Lipids are essential for health. There are three types of lipids typically found in foods:
triglycerides, phospholipids and sterols. Triglycerides are the most common lipid found in food.
A triglyceride is made up of glycerol and three fatty acids. These fatty acids can be classified
based on chain length, level of saturation and shape. Phospholipids combine two fatty acids and
a glycerol backbone with a phosphate group, making them soluble in water. Sterols have a
multiple ring structure; cholesterol is the most commonly occurring sterol in our diets and is
found only in animal products.
Lipid digestion begins when triglycerides are broken into droplets by bile. Pancreatic lipases
subsequently digest the triglycerides into two free fatty acids and one monoglyceride. These end
products of digestion, as well as other lipids, are then transported into the intestinal mucosal cells
with the help of micelles. Once inside the mucosal cells, triglycerides are re-formed and
packaged into lipoproteins called chylomicrons. Their outer layer is made up of proteins and
phospholipids, which allows them to travel in the blood. Dietary fat, in the form of triglycerides,
is transported by the chylomicrons to cells within the body that need energy. Triglycerides stored
in the muscle tissue are used as a source of energy during physical activity. Excess triglycerides
are stored in the adipose tissue and can be used whenever the body needs energy.
Dietary lipids play a number of important roles within the body. (1) They provide more than
twice the energy of protein and carbohydrate, at 9 kcal (37 kJ) per gram, and provide the
majority of energy required at rest. Lipids are also a major fuel source during exercise, especially
endurance exercise. (2) Dietary fats provide essential fatty acids (linoleic and alpha-linoleic
acid). (3) Dietary fats help transport the fat-soluble vitamins into the body. (4) Dietary fats help
regulate cell function and maintain membrane integrity. (5) Stored body fat in the adipose tissue
helps protect vital organs and pad the body. (6) Fats contribute to the flavour and texture of foods
and the satiety we feel after food.