The Importance Of Fats (Lipids)
Fats, oils, and waxes belong to the group of naturally occuring organic materials called lipids. Lipids are those constituents of plants or animals which are insoluble in water but soluble in other organic solvents.
Amongst the various foodstuffs, fats provide the body with maximum energy (9 kcal per gram), approximately twice that for an equal amount of protein or carbohydrates.
Lipids are concentrated sources of energy as well as structural components of cell membranes. Everybody needs a certain amount of dietary fat for normal body functions. When fats are digested, emulsified, and absorbed, they facilitate the intestinal absorption and transport of fat soluble vitamins A, D, E, and K. They are also used to cushion and protect the heart, kidneys and liver. In certain climates subcutaneous body fat helps to insulate the body from the cold and prevent heat loss through the skin. These functions can be met by a daily intake of 15 to 25 grams of fat.
Lipids enter the body through the mouth and pass to the stomach, but are little affected by its acidic environment. They are absorbed primarily in the small intestines, where they are emulsified by salts of the bile acids and are hydrolyzed to fatty acids and glycerol by various water-soluble enzymes (lipases). From the intestines, the hydrolyzed lipids enter the bloodstream and are transported to other organs, mainly the liver, for further metabolism. Ultimately the fatty acids may be degraded to carbon dioxide and water to furnish energy.
There are many types of fatty acids, but they can be grouped into three divisions - saturated fats, monounsaturated fats and polyunsaturated fats. Polyunsaturated fats consist of two classes, omega-3 and omega-6.
Saturated fats have a profound hypercholesterolemic (increase blood cholesterol levels) effect. They are found predominantly in animal products (butter, cheese and meat) but coconut oil and palm oil are common vegetable sources. Saturated fat raises blood cholesterol levels more than anything else in the diet, even more than dietary cholesterol.
Intake of monounsaturated fats in oils such as olive oil is thought
to be preferable to consumption of polyunsaturated fats in oils such as
corn oil because the monounsaturated fats apparently do not lower
high-density-lipoprotein (HDL) cholesterol levels. Evidence for this
belief is derived primarily from metabolic ward studies of relatively
short duration. [
Dreon, D.M., Vranizan, K.M., Krauss, R.M.,
Austin, M.A., and Wood, P.D., The effects of polyunsaturated fat vs
monounsaturated fat on plasma lipoproteins, Journal Of The American
Medical Association, 263:2462, 1990]
Unsaturated fats come primarily from vegetable oils (safflower, corn, soyabean, cottonseed, sesame, and sunflower oils), nuts and seeds, although fish is a good source of unsaturated fatty acids. Most of the essential fatty acids are found in unsaturated fat, so foods high in saturated fat and cholesterol (animal fat, dairy products and eggs) should be eaten sparingly.
Fats should make up only 10 to 25 percent of the calories (not weight) in your diet. There is an overwhelming consumption of fats in the average Western Diet, this has lead to a huge health problem among population groups with a diet high in animal fats (ice-cream, chocolates, fast foods and desserts). There are many diet related human disorders that are found almost exclusively in the Western World, Coronary Heart Disease (CHD) and cancer of the colon, are a few of the more severe.
Cholesteryl esters are composed of a single fatty acid esterfied to cholesterol, in which the polar component is an alcohol.
Cholesterol is a major component of all cell membranes. It is required for synthesis of sex hormones, bile acids, and vitamin D. It is also a precursor of the steroid hormones produced by the adrenal cortex and gonads.
Dietary cholesterol is found only in foods derived from animals (meat, fish, poultry, eggs and dairy products); it is not present in plants.
Table 1.2 - Sources of dietary cholesterol
All offal, pat?, egg yolk, fish roes, mayonnaise and shell fish.
Fat on meat, duck, goose, cold cuts, whole milks, cream, ice cream, cheese, butter and most commercially made cakes, biscuits and pastries.
All fish and fish canned in vegetable oil, very lean meats, poultry without skin, skimmed milk, low fat yoghurt and cottage cheese.
All vegetables, and vegetable oils, fruit (including avocados and olives), nuts, rice, egg white and sugar.
The amount of cholesterol synthesized and metabolized by the body is
far greater than the amount usually consumed in the diet. It must also
be noted that in healthy people little correlation has been found
between the intake of cholesterol and blood cholesterol levels. Yet the
level of cholesterol in the blood is increased with high intakes of
dietary saturated fat and can be lowered by increasing the intake of
linoleic acid and fiber, which leads to a reduction of cholesterol
absorption from theintestine and an increased faecal excretion of
[Manual of Dietetic Practice, 1989. Edited for the British Dietetic Association by Briony Thomas.]
There is some evidence indicating that other nutrients can lower blood cholesterol levels. Choline emulsifies cholesterol thus helps to control a build up. Inositol metabolizes fats and cholesterol. Vanadium inhibits the formation of cholesterol in the blood vessels and aids in preventing heart attacks. Zinc also helps to decrease cholesterol levels.
If you are on a cholesterol reducing drug, you will suffer a decrease in the absorption of Vitamin A, thus needing to increase the amount available in your diet.
The Food and Nutrition Board's Committee on Diet and Health recommend that dietary cholesterol should be less than 300 milligrams per day.
Linoleic Fatty Acid
Linoleic acid is an omega-6, polyunsaturated fatty acid, can not be produced by the body and must be consumed in the diet, it is thus an essential fatty acid. A minimal adult intake of 3 to 6 grams per day is sufficient to prevent both biochemical and clinical evidence of deficiency.
It is important for maintaining the structure and function of
cellular and subcellular membranes. Linoleic fatty acid lowers
cholesterol levels in the blood and helps in the prevention of heart
Manual of Dietetic Practice, 1989. Edited for the British Dietetic Association by Briony Thomas]
A deficiency has been associated with scaly skin, hair loss, and impaired wound healing in hospital patients.
The best natural sources are vegetable oils, wheat germ, sunflower, peanuts, pecans, almonds, avocados and eggs.
Omega 3 Fatty Acids
Emerging research has suggested possible health benefits associated with moderate increases in dietary alpha-linolenic acid, including, reduced blood clotting tendency and reduced blood pressure. Data from the first prospective intervention trial to investigate the effects of dietary fat, fish and fiber, which was conducted in Cardiff, Wales, supported the idea that omega-3 fatty acids have a positive effect on reducing mortality from cardiovascular disease.
The principal sources of the omega-3 fatty acid alpha-linolenic acid are salad and cooking oil, salad dressing, shortening, margarine and products made from canola or soybean oils.