Fat (also known as lipid), is one of the three classes of nutrients; the other two are proteins and carbohydrates. The major function of fat is to provide energy for the body. Pound per pound, fat contains more energy (calories) than protein and carbohydrates. There are three types of fat:
Triglycerides are where most of the fat calories are stored. In addition to providing energy, fat also serves other functions, for example, cholesterol and phospholipids are components of cell membranes and sheaths surrounding nerve cells. Cholesterol is also important for the production of bile acids and other hormones (such as sex hormones and adrenal hormones).
Fatty acids consist of chains of carbon atoms linked together by chemical bonds. On one end (terminal) of the carbon chain is a methyl group (a cluster of carbon and hydrogen atoms), the other terminal is a carboxyl group (a cluster of carbon, oxygen and hydrogen atoms). The chemical bonds between carbon atoms can be either single or double bonds. Single bonds have more hydrogen molecules around them than double bonds. These chemical bonds determine whether a fatty acid is saturated or unsaturated (see discussion below). Fatty acids also come in different lengths: short chain fatty acids have less than 6 carbons, while long chain fatty acids have 12 or more carbons.
Fatty acids serve as energy for the muscles, heart, and other organs, as building blocks for cell membranes, and as energy storage for the body. Those fatty acids not used up as energy are converted into triglycerides. A triglyceride is a molecule formed by attaching three fatty acids onto a glycerol compound that serves as a backbone. Triglycerides are then stored in the body as fat (adipose) tissue.
Saturated fatty acids contain single bonds only. Fats containing saturated fatty acids are called saturated fats. Examples of foods high in saturated fats include lard, butter, whole milk, cream, eggs, red meat, chocolate, and solid shortenings. Excess intake of saturated fat can raise one's blood cholesterol and increase the risk of developing coronary artery disease.
Monounsaturated fatty acids contain one double bond. Examples of foods high in monounsaturated fat include avocados, nuts, and olive, peanut and canola oils. Scientists believe that increased consumption of monounsaturated fats (for example eating more nuts) is beneficial in lowering LDL cholesterol (the "bad" cholesterol) and lowering the risk of coronary heart disease, especially if monounsaturated fats are used to substitute for saturated fats and refined sugars.
Polyunsaturated fatty acids contain more than one double bond. Examples of foods high in polyunsaturated fats include vegetable oils, corn, sunflower, and soy.
Essential fatty acids are polyunsaturated fatty acids that the human body needs for metabolic functioning but cannot produce, and therefore has to be acquired from food.
Omega-3 fatty acids are a class of essential polyunsaturated fatty acids with the double bond in the third carbon position from the methyl terminal (hence the use of "3" in their description). Foods high in omega-3-fatty acids include salmon, halibut, sardines, albacore, trout, herring, walnut, flaxseed oil, and canola oil. Other foods that contain omega-3-fatty acids include shrimp, clams, light chunk tuna, catfish, cod, and spinach.
Omega-6 fatty acids are a class of essential polyunsaturated fatty acids with the initial double bond in the sixth carbon position from the methyl group (hence the "6"). Examples of foods rich in omega-6 fatty acids include corn, safflower, sunflower, soybean, and cottonseed oil.
These are synonyms for omega-3 and omega-6 fatty acids, respectively.
Trans fatty acids (trans fats) are made through hydrogenation to solidify liquid oils. Heating omega-6 oils such as corn oil to high temperatures creates trans fats. Trans fats increase the shelf life of oils and are found in vegetable shortenings and in some margarines, commercial pastries, fried foods, crackers, cookies, and snack foods. Intake of trans fatty acids increases blood LDL-cholesterol ("bad" cholesterol), decreases HDL cholesterol ("good cholesterol"), and raises the risk of coronary heart disease.
Scientific evidence is mounting that fish oil (predominantly omega-3-fatty acids) can reduce the risk of sudden cardiac death. Some scientists also believe that omega-3 fatty acids can improve one's blood lipid (cholesterol and triglyceride) levels and decrease the risk of coronary heart disease.
Coronary heart diseases are heart conditions caused by atherosclerosis (hardening and narrowing) of the coronary arteries, arteries that supply blood and oxygen to the heart muscle. Coronary arthrosclerosis is typically silent; people can have coronary atherosclerosis for years to decades without any symptoms or signs of heart disease. Symptoms and signs of coronary heart disease develop when the diseased arteries become critically narrow or completely blocked so that they can no longer deliver adequate blood and oxygen to the heart muscle. Obesity, high blood cholesterol, high blood pressure, cigarette smoking, and diabetes mellitus can accelerate the arthrosclerosis process, and increase the risks of developing coronary heart disease. Coronary heart diseases include heart attacks, sudden cardiac death, angina, heart failure and heart rhythm disturbances.
Coronary heart disease is an important cause of death in the United States. Death from coronary heart diseases can be due to heart failure, arrhythmias (abnormally fast or slow heart rhythm) or sudden cardiac death. Each year more than five hundred thousand Americans die of heart attack, and approximately 250,000 die of sudden cardiac death.
Most sudden cardiac death is due to ventricular fibrillation. Ventricular fibrillation is a chaotic electrical disturbance that causes the heart to stop beating. Ventricular fibrillation typically occurs at the onset of a heart attack, usually before the victim can reach the hospital. Ventricular fibrillation will lead to brain death within minutes unless effective CPR can be delivered and/or the normal heart rhythm is restored by electric shocks (called defibrillation). More than 50% of people who die of sudden cardiac death have no signs or symptoms of coronary heart disease.
There are several types of evidence suggesting that omega-3-fatty acids prevent death from heart disease; epidemiological evidence, archeological evidence, evidence from animal studies, observation studies, and interventional studies.
Epidemiology is the study of populations in order to determine the frequency and distribution of disease and measure risks. Scientists have observed that populations with high fish intake (Alaskan natives, Greenland Eskimos, and Japanese living in fishing villages) have low rates of cardiovascular disease and low rates of sudden cardiac death.
There is archeological evidence that the early ancestors in the Paleolithic period were lean, fit and free of coronary heart disease. Their physical fitness was in part due to their active hunter-gatherer life style (they exerted themselves daily for food, water, and physical security) and in part due to their diet.
The diet of the Paleolithic ancestors consisted mainly of natural and unprocessed food. Compared to the modern American diet, it contained more fruits, vegetables, nuts, lean protein, polyunsaturated and monounsaturated fats, and omega-3-fatty acids. Their diet also contained little saturated fats, no trans fats, and no refined grains and sugars.
The animal meats in the Paleolithic period were different from the meats of today's domesticated animals. The animal meats then were rich in omega-3-fatty acids because the algae, plants, and grass (foods of the grazing animals and fish in those days) were rich in omega-3-fatty acids. Today's domesticated animals are corn and grain fed. Consequently meat from these domesticated animals is high in saturated fat but low in omega-3 fatty acids. Today's farm fish meat is often lower in omega-3-fatty acids than non-farmed fish.
Americans today lead generally sedentary lives, not the active hunter-gatherer lifestyle our genetics were designed for. The average American diet today is high in saturated foods, trans fats, and foods rich in grains and refined sugars. Our modern diet is also poor in omega-3 fatty acids. The combination of sedentary life and our modern diet is the prime cause for our epidemic of obesity, high blood pressure, diabetes, and coronary heart disease.
In studies involving animals (dogs, rats, and marmosets), omega-3-fatty acids were found to prevent ventricular fibrillation (thus preventing sudden death) when given to animals just prior to experimentally induced heart attacks. Omega-3-fatty acids were found also to terminate ventricular fibrillation in animals suffering experimentally induced heart attacks. Therefore scientists suspect that omega-3-fatty acids may prevent ventricular fibrillation of the heart in event of a heart attack in humans.
Two large, long-term observational studies have been published on the relationship between dietary intake of fish oil and omega-3-fatty acids and risk of heart disease and sudden cardiac death: The Nurses' health study and the Physician's health study.
The Physician's health study began in 1982 when more than 20,000 healthy male physicians were followed for 11 years. Life style, coronary risk factors and diet data were collected at entry, and life style and diet data were collected via questionnaires at 12 months and 18 months. The results of the study were published in JAMA 1998, vol 279, p23. The title of the article is "Fish consumption and risk of sudden cardiac death". The study found that men who consumed one or more fish meals per week had a 50% lower risk of developing sudden cardiac death than men who rarely ate fish (less than one fish meal per month).
In a separate article, scientists compared blood levels of omega-3-fatty acids in 94 of these men who died of sudden cardiac death against living men matched for age and smoking habits. They found that high levels of omega-3-fatty acids in the blood were associated with low risk of cardiac sudden death. Men with the highest blood levels of omega-3 fatty acids had an 80% lower risk of sudden cardiac death than men with the lowest blood levels. (High omega-3-fatty acids in the blood is usually due to a high consumption of fish).
The Physician's health study found no association between fish and omega-3-fatty acid consumption and risk of developing nonfatal coronary heart diseases such as heart attacks without sudden death. Thus the scientists suspected that the major benefit of fish oil and omega-3-fatty acid was in preventing ventricular fibrillation in event of a heart attack.
The Nurses' Health study began in 1976, when more than 80,000 female nurses completed life style and diet questionnaires. They were followed for 16 years for the development of coronary heart disease. Omega-3 fatty acid consumption was calculated from the questionnaires. The result of the study was published in JAMA vol 287. No.14, p 1815. The title of the article is "Fish and omega-3-fatty acid intake and risk of coronary heart disease in women". They found that compared to women who rarely ate fish (less than one fish meal per month), women who ate fish once a week had a 29% lower risk of developing coronary heart disease. Those who ate fish five times a week had a 34% reduction in the risk of coronary heart disease and a 45% reduction in the risk of death from heart disease (usually sudden cardiac death).
What are the shortcomings of observational studies?
Even though carefully conducted observational studies like the Nurses' health study and Physician's health study can provide useful scientific information, observational studies cannot differentiate between cause and association. In other words, the Physician's heath study found only an association between eating fish and lower risk of sudden cardiac death. The Nurses' health study only found an association between eating fish and lower risk of coronary heart disease and death from heart disease. Neither study could conclusively prove that eating fish caused these health benefits. Therefore, carefully conducted interventional studies are necessary to confirm findings from observational studies.
Interventional studies are prospective studies that randomly (by chance, e.g., by a flip of a coin) assign subjects to two groups, a treatment group and control group. Subjects in the treatment group will be given the medication (or diet) being tested, while the subjects in the control group will usually be given an inert substance (placebo). Subjects are assigned to each group randomly to prevent either group from having sicker (or older) subjects that might bias the study. Subjects in both groups are evaluated the same way before treatment and during treatment. This way the beneficial or harmful effects of treatment can be determined by comparing the results of treatment against the results of the placebo.
Two interventional studies on omega-3-fatty acids and fish oil have recently been published. The GISSI-Prevention trial studied the effect of omega-3-fatty acids on sudden death and recurrence of coronary heart disease in patients who recently survived a heart attack. The Lyon Diet Heart Study studied the effect of Mediterranean diet (see discussion below) on the recurrence of coronary heart disease in patients who recently survived a heart attack.
The GISSI-Prevention trial randomly assigned more than 11,000 patients with recent heart attacks to four treatment groups; omega-3-fatty acids (850 mg capsule daily), vitamin E, both, or neither (control). The study subjects in all four groups were followed for 3.5 years. Study results were published in Circulation, 2002; 105:1897-1903. Subjects given omega-3 fatty acids had a 20% lower death rate from coronary heart disease than subjects in the vitamin E and control groups. Omega-3-fatty acids were particularly effective in preventing sudden cardiac death (45% reduction in sudden cardiac death). The scientists believed that the benefits of omega-3-fatty acids are in the prevention of ventricular fibrillation. Vitamin E was found to have no benefit compared to controls.
The Lyon Diet Heart Study randomly assigned 600 subjects who survived recent heart attacks to either the Mediterranean diet or a prudent Western diet (diet low in saturated fat and cholesterol). The Mediterranean diet is rich in omega-3-fatyy acids, as well as monounsaturated fats, fruits, vegetables, and nuts. The study subjects were followed for 4 years. The results of the study were published in Circulation, 1999; 99:779-785. The subjects eating the Mediterranean diet had more than 50% reduction in sudden cardiac death and in repeat fatal or nonfatal heart attacks.
The GISSI-Prevention trial found that omega-3-fatty acids prevented sudden cardiac death, but did not prevent repeat heart attacks. The Lyon Diet Heart Study found that the Mediterranean diet not only prevented sudden cardiac death, but also prevented recurrence of both fatal and nonfatal heart attacks. The reasons for the difference in the two studies are not clear; maybe there are additional cardiac protective factors in the Mediterranean diet.
Omega 3 fatty acids may play a role in the prevention and/or treatment of the following health conditions:
Omega 3 Fatty Acids
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