How Safe are Trans Fatty Acids?
With the discovery that saturated fats have adverse effects upon blood lipids, people turned to vegetable oils as a safe replacement for the highly saturated animal fats – the lard, beef tallow and butter used in cooking and in table spreads. Now some researchers are warning us that the plant oils may not be as safe as we once thought they were.
While few would question the safety of a judicious use of plant oils, it is the hydrogenated oils that have come under fire. The partial hydrogenation of vegetable oil improves the stability of the oil and render it less likely to be oxidized. This process also converts the vegetable oil into a soft, semi-solid fat that somewhat resembles the texture of butter. During the hydrogenation process there is produced a variety of trans fatty acids, which were also developed for frying purposes since poly-unsaturated-rich vegetable oils can readily undergo oxidation at the elevated temperature of frying.
While margarine was once touted as being better than butter, some are now questioning this and even saying that the reverse is true. This has led many to switch to butter. This, however, is not a wise move due to the issues that are discussed below.
Trans Fatty Acids Called into Question
The basis for all of the heated talk about margarine and other hydrogenated oils centers around trans fatty acids, which some researchers report to have an adverse effect on blood lipids that is more detrimental than the effect of saturated fat. Some scientists have expressed concern that trans fatty acids are a new artificial element in our food supply and since we do not understand the metabolic and health implications of ingesting this new type of fat, we should avoid using foods containing the trans fats.
In reality, trans fatty acids have been consumed for centuries, since they occur naturally in beef, mutton, butter, milk and other dairy products. They appear in animal fats largely due to the microbial hydrogenation of polyunsaturated fats in the animals forestomach. Beef tallow and butter contain about 3-10% trans fatty acids. In fact, milk fat has over 500 different fatty acid isomers, although most of them are present in trace amounts. Trans fatty acids have also been identified in very small amounts in some seeds and leafy vegetables.
Margarine Versus Butter
Table margarines vary in their content of trans fatty acids from 0 to about 30%. The soft (tub) margarines have lower levels (0 -15%) due to a higher content of liquid vegetable oil while stick margarines more commonly have 15-30% trans fat since they are more hydrogenated. If the first ingredient listed on the label is a liquid vegetable oil the trans fat content of the margarine will tend to be low, while a label that declares a partially hydrogenated vegetable oil as the first ingredient will have a higher level of trans fat. Because diet margarines and some liquid spreads contain considerable amounts of water they have greatly reduced fat and hence energy content. They contain little, if any, trans fat. While some margarines contain a higher trans fat content, butter is very high in both saturated fat and cholesterol (see Table 1). In fact, butter is even more atherogenic than beef since butter contains a greater content of those saturated fatty acids which are responsible for elevating blood cholesterol levels. Margarines also contain 3 to 10 times as much of the natural antioxidant, vitamin E, as butter. Since partially hydrogenated vegetable oils do not contain cholesterol or high levels of saturated fat, they are often promoted as heart-healthy. This could be considered a legitimate claim only for those soft tub margarines and diet margarines whose first ingredient is a liquid vegetable oil.
The European Difference
Some European margarines are available that do not contain partially hydrogenated oils. Instead, they are prepared from a mixture of liquid vegetable oil and saturated fats such as tropical oils, lard or beef tallow. While these margarines are free of trans fatty acids they may contain some cholesterol and a greater content of saturated fat. Some margarines in Europe may also contain hydrogenated marine oils.
Comparison of the Contents of Butter and Margarine
|Saturated Fat (%)||62||18-20|
|Trans Fat (%)||1-7||0-18|
|Saturated & trans fat (%)||63-69||18-38|
North Americans consume on average about 35% of their daily calories as fat. Saturated fat comprises about 13% of their daily energy intake, while trans fat makes up only 3-4% of the daily calories. This translates into a daily consumption of about 7-10 g of trans fatty acids. Much higher intakes of trans fatty acids may have adverse effects upon patients with elevated blood cholesterol levels, but there is little evidence to suggest that current levels of dietary trans fatty acids are harmful.
Hydrogenated vegetable oil, containing trans fatty acids, is used in a variety of commercial food products other than margarines, shortenings, salad and cooking oils. Actually, the fat in commercial bakery products such as cakes, cookies, crackers, doughnuts and pastries may contain up to 33% trans fatty acids. The major sources of trans fatty acids in the North American diet today are stick margarines, shortenings, french fries, commercial frying fats, and high-fat baked goods.
Effects of Trans Fatty Acids
Structurally, trans fatty acids are similar to saturated fatty acids and hence they influence cell membranes in the same way. Dietary fatty acids are readily absorbed and incorporated into most human tissues at levels that reflect the diet. Their conversion into energy occurs at about the same rate as that of the saturated fatty acids, so that they do not preferentially accumulate in the tissues.
Saturated fatty acids elevate LDL cholesterol levels by inhibiting the removal of cholesterol from the blood by inhibiting LDL receptors. Trans fatty acids can also raise LDL cholesterol levels in the blood, but usually not to the same degree as saturated fatty acids. The mechanism by which trans fat elevates blood cholesterol levels is as yet unknown.
Trans fatty acids also apparently elevate lipoprotein[a], a risk factor for heart disease. At high levels of intake (3-4 times the normal US consumption levels), but not at levels that are currently consumed, trans fatty acids can modestly reduce blood levels of HDL cholesterol (the good cholesterol) and also interfere with the metabolism of the essential fatty acids.
In the Nurses Health Study in Boston it was reported that the highest intakes of trans fatty acids were associated with an increased risk of coronary heart disease compared with low intakes. This data has been severely criticized by a number of scientists who point out that the increased risk of heart disease in the nurses was inconsequential, and the dietary fat intakes were collected in the early 1980’s when margarines had much higher levels of trans fatty acids than the current levels.
Since trans fat may act somewhat like saturated fat with respect to the effect on blood lipids, it is important for a heart patient to know the quantity of both saturated and trans fat that is being consumed. Unfortunately, the present labeling laws do not mandate that trans fatty acid content of a food be disclosed on the label.
While there are some concerns regarding the effects of trans fatty acids upon blood lipid levels, any concerns regarding an increased risk of cancer are unfounded. Extensive animal studies using vegetable oils that contain trans fat have not found any effect of trans fatty acids upon the occurrence of cancer.
Need to Reduce Total Fat and Saturated Fat
The World Health Organization recommends a diet that contains 15-30% of the dietary calories as fat. Those consuming in excess of this level should reduce their fat intake in general, and saturated fat and cholesterol in particular. This is largely achieved by reducing the intake of animal products, including butter. In addition, many should cut down their intake of all fats, including trans fat and polyunsaturated fat.
A tablespoon of vegetable oil provides 14 grams of fat and 125 calories (525 kJ). A tablespoon of either butter or regular margarine has about 10-11 grams of fat and 90-100 calories (375-420 kJ) due to the water content. Diet margarines contain a higher percentage of water, hence have they have less fat and fewer calories.
Is butter a better dietary choice than margarine? The American Heart Association recently pronounced margarine better than butter because butter is rich in saturated fat and cholesterol. They recommended that consumers:
- use those margarines that contain no more than 2g of saturated fat per tablespoon
- use soft margarines rather stick margarines
A low-fat diet in which animal fat is limited and vegetable fat is obtained largely from foods such as avocados, nuts, seeds, olives, soybeans and unrefined grains is the healthiest diet. A soft vegetable margarine or vegetable oil is generally considered better than butter. The need to reduce consumption of saturated fatty acids far outweighs any cause for alarm about trans fatty acids in foods at present levels of consumption.