Essential fatty acids & phospholipids are primary constituents of cell membranes, and as such they are vital to the makeup of the human body. Essential fatty acids are used to generate certain intra-cellular hormone-like substances, including prostaglandins and leukotrienes, which are responsible for regulating key bodily processes.
Almost by definition, life is composed of cells, and cells are defined by membranes. One theory suggests that, around four billion years ago, self-replicating molecules, similar to the ribonucleic acid or RNA in our own cells, were synthesized from organic molecules. These self-replicating molecules adapted to changes in their environment to increase their potential for survival. Thus began the process of evolution that has led, over the eons, to us. One turning point was when these molecules developed membranes - envelopes which could help concentrate chemicals needed for the cell's survival. There existed in the "primordial soup"substances uniquely suited to this purpose: a class of organic compounds we call lipids . Lipids are more commonly called fats, and in this health and image-conscious age people often think of them as something to be avoided. However, the word fat refers to a variety of substances with a diverse range of chemical properties, which are essential for survival and well-being . The simplest lipids, fatty acids such as palmitic acid, consist of a hydrocarbon "tail" connected to a carboxyl group (COOH). The majority of lipids in food and in the human body occur in the form of triglycerides - a molecular configuration in which three fatty acid chains are attached to a 'backbone' of glycerol (an organic alcohol composed of a 3-carbon chain with an alcohol group attached to each carbon). The major roles of lipids can be described as energy and storage, structural, and metabolic.
Molecules can contain more or less chemical energy. In living systems most of the energy needed to drive chemical reactions is derived from oxidation. Oxygen, the ultimate electron acceptor,is a strong oxidant:it has a marked tendency to attract electrons, becoming reduced in the process. When a molecule undergoes a chemical reaction from a high-energy reduced state to a low-energy oxidized state, energy is released. This is what happens in a fire: the high-energy carbohydrates in wood, such as glucose, react with oxygen, releasing heat and the low-energy molecules of carbon dioxide and water. This is similar to what happens in metabolism. Most of the carbon in a fatty acid chain is highly reduced, which makes fats more energy-rich than the other organic molecules that can be burned as food. This is what we mean when we say fats are high in calories - a measure of the amount of energy released when a substance is oxidized. Fats contain more than twice as many calories as carbohydrates. This makes fats an important storage fuel for most of the body.
Another important class of lipids in the human body consists of the phospholipids. Like triglycerides, phospholipids contain fatty acid chains- in this case two, one saturated and one unsaturated, attached to a glycerol backbone. Unlike triglycerides, in phospholipids the third carbon of the glycerol molecule is attached to a phosphate (a molecular group that contains phosphorus and oxygen), which is in turn attached to either an amino acid or, in the case of phosphatidyl choline, a molecule of the B-vitamin - like substance, choline. Their unique molecular structure makes phospholipids amphipathic, which means 'likes both':
Fats, being hydrophobic, tend to separate out from water. When fat is mixed with phospholipids in the presence of water, the phospholipid molecules attach themselves to the molecules of fat and bring them into the water solution, enabling the fats to dissolve in water.
Phospholipids form a structure called a lipid bilayer, a two-ply sheet of phospholipid molecules in which the hydrophilic head groups face outward and are in contact with the water, and the hydrophobic tails face each other on the inside of the bilayer. This structure is one of the key constituents of the cell membranes that surround every living cell.
The lipid bilayer of cell membranes is a fluid in which membrane-embedded proteins "float." These proteins serve a wide variety of different functions. Some are enzymes, serving to carry out chemical reactions in the adjacent solution. Some are involved in signaling, in which a biochemical action in a cell is 'commanded' by means of a hormone or some such other signaling molecule. Still others are involved in transporting substances across the membrane, into or out of the cell.
The functions of membrane-embedded proteins are dependent on a very precise balance of phospholipids for their function. Phosphatidyl serine, for instance, has a negatively-charged head group that associates preferentially with a class of membrane-bound proteins called ATPases. ATPases regulate, among other things, the balance of sodium and potassium in intra- and extracellular fluids, a balance that is necessary for the integrity of our cells and also for the electrochemical impulses that make up our thoughts and feelings. Without phosphatidyl serine, these vitally important membrane-embedded proteins could not function.
Cholesterol is a waxy substance that is essential to the structure of cell membranes, which depend for their function on a delicate balance between fluidity and solidity. Cholesterol provides a semifluid matrix, as well as enhancing membrane fluidity. About 80% of the cholesterol the body uses is manufactured by the liver; the other 20% is consumed in food. Elevated blood cholesterol levels are associated with heart disease. Saturated fats are converted into cholesterol more readily than unsaturated fats, and polyunsaturated fats usually depress blood cholesterol concentration to some degree. Researchers have thus recommended that people lower their consumption of saturated fats and increase their consumption of polyunsaturated fats. A process called hydrogenation , in which hydrogen molecules are added, is used to harden these unsaturated fats to create solid spreads, such as margarine. This process causes formation of altered fats called trans fatty acids. Although the results are not conclusive, human and animal studies have pointed to possible deleterious effects from consumption of trans - fatty acids, which are estimated to account for 5.5% of all fats consumed by Americans. These studies include one in men and women that showed harmful effects of trans - fatty acids on blood cholesterol ratios.
When each link of a fatty acid chain contains an atom of hydrogen, as in palmitic acid, that fatty acid is said to be saturated . If two carbon links are double bonded to each other, each has one less hydrogen atom, and the fatty acid chain is said to be unsaturated. If a fatty acid contains one double bond, it is said to be monounsaturated, and if it has two or more double bonds it is said to be polyunsaturated. Certain polyunsaturated fatty acids cannot be manufactured by the body and must be obtained from the diet. These nutrients are called essential fatty acids and are necessary for the normal function of all tissues. The essential fatty acids fall into two categories:
(1) Those with an unsaturated double bond between the 6th and 7th carbon in the chain, called omega-6 fatty acids, which include linoleic acid (LA), gamma-linolenic acid GLA), and arachidonic acid (AA).
(2) Those with a double bond between the 3rd and 4th carbons, called omega-3 fatty acids, which include alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA).
In addition to being phospholipid precursors, essential fatty acids can be converted to a class of hormone like intracellular messengers called eicosanoids. The physiologic effects of eicosanoids are potent in minute quantities. Their effects are so powerful that they need to be produced near the site of their action and are quickly inactivated. The important eicosanoids include the thromboxanes, leukotrienes and prostaglandins (PGs ). Prostaglandin molecules consist of a five-carbon ring with two side chains. They can be distinguished from each other by numbers that refer to the number of double bonds in their molecular side chains: 1-series PGs have one double bond, 2-series have two double bonds, and so on. Prostaglandins mediate a variety of bodily processes, including inflammatory reactions, blood vessel contraction and dilation, and platelet aggregation. The different PGs have different effects on the body, and different essential fatty acids act as precursors for different PGs.
Important essential fatty acids in humans are the omega-6 fatty acids, which include linoleic acid (LA), gamma-linolenic acid (GLA), and arachidonic acid (AA). 1-series PGs are derived from GLA and tend to cause blood vessels to dilate and reduce the stickiness of platelets (cell fragments in the blood that help initiate blood clotting). 2-series PGs are derived from arachidonic acid and tend to increase platelet stickiness and cause blood vessels to constrict. Meat and dairy products are dietary sources of the PG2 precursor, arachidonic acid; American diets tend to be rich in these foods. The rate-limiting step for production of GLA in the human body is an enzyme called delta-6-desaturase (D6D). The action of this important enzyme can be blocked by a number of different lifestyle factors, including a diet high in saturated or trans- fatty acids and chronic alcohol consumption. A modest increase in consumption of GLA will significantly increase the ratio of GLA to AA in the tissues, which may have a beneficial effect on the homeostasis of the cardiovascular system. Supplementation with omega-3 fatty acids, such as flaxseed oil or fish oil, is beneficial for similar reasons. Omega-3 fatty acids are precursors for 3-series PGs, which reduce platelet stickiness. Series-3 PGs also tend to inhibit conversion of AA into its metabolites, the 2-series PGs.
The lipid composition of our diets has changed radically in the 20th century. Our intake of saturated fats has increased dramatically, and trans fatty acids, which did not exist before the advent of modern food processing technology, now form a major part of our diets. We eat less fish and green leafy vegetables, important sources of omega-3 fatty acids, than our ancestors did. Far from being an inert, homogeneous substance, fat is dynamic and varied - a subtle and interactive matrix for many of the biological processes taking place in our bodies, minute by minute.