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Dietetic Internship

Food for Thought

Diane Reed

Another Fat to Watch
Coronary artery disease (CAD) is a killer of Americans.  Each year, more is discovered about this disease.  Research has identified risk factors for CAD and ways of preventing or delaying the onset of this fatal disease.  Diet is important as a risk factor for CAD and is a risk factor that can be controlled.  Serum cholesterol levels as well as the complete lipid profile, which includes total cholesterol, low density lipoprotein (LDL), high density lipoprotein (HDL), total cholesterol to HDL ratio, and triglycerides, are used in practice to assess risk for CAD.  A low fat, low cholesterol, no added salt diet is the diet prescription used for those with CAD or those who are at risk of developing CAD.

Saturated fat, in particular, has made it to the spotlight as the culprit for raising total serum cholesterol levels.  Intake of saturated fat has a more pronounced effect on serum cholesterol than does actual cholesterol intake.  Accordingly organizations such as the American Heart Association (AHA) and the National Cholesterol Education Program (NCEP) have been advising Americans to limit their intake of saturated fat and cholesterol.  The NCEP developed the Step I and Step II diets in order to help those with CAD and to lower the risk for those prone to this chronic disease.  The Step I diet calls for less than 10% of calories from saturated fat and less than 300 mg of cholesterol daily.  The Step II diet is more aggressive, with less than 7% of calories from saturated fat and no more than 200 mg of cholesterol per day (1).

With all the attention saturated fat has received, many Americans have made the switch from butter to margarine.  Margarine serves as a low saturated fat, cholesterol free alternative to butter (2).  However, with the discovery of “trans unsaturated fatty acids” the public is beginning to question if this switch is for the best.  Trans unsaturated fatty acids are now sharing center stage with saturated fatty acids.  This article will review trans unsaturated fatty acid composition, sources, effect on blood lipid levels, impact on coronary artery disease, and use in the American diet. 

Trans unsaturated fatty acids have a chemical make-up different from other unsaturated fats.  In a trans fat the adjacent carbon atoms on either side of the double bond are on opposite sides from each other (3).  This is called a trans configuration, and hence the name, “trans unsaturated fatty acid.”  Other unsaturated fats have adjacent carbon atoms on the same side around the double bonds… a cis configuration (3).  The process of partial hydrogenation is responsible for the chemical change to the trans configuration that occurs in unsaturated fats.  Partial hydrogenation is the process of heating vegetable oil in the presence of metal catalysts and hydrogen to form a hard fat (3).  Partial hydrogenation also removes the essential fatty acids, linolenic and linoleic.  These fatty acids cause the solid fat to spoil after prolonged storage and break down at high temperatures due to their low smoke points (3).

Trans fats are found in products that have undergone partial hydrogenation, such as stick margarine and shortening.  Trans fats are also produced in the rumen of cattle, thus dairy products and beef have small amounts of trans fats.  The trans fats are produced as a by-product of the bacterial fermentation that occurs in the rumen (3).  Most of the trans fats consumed in western society come from baked goods, fried fast foods, and commercially prepared foods.  Some manufacturers are producing trans-free spreads. However since average intake of trans fats from margarine accounts for only 25-37% of the average total daily intake of trans fat (3), a trans free margarine product is not the magic bullet for decreasing trans fat intake.

Trans fats have received considerable attention due to their effect on the lipid profile.  Studies have shown that trans unsaturated fatty acids raise serum total and LDL cholesterol and may actually lower HDL cholesterol (2).  In so doing, trans fats raise the total cholesterol to HDL ratio (4).  These findings make the consumption of trans fats a risk for heart disease.  The AHA recommends that Americans limit their intake of trans fats, particularly by reducing intake of commercially prepared baked goods and fried foods.  Collectively, the AHA wants Americans to limit total intake of “cholesterol-rising fatty acids” to less than 10% of total daily caloric intake (5).  This would include saturated and trans fats.

The quantity of trans fats being consumed by Americans is a big concern.  A study conducted by Allison and colleagues sought to determine trans fat intakes of Americans (6).  The study used data collected from the Continuing Survey of Food Intakes by Individuals (CSFII), which was conducted between 1989-1991.  The subjects completed a 24- hour recall of dietary intake, and used a two-day food record to document dietary habits.  The trans fat levels of food items were determined using a database from the United States Department of Agriculture released in 1995 (6).  The results of this study found that on average, Americans consume 2.6% of their energy as trans unsaturated fatty acids.  Of the total calories consumed daily from fat, 7.4% are from trans fats. The average intake in grams of trans fat was 5.3 grams.  Of the 5.3 grams of trans fats consumed on average, 20-25% (1.06 – 1.3 grams) of this amount come from natural occurring sources, in particular beef and dairy products (6).

This study also reported average saturated fat intakes. The average intake of saturated fatty acids was 12.5% of calories, while the amount of total calories from fat was 35.2%.  These intakes of saturated and total fat are above the USDA’s Dietary Guidelines for Americans, of less than 30% of total calories from fat and less than 10% from saturated fat. 

The authors of this study stress the average American intake of saturated fat and total fat, “not” trans fats as being the major problem.  Thus the figures for saturated fat and total fat intake are deemed more alarming and indicative of the lipid profile and risk for CAD (6).  However, trans fats do affect the lipid profile and are important in CAD.

In a study similar to that of Allison, Lemaitre and colleagues determined average intakes of trans fats in the western diet and compared individual intakes of trans fats to the adipose tissue of the subjects (7).  Using samples of adipose tissue from the subcutaneous fat of the upper buttock, the researchers determined the amount of trans unsaturated fatty acid tissue in the sample.  The technique used, gas chromatography, allowed for 10 different trans fats to be identified.  The study employed the same USDA database used in Allison’s study (7).  A food frequency questionnaire was used to obtain diet histories from the subjects.

The mean intake of trans unsaturated fatty acids based on the food frequency questionnaire was 2.24 grams per day, lower than the results reported by Allison and colleagues (7).  Trans fats represented 5.2% of the calories from dietary fat.   The mean level of trans fatty acids in the adipose tissue samples was 4.65%. Of interest, the researchers reported that the weight of the subjects was the biggest predictor of the amount of trans unsaturated fatty acids in the sample (7).

The preceding studies focused on the intake of trans fats in the typical American diet.  Both studies raised concern about intake of trans fats, however Allison and colleagues placed greater emphasis on the high intakes of saturated fat and total fat.   The following studies focus on trans unsaturated fatty acids and their effect on the lipid profile.

Lichtenstein and colleagues examined the effects of diet and fat intake on the lipid profile. All subjects consumed 6 different diets for a time period of 35 days per diet.  Blood lipid levels were drawn before and at the end of each diet to assess the impact of the specific fat intake on the lipid profile.  Each diet contained 30% of calories from fat, the recommended level for healthy Americans.  However, the type of fat used predominantly in each diet was different.  Types of fat included soybean oil (less than 0.5 grams of trans fat per 100 grams), semi-liquid margarine (less than 0.5 grams of trans fat per 100 grams), soft margarine (7.4 grams of trans fat per 100 grams), shortening (9.9 grams of trans fat per 100 grams), stick margarine (20.1 grams of trans fat per 100 grams), and butter, which contains only a small amount of naturally occurring trans unsaturated fatty acids (4).

The results of the study showed differing changes in total and LDL cholesterol depending on the predominant fat in the diet.  Levels were the lowest following the soybean oil and semi-liquid margarine diet.  The soft margarine, shortening, stick margarine and butter diets resulted in progressively higher total cholesterol and LDL levels.  Little difference in HDL levels among the various diets was observed, however the stick margarine diet did have the lowest HDL levels.

This study recommends the use of soybean oil or semiliquid margarine based on the positive effects these two fats had on the lipid profile.  Stick margarine and butter had the least favorable effects on total cholesterol and LDL.  While this study points out the effects of trans fats in the stick margarine on cholesterol and LDL levels, it is important to recognize that butter exhibited the same effects.  The switch back to butter in order to reduce trans unsaturated fatty acid intake is not the correct choice, according to this study.  Semi-liquid margarines and oils are the most beneficial fat sources and should comprise a large part of the 30% of calories consumed daily from fat.

In yet another study designed to uncover the mystery behind trans fats and their effect on serum cholesterol, Judd and colleagues compared the effect of butter and margarine consumption on the lipid profile (2). Three diets were used: a butter diet, a margarine diet that contained the average amount of trans fats in commercial margarine (17% dry weight), and another margarine that was trans fat free.  The trans free margarine was made of polyunsaturated fatty acids that had not undergone partial hydrogenation.

Fasting lipid profiles were taken prior to each diet, after the diet ended, and before the next one began.  Compared with those on the butter diet, cholesterol was lower in those on the margarine diets. The butter diet also raised LDL levels, while the LDL levels following the margarine diets were lower.  Judd and colleagues’ findings support the previous study by Lichtenstein.  Butter has a greater impact on the total cholesterol and LDL cholesterol levels.  While trans unsaturated fatty acids do have an effect on the lipid profile, butter was shown to have a more pronounced effect in these studies.

Additional epidemiological evidence supporting the harmful effects of trans fats in the diet can be found in the Health Professionals Follow-up Study, the Alpha-Tocopherol Beta-Carotene Cancer Prevention Study and the Nurse’s Health Study. In each of these studies, trans fat intake was assessed through food frequency questionnaires.  All found an adverse effect of trans fats and an association with increased risk of heart disease (3). 

The data presented in these studies can serve as the guidelines for use of trans unsaturated fatty acids in the diet.  Trans fats do clearly impact the lipid profile, putting one at a greater risk for developing CAD.  However, butter has a similar affect on the lipid profile.  HDL is the only component from the lipid profile that is not negatively effected by butter but that is negatively effected by trans fats (4).  On a per gram basis, trans fat appears to have a stronger adverse effect on lipid levels (3).  The studies presented support using products that are low in both trans unsaturated fatty acids and saturated fatty acids.  Therefore, the switch back to butter is ill advised due to its high saturated fat content.  Liquid oils, semi-liquid margarines and soft tub margarines are the best fats to consume for heart health.  The more solid a fat, the more saturated fat and trans fat it will contain.  It is almost impossible to completely avoid trans unsaturated fatty acids in the diet.  However, the small amount of trans fatty acids that would be consumed by a person using primarily semi-liquid margarine and oils would most likely not have a major impact on serum lipid levels. 

Because of potential health risks, some researchers feel that there is a need for trans unsaturated fatty acids to be included on the food label.  Ascherio and colleagues state that when something with no nutritional value is added to a food, manufacturers need to use a low threshold for harm in order to show that products are safe (3).  Currently, because trans fatty acid labeling is not required, manufacturers can use a large amount of trans fatty acids in their products without the consumers’ knowledge.  Unfortunately many products marketed as being cholesterol free are loaded with trans fatty acids.  There is some discussion on including trans fat with saturated fat on the food label.  However trans fat content on food labels would not necessarily help in this battle since many of the foods highest in trans fats are exempt from labeling regulations, such as fast foods. The AHA has recently stated that they are expecting future inclusion of trans fats on the food label as well as an increase in trans-free foods (5).

In conclusion, by adopting the use of unhydrogenated oils in food manufacturing and by switching to semi-liquid margarine, the risk for CAD may be decreased.  Trans unsaturated fatty acid intake as well as saturated fatty acid intake would be decreased, resulting in improved lipid profiles and healthier hearts.

References

  1. Mahan K, Escott-Stump S. Krause’s Food, Nutrition and Diet Therapy. 10th ed. Philadelphia:Saunders, 2000:584.
  2. Judd JT, Baer, DJ, Clevidence BA, et al. Effects of margarine compared with those of butter on blood lipid profiles related to cardiovascular disease risk factors in normolipemic adults fed controlled diets. Am J Clin Nutr. 1998;68:768-77.
  3. Ascherio A, Katan MB, Zock PL, Stampfer MJ, Willett WC. Trans fatty acids and coronary heart disease. N Engl J Med. 1999;340:1994-1998.
  4. Lichtenstein AH, Ausman, LM, Jalbert SM, Schaefer EJ. Effects of different forms of dietary hydrogenated fats on serum lipoprotein cholesterol levels. N Engl J Med. 1999;340:1933-1939.
  5. Krauss RM, Eckel, RH, et al. AHA guidelines revision 2000: A statement for healthcare professionals from the nutrition committee of the American Heart Association. Circulation. 2000;102:2284-2299.
  6. Allison DB, Egan SK, Barraj LM, Caughman C, Infante M, Heimbach JT. Estimated intakes of trans fatty and other fatty acids in the US population. J Am Diet Assoc. 1999;99:166-174.
  7. Lemaitre RN, King IB, Patterson RE, Patsy BM, Kestin M, Heckbert SR. Assessment of trans fatty acid intake with a food frequency questionnaire and validation with adipose tissue levels of trans fatty acids. Am J Epidemiol. 1998;148:1085-1092.