Lipids are fats that have important roles in the body. Although most people think that fat is harmful to health, the truth is that fat is important to life. Fat provides energy and depending on the classification of lipids, it plays an important role in metabolism.
What Are Lipids?
Lipids are insoluble organic compounds that consist of fat and oil. The chemical composition of these molecules includes hydrogen, carbon, and oxygen. They provide high energy and perform three important biological functions in the body: to provide structure to cell membranes, to store energy, and to function as signaling molecules. The classification of lipids includes phospholipids, triacylglycerols, and sterols.
What Are the Functions of Lipids in the Body?
Lipids play important roles in the normal function of the body:
- To serve as structural building materials for cell membranes and organelles
- To provide energy for the organisms, which is greater than what’s provided by carbohydrates or proteins.
- To serve as signaling molecules and molecular messengers in the body.
However, lipids also serve as biomarkers of certain diseases and play some role in genetic modification and chronic disease.
What Are the Food Sources of Lipids?
Lipids or fats are macronutrients that are important in human nutrition. They are found mainly in dairy foods and meats, as well as other foods. Rich sources of dietary fats include soybeans, nuts and seeds, olives, and avocados, which contain essential fatty acids (EFAs). Contrary to popular belief, lipids are an important part of our diet, and a minimum intake is necessary. However, many health problems are linked to excessive dietary fat intake, such as obesity, heart disease, and cancer.
Classification of Lipids
1. Simple Lipids or Homolipids
Simple lipids are esters of fatty acid linked with various alcohols.
- Fats and oils (triglycerides, triacylglycerols)
These esters of fatty acid have glycerol, a trihydroxy alcohol. Fat is solid at room temperature, while oil is in liquid form.
Triglycerides are abundant and constitute about 98 percent of all dietary lipids. The rest consists of cholesterol, its esters and phospholipids. Unlike carbohydrates, which can be stored only for a short time in the body, triglycerides are stored in the body in large amounts as body fat, which can last for years.
An average man weighing about 70 kg, has at least 10 to 20 percent of his body weight in lipid, most of which is triacylglycerol. This is found in adipose (fat) tissue, as well as all other organs of the body. Body fat is a reservoir of chemical energy.
Waxes are long-chain saturated and unsaturated fatty acid esters with monohydroxy alcohols, which have high molecular weight.
Waxes are produced naturally by skin glands as a protection, to keep it lubricated, pliable, and water-proof. Wax also covers hair, feathers, and wool.
2. Compound Lipids or Heterolipids
Heterolipids are fatty acid esters with alcohol and additional groups.
- Phospholipids (phosphatids)
Phospholipids contain fatty acids, glycerol, nitrogen bases, phosphoric acid, and other substituents. They are most abundant in cell membranes and serve as structural components. They are not stored in large quantities. As their name implies, phospholipids contain phosphorus in the form of phosphoric acid groups. Their molecular structure is polar, consisting of one hydrophilic head group and two hydrophobic tails.
- Glycolipids (cerebrosides)
Glycolipids are fatty acids with carbohydrates and nitrogen but without phosphoric acid. Glycolipids also include some compounds like sulfolipids, gangliosides, and sulfatids which are structurally-related.
These cerebrosides are important constituents of the brain and other tissues. They consist of at least one sugar unit, so they are also called glycosphingosides. They are like phospholipids because they have a hydrophobic region, with a polar region and two long hydrocarbon tails. Like phospholipids, glycolipids form lipid bilayers that are self-sealing and form the structure of cellular membranes.
3. Derived Lipids
These substances are derived by hydrolysis from compound and simple lipids. These fatty acids include alcohols, mono- and diglycerides, carotenoids, steroids, and terpenes.
The steroids are biological compounds that are some of the most studied types of fat. They contain no fatty acids and unlike fats, are nonsaponifiable (cannot be hydrolyzed to yield soap).
Cholesterol is a well-studied lipid, because of its strong correlation with the incidence cardiovascular disease. It is an important component of cell membranes and plasma lipoproteins, and is an important precursor of many biologically important substances like bile acids and steroid hormones. It is abundant in nerve tissues and is associated with gallstones.
Dietary cholesterol is found in saturated fats of animals (as butter and lard), but vegetable oils do not contain cholesterol. Only a small portion of your body cholesterol comes from the diet. Most of it is produced in the body. Eating unsaturated fatty acids from vegetable oil helps lower blood cholesterol levels by reducing cholesterol synthesis in the body. However, eating saturated fats from animal fat elevates blood cholesterol and triglycerides and reduce the ratio of your good to bad cholesterol.