Most natural fats are found in vegetable oils, dairy products and in animal fat. Vegetable oils such as olive oil are liquid at room temperature because the are composed largely of triacylglycerols and unsaturated fatty acids. Most fats are complex mixtures of simple and mixed triacylglycerols. Beef fat is a white, greasy solid at room temperature because it is composed of triacylglycerols containing only saturated fatty acids.
The chief storage form of metabolic fuel is fat in living organisms. However, this fat is in different forms in different organisms. The fats and oils used as forms of stored energy are derivatives of fatty acids. Fatty acids circulate in the blood bound to a protein carrier, serum albumen and in blood plasma as carboxylic acid derivatives. Specialized cells called adipocytes, or fat cells, store large amounts of triacylglycerols as fat droplets that nearly fill the cell. Triacylglycerols yield twice as much energy gram per gram as carbohydrates. Also, they are unhydrated unlike carbohydrates and hence , the organism that carries fat as fuel, does not have to carry the extra water of hydration. Humans have fat tissues composed of adipocytes. These are located under the skin, mammary glands, and in the abdominal cavity. Moderately obese people can draw for months on their fat stores to supply their energy needs. Carbohydrates can only provide one day’s energy, although they offer a quick source of energy. Some lipids while essential for cell structure (omega- 3 and omega -6 oils) cannot be produced by the body and require dietary sources. In a study conducted in rats showed that a critical period of rat brain development occurs during late trimester of pregnancy and early lactation months. During this time, large amounts of omega- 3 fatty acids are deposited in the brain lipids of the developing foetus and baby rat brain. So, during this time the mother’s diet must be provided with a suitable dietary source of omega – 3 fatty acid.
The fatty acids can be of a wide variety. Heart tissue is uniquely composed of certain lipids such as ether lipids and plasmalogens. The functional significance and their difference is unknown. However, if you eat a fat-free diet, you are certain to ‘starve’ your heart tissue of much needed nutrients to remain healthy. Anorexic girls soon develop weak heart tissue precisely because they eat a fat free diet. Overzealous adults starve their own hearts and sometimes that of their growing children by feeding a fat-free diet. Fatty food should be in moderation included in a healthy diet.
At least one fatty lipid, platelet-activating factor is a potent molecular signal. It plays an important role in inflammation and the allergic response. It is released from leukocytes and stimulates the release of serotonin from platelets and exerts other effects on liver, heart, uterine, lung and smooth muscles. A fat-free diet could potentially make one more prone to allergies.
Humans have at least 60 different fatty acids called Spingolipids in their cellular membranes. They are especially important in neurons and some are clearly recognition sites on the cell surface and can begin a cascade reaction. Certain variations of spingolipids define human blood groups and therefore determine safe blood transfusions. This area needs to be researched further. However, they emphasize the importance of moderate fat in a food menu. Credit for this initial research goes to Johann Thudichum (1829-1901).
Many new cells die each day and are replaced by new cells, each having a new cell membrane, which is composed of membrane fatty acid lipids. A fat-free diet could reduce the healthy replacement of new cells. Sterols are structural lipids present in the membranes where they serve as a precursor to certain biological activities. For example, steroid hormones and cholesterol are major sterols, which have various functions including giving a structural role to cell membranes. Trust me, if your cell membrane lacks structure, your cell has no structure and results in a body with no form or structure! You need structure, hence you need sterols. A fat-free diet with no sterols or cholesterol can be deadly.
Thus, fatty acids considered above have shown that lipids form a structural role in cell formation (about 5%) and a fuel storage role (80%). These lipids play a passive role in the cells. Another group of lipids have an active role in the cells, as hormones, and intracellular messengers generated in response to an exterior cell signal. A third group consists of conjugated lipids that absorb light in the visible band. Some of these pigments have a significant role in vision. Specialized lipids are important because they are useful to carry vitamins A, D, E and K as fat soluble vitamins. So, these are biologically active fatty acid lipids. Imagine a fat-free diet would be deficient in such biological activity. Membrane spingolipids have a role as intracellular messengers and as regulators. This can be important in any number of activities such as mounting an inflammatory response or in wound healing.