Lipids

The nutritionally important lipids are fats (solids) and oils (liquids) that consist of fatty acids with 12- 20 carbons. Most of the lipid found in food is in the form of triglycerides which are fatty acid esters of glycerol. Other types of dietary lipids are cholesterol and phospholipids. The dietary requirement for lipids can be satisfied entirely by consumption of sufficient amounts of linoleic and linolenic acids.

Both fatty acids are 18-carbon polyunsaturated fatty acids differing in the number and position of the double bonds. Linoleic acid has two double bonds with placement of the first double bond at the sixth carbon numbering from the omega carbon, which is the carbon on the opposite end of the carboxylic acid in the fatty acid chain. Linolenic acid has three double bonds with the first double bond on the third carbon from the omega carbon. The placement of the double bonds relative to the omega carbon is significant because it determines the capacity for endogenous synthesis of the fatty acid. Omega-6 and omega-3 fatty acids cannot be synthesized by human enzymes, while the omega-9 fatty acid, oleic acid, can be synthesized endogenously. Thus linoleic and linolenic acid are essential fatty acids in human nutrition. Neither cholesterol (after age 2) nor phospholipids are essential nutrients.