Unsaturated fats exposed to air oxidize to create compounds that have rancid, stale, or unpleasant odors or flavors. Hydrogenation is a commercial chemical process to add more hydrogen to natural unsaturated fats to decrease the number of double bonds and retard or eliminate the potential for rancidity. Unsaturated oils, such as soybean oil, which contain unsaturated fatty acids like oleic and linoleic acid, are heated with metal catalysts in the presence of pressurized hydrogen gas. Hydrogen is incorporated into the fatty acid molecules and they become saturated with hydrogen. Oleic acid (C18:1) and linoleic acid (C18:2) are both converted to stearic acid (C18:0) when fully saturated. The liquid vegetable oil becomes a solid saturated fat (shortening with a large percentage of tristearin). By comparison, animal fats seldom have more than 70% saturated fatty acid radicals. Lard, for example, has 54% unsaturated fatty acid radicals.
Fully saturated fats are too waxy and solid to use as food additives, so manufacturers use partially hydrogenated oils. These oils are also produced at high temperatures with metal catalysts and pressurized hydrogen, but the process is stopped when the oil has the proper consistency for its application. The high temperatures and catalysts used for this chemical reaction weaken the double bonds and, as a side effect, cause a large percentage of the natural Cis double bonds to change to Trans double bonds. Trans fatty acids are present mainly in partially hydrogenated fats, but they are also present in hydrogenated fats because chemical reactions never achieve 100% efficiency.
Metabolism of natural 20-carbon polyunsaturated fatty acids like arachidonic acid results in the biosynthesis of mediators with potent physiological effects such as prostaglandins, prostacyclins, thromboxanes, leukotrienes, and lipoxins. These substances are known collectively as eicosanoids because they contain 20 carbon atoms (Greek eikosi = 20). However, polyunsaturated trans fatty acids cannot be used to produce useful mediators because the molecules have unnatural shapes that are not recognized by enzymes such as cyclooxygenase and lipoxygenase.[21,22] Although low levels of trans-vaccenic acid occur naturally in some animal food products, partially hydrogenated oils contain a large proportion of diverse trans fatty acids. Trans fatty acids that are incorporated into the cell membranes create denser membranes that alter the normal functions of the cell.
Effect of trans fats on the heart. Dietary trans fats raise the level of low-density lipoproteins (LDL or "bad cholesterol") increasing the risk of coronary heart disease. Trans fats also reduce high-density lipoproteins (HDL or "good cholesterol"), and raise levels of triglycerides in the blood. Both of these conditions are associated with insulin resistance which is linked to diabetes, hypertension, and cardiovascular disease. Researchers have reported that people who ate partially hydrogenated oils, which are high in Trans fats, worsened their blood lipid profiles and had nearly twice the risk of heart attacks compared with those who did not consume hydrogenated oils.[1,2,3,4] Because of the overwhelming scientific evidence linking trans fats to cardiovascular diseases, the Food and Drug Administration started requiring all food labels to disclose the amount of trans fat per serving in 2006.
Effect of trans fats on the brain. Trans fats also have a detrimental effect on the brain and nervous system. Neural tissue consists mainly of lipids and fats. Myelin, the protective sheath that covers communicating neurons, is composed of 30% protein and 70% fat. Oleic acid and DHA are two of the principal fatty acids in myelin. Studies show that trans fatty acids in the diet get incorporated into brain cell membranes, including the myelin sheath that insulates neurons. These synthetic fats replace the natural DHA in the membrane, which affects the electrical activity of the neuron. Trans fatty acid molecules alter the ability of neurons to communicate and may cause neural degeneration and diminished mental performance. Neurodegenerative disorders such as multiple sclerosis (MS), Parkinson's Disease, and Alzheimer's Disease appear to exhibit membrane loss of fatty acids.[12,19] Unfortunately, our ingestion of trans fatty acids starts in infancy. A Canadian study showed that an average of 7.2% of the total fatty acids of human breast milk consisted of trans fatty acids which originated from the consumption by the mothers of partially hydrogenated vegetable oils.
High blood serum cholesterol levels are associated with increased risk of cardiovascular diseases. Blood cholesterol levels can be lowered by reducing the sources of dietary cholesterol, increasing the amount of fiber in the diet, and by consuming oils high in polyunsaturated fatty acids while reducing the intake of saturated fats. Research on dietary fats by Hegsted and others has shown that myristic acid (C14:0), and palmitic acid (C16:0) increase cholesterol levels, whereas polyunsaturated fats such as linoleic acid (C18:2) reduce cholesterol levels.
The chart above shows the effects of individual dietary fatty acids on Total Serum Cholesterol, LDL cholesterol, and HDL cholesterol when l% of the energy from carbohydrates in the diet is replaced by 1% of energy of the specific fatty acids. The chart shows cholesterol increases from lauric acid (C12:0), myristic acid (C14:0), and palmitic acid (C16:0) which are found in coconut oil, palm oil, and butter. Elaidic acid (trans-C18:1), which is present in hydrogenated fats, is the worst because it increases LDL and decreases HDL. The saturated fatty acid stearic acid (C18:0), the monounsaturated oleic acid (C18:1), and the polyunsaturated linoleic acid (C18:2) decrease LDL and increase HDL to various degrees.
The following table shows the relative effect on blood cholesterol of several common dietary fats when included as 10% of the total calories. This corresponds to 200 Calories of a 2000-Calorie diet, which is approximately 22 grams or slightly less than 2 Tablespoons of fat. The dietary cholesterol is given in mg/100g and the fatty acid numbers are percentages by weight. Delta TC represents the relative change in Total Cholesterol calculated using the Hegsted equation.
|Oil or Fat||
|Grape seed oil||0||8||73||0||0||-1196|
Animal fats and tropical oils which are high in myristic acid and low in linoleic acid increase cholesterol levels. We can also conclude from this table that oils high in linoleic acid such as grape seed oil, sunflower oil, and safflower oil can play a significant role in reducing blood cholesterol levels when consumed regularly as part of the diet. Keep in mind that "high-oleic safflower oil" or safflower oil refined for high-temperature cooking does not have any cholesterol-lowering effect because its fatty acid composition is similar to olive oil. The effect of olive oil on cholesterol is relatively neutral.
Read labels carefully when selecting oils high in linoleic acid. The best quality oils are unrefined cold pressed or expeller pressed oils packed in dark glass bottles filled with an inert gas. These oils recommend refrigeration after opening. The labels may contain the words "high in polyunsaturated fats" and sometimes include the percentage of linoleic acid. If the label says "high in monounsaturated fats", the oil is high in oleic acid and may not lower blood cholesterol levels. An oil labeled "safflower oil" or "sunflower oil" is not necessarily high in polyunsaturated fats.
The nutrition label provides the best guide for choosing the oils because it lists the grams of saturated, polyunsaturated, and monounsaturated fat per serving. Make sure that the quantity of polyunsaturated fat is at least 3 times greater than the monounsaturated fat. For example, in a serving of 14 grams, if there are 3 grams of monounsaturated fat, there should be at least 9 grams of polyunsaturated fat. Examination of 12 different brands of oils at a Whole Foods supermarket to check if they met this requirement produced the following results:
The oils that did not meet the requirement had 5 times less polyunsaturated fat than monounsaturated fat, approximately the same as olive oil (2 grams of polyunsaturated fat, and 10 grams of monounsaturated fat in a 14 gram serving).
Phospholipids or phosphatides are natural surfactants and emulsifiers consisting of an alcohol such as glycerol, one or two molecules of fatty acid, and a phosphoric acid compound. They are found in all plants and animals and include such substances as lecithin, cephalin, and sphingomyelin. Lecithin, also called phosphatidylcholine, is a significant constituent of brain and nervous tissue consisting of a mixture of diglycerides of stearic, palmitic, and oleic acids, linked to the choline ester of phosphoric acid. The chemical structure of dipalmitoyl lecithin illustrated here is typical of the phosphatides found in the brain, lung, and spleen.
Cell walls and other biological membranes consist of two layers of phospholipids where the fatty acid tails of the phospholipids are oriented toward each other and the phosphate groups form the outer surfaces of the membrane. These bilipid layers are semipermeable, allowing some molecules to pass freely through the membrane while blocking others. The most common phospholipids are lecithin, phospatidylethanolamine, phosphatidylserine, and phosphatidylinositol.
Sterols, such as cholesterol, are alcohols with the cyclopentanophenanthrene ring system (atoms 1 through 17 in the structure below). This substructure is also found in steroid hormones such as testosterone, progesterone, and cortisol. Cholesterol is classified as an alcohol because it has a hydroxyl group (-OH) in position 3 of the ring system. Cholesterol is produced by the liver and is found in all body tissues where it helps to organize cell membranes and control their permeability. Cholesterol derivatives in the skin are converted to vitamin D when the skin is exposed to sunlight. Vitamin D3 mediates intestinal calcium absorption and bone calcium metabolism. A high level of cholesterol in the blood is considered to be a risk factor for cardiovascular diseases. Cholesterol and lipoprotein levels can be normalized through exercise and reduced Calorie diets that eliminate hydrogenated fats and add sources of polyunsaturated fatty acids such as grape seed oil.
Sterols of vegetable origin are called "phytosterols". They have the same basic structure as cholesterol, but differ in the side chains attached to carbon 17. Phytosterols, such as stigmasterol from soybean oil, are of current interest because they lower blood cholesterol levels. Sterols that are fully saturated (no double bonds) are called "stanols". For example, stigmastanol has the same structure as stigmasterol, but without the double bonds. When fatty acids react with the hydroxyl at carbon 3 they form "sterol esters".
The production of edible fats is a large commercial enterprise worth millions of Dollars that involves many segments of our society, including farmers who cultivate oil-bearing plants, ranchers and fishermen who provide sources of animal fats, and a variety of industries that extract, process, store, and distribute the resulting fats and oils. There is a constant tug-of-war between manufacturers, consumers, special interest groups, and government regulatory agencies, each trying to advance their own agenda. Fats and sugars have been blamed for the epidemic increase of obesity during the last thirty years, and well-intentioned government agencies and public advocacy groups have influenced official policy decisions, but not always based on a sound scientific foundation because it takes many years to determine the health consequences of specific products or policies. Many times, policies are enacted for political reasons to benefit farmers and manufacturers at the expense of public health.
By now, it is a well-established fact that trans fats are harmful and are responsible for causing thousands of deaths per year from cardiovascular diseases, but hydrogenated fats continue to be added to so many food products that it almost impossible to avoid them. Frequently, even lard is hydrogenated! Why aren't these products banned? Because manufacturers with a lot of political influence don't want to lose money on products that might turn rancid before they are sold. Fast modern distribution methods, good packaging, and controlled temperature storage could solve this problem and deliver healthier products to the consumers, but it would increase costs. One of the latest trends used by manufacturers is to avoid the word "hydrogenated" and to obtain oils from foreign sources where our regulations do not apply. Some products use "Modified Palm Oil" without mentioning the process used to modify the oil. The modification could be a simple fractionation to separate high-melting from low-melting triglycerides, but it could involve hydrogenation.
The Fat Free Craze
The concept that some fats are essential for good health is just emerging in the awareness of the general public, but the aversion to fat resulting from many years of indoctrination against fat has resulted in great consumer demand for low-fat or nonfat products. Some manufacturers, eager to increase their sales, concoct products that use monoglycerides, diglycerides, or fatty acid esters of polyglycerol, and argue that these products are "Fat Free" because only triglycerides are fats. Average consumers eat these products under the illusion that they are low in calories because the manufacturers do not disclose the calories of these components in the Nutrition Facts. In addition, most products containing monoglycerides, diglycerides, or artificial fats do not state whether the constituent fatty acids are saturated or hydrogenated. New regulations are often adopted as a reaction to abuses like this, but it is a slow process that may be further delayed by lobbying and enables manufacturers to continue reaping profits in the meantime.
For many years, saturated fats were equated with the "artery clogging" deleterious health effects associated with trans fats. Many authoritative medical sources advise decreasing the consumption of saturated fats because there is a relationship between serum total cholesterol concentrations and saturated fatty acid intake. However, several scientific studies indicate that saturated fats have beneficial biological effects and differ significantly from trans fats which are always bad. It is worthwhile noting that the butterfat in human milk has 20% more saturated fats than lard. Saturated fatty acids, particularly medium chain fatty acids such as lauric and capric acids, have been found to play an important role in supporting the immune system. Studies of the effects of specific fatty acids on serum cholesterol levels have shown that of the three most common saturated fatty acids in tallow and lard, only myristic acid increases the level of cholesterol in the blood substantially, whereas stearic acid has no effect, and the polyunsaturated linoleic acid decreases it. The quantitative relationship expressed by the Hegsted equation suggests that blood cholesterol levels may be lowered naturally by adding to our diet sources of polyunsaturated fatty acids such as safflower, sunflower, or grape seed oils.
Our knowledge of the metabolism of fats continues to increase and it will take many years of research and expensive long-term studies to establish objective facts to clarify the conflicting statements from government agencies, short-term studies, advocacy groups, commercial interests, and the latest diet fads. If you are confused about what fats to eat because of the barrage of advertisements, inflammatory language, and misinformation, here is some simple advice: consume unaltered natural oils and fats that have been used traditionally for hundreds or thousands of years, and avoid oils that have been chemically modified or created in a laboratory. If your weight is in the normal range, animal fats such as lard can be used to meet a portion of your caloric needs, as long as the lard originates from organically raised animals and it is not hydrogenated. Many pesticides used in farms concentrate in fatty tissues and can result in high pesticide residues in fat from animals that are not raised organically.