Fatty acids are a group of non-polar (non-water soluble) molecules that comprise of a carbon chain with a hydroxyl group and oxygen molecule at one end and a methyl group at the other. The chain lengths vary, but generally the shorter the chain length, the more polar (water soluble) the fatty acid becomes. Formic acid and acetic acid with 1 and 2 carbon are actually short enough to be water soluble. Nutritionally important fatty acids have even numbers of carbons in their chain, with chain lengths of common dietary fatty acids varying in chain length from short chain fatty acids with fewer than 6 carbons, to medium chain fatty acids having between 6 and 12 carbons, to long chain fatty acids having between 12 and 22 carbons and to very long chain fatty acids having more than 22 carbons in their chain. Adipose tissue is composed primarily of stearic acid, an 18 carbon saturated fatty acid, the same fatty acid that predominates in the fat tissue of most animal, which can be seen on red meat as a white solid fat.
Fatty acids are stored in adipose tissue in the form of triglycerides. Triglycerides (also called triacylglycerols) comprise of three fatty acids joined to a molecule of glycerol. Triglycerides provide 9 kcals per gram when oxidised, which is higher than both carbohydrate and protein, which yield 3.75 and 4.1 kcal per gram, respectively. This makes triglycerides a highly efficient storage substance from an energy perspective. The fact that triglycerides can also be stored in the absence of water, further increases the efficiency of storage when compared to glycogen, with every gram of glycogen having 2 to 3 grams of water associated with it. Although many consider body fat to be unsightly, body fat is necessary, for as adipose tissue falls below 3 % of total body weight the risk of death increases significantly. The disadvantage of storing fatty acids is that they cannot be oxidised in the absence of oxygen like carbohydrate, and for this reason are not a good source of instant energy in short duration activities of high intensity.