes functioning properly, to insulate body organs against shock, to keep body temperature stable, and to maintain healthy skin and hair. The body does not manufacture certain fatty acids (termed essential fatty acids) and the diet must supply them.
Although alcohol provides energy, and can thus be compared to macronutrients, it is not a substance that is essential for normal function. The acetic acid in vinegar also provides a similar amount of energy per gram, but again, it is not a nutrient because it is not essential for normal function.
Fat has an energy content of 9 kcal/g (~37.7 kJ/g); proteins and carbohydrates 4 kcal/g (~16.7 kJ/g). Ethanol (grain alcohol) has an energy content of 7 kcal/g (~29.3 kJ/g).