Triglycerides, consisting of a glycerol bonded to three fatty acids via ester linkages, are considered fats because of this specific chemical structure. Other compounds with different numbers of hydroxyl groups, like butane-1,2,3,4-tetraol or ethane-1,2-diol, can form ester linkages but don't match the triglyceride structure, thus are not fats.
Fat molecules are mainly referred to as triglycerides, which are esters composed of glycerol and fatty acids. The glycerol in a triglyceride is an organic compound (alcohol) with three carbons and three hydroxyl (-OH) groups. During the formation of a triglyceride, each of these hydroxyl groups from the glycerol forms an ester linkage with a fatty acid through a dehydration synthesis reaction, releasing three water molecules.
Glycerol is specifically a trihydroxy alcohol, meaning it contains exactly three hydroxyl groups, which allows it to bond with three fatty acids to form a triglyceride. This is why compounds such as butane-1,2,3,4-tetraol (with four hydroxyl groups) and ethane-1,2-diol (with two hydroxyl groups) are not classified as fats in biological systems, despite their ability to form ester linkages with fatty acids. They do not fit the specific chemical structure that is characteristic of a triglyceride, which is the esterification of exactly three fatty acids to one glycerol molecule.
Fatty acids can range from 4 to 36 carbons in length, with the most common containing 12-18 carbons. Triglycerides found in nature often contain mixed fatty acids rather than three identical ones, making the natural occurrence of simple triglycerides rare.