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BACKGROUND AND SCIENTIFIC REVIEW

For further information contact: Robin Herman

(617)432-4752 rherman@hsph.harvard.edu

TRANS FATTY ACIDS AND CORONARY HEART DISEASE

Alberto Ascherio

Meir J. Stampfer

Walter C. Willett

Departments of Nutrition and Epidemiology, Harvard School of Public Health

The Channing Laboratory, Department of Medicine, Brigham and Women’s Hospital

On November 12, 1999, the Food and Drug Administration announced its proposal to

include the trans-fatty acid (trans fat) content of foods on the standard food label. At present,

only saturated fats are listed. Because many persons will be unfamiliar with trans fat and its

health effects, we have posted the following review.

• What are trans fatty acids?

Trans unsaturated fatty acids, or trans fats, are solid fats produced artificially by heating liquid

vegetable oils in the presence of metal catalysts and hydrogen.

1

This process, partial

hydrogenation, causes carbon atoms to bond in a straight configuration and remain in a solid state

at room temperature. Naturally-occurring unsaturated fatty acids have carbon atoms that line up

in a bent shape, resulting in a liquid state at room temperature.

• Which foods contain trans fatty acids?

Trans fats are produced commercially in large quantities to harden vegetable oils into shortening

and margarine. Food manufacturers also use partial hydrogenation of vegetable oil to destroy

some fatty acids, such as linolenic and linoleic acid, which tend to oxidize, causing fat to become

rancid with time. The oils used to cook french fries and other fast food are usually this kind of

partially hydrogenated oil, containing trans fats. Commercial baked goods frequently include

2

trans fats to protect against spoilage. A small amount of trans fat is also produced in the

gastrointestinal tract of cattle, so that low levels of these isomers are found in dairy and beef fat.

Commercial production of partially hydrogenated fats began in the early 20th century and

increased steadily until about the 1960's as processed vegetable fats displaced animal fats in the

diets of the U.S. and other Western countries. Lower cost was the initial motivation, but health

benefits were later claimed for margarine as a replacement for butter.

Although the average level of trans fat in margarines has declined with the advent of

softer versions, per capita consumption of trans fatty acids has not changed greatly since the

1960’s because of the increased use in commercially-baked products and fast foods.

• What are the health effects of trans fats?

Concerns have been raised for several decades that consumption of trans fatty acids

might have contributed to the 20th century epidemic of coronary heart disease.

2

Metabolic studies have shown that trans fats have adverse effects on blood lipid levels—

increasing LDL (“bad”) cholesterol while decreasing HDL (“good”) cholesterol. This combined

effect on the ratio of LDL to HDL cholesterol is double that of saturated fatty acids.

3

Trans fats have also been associated with an increased risk of coronary heart disease in

epidemiologic studies.

4

Based on the available metabolic studies, we estimated in a 1994 report that

approximately 30,000 premature coronary heart disease deaths annually could be attributable to

consumption of trans fatty acids.

4

In response to these reports, a 1995 review sponsored by the food industry concluded

that the evidence was insufficient to take action and that further research was needed.

5

Since that

3

time many more metabolic studies have been conducted and additional prospective

epidemiologic studies have been reported.

Because of the weight of the evidence, the FDA has recently issued a proposal for

including trans fatty acid content on the food label. One important issue is whether to list trans

fat as a separate constituent or to combine it with saturated fat.

• What are the arguments for listing trans fat separately from saturated fat?

The combined results of metabolic and epidemiologic studies strongly support an adverse

effect of trans fat on risk of CHD. Furthermore, two independent methods of estimation indicate

that the adverse effect of trans fat is stronger than that of saturated fat. By our most conservative

estimate, replacement of partially hydrogenated fat in the U.S. diet with natural unhydrogenated

vegetable oils would prevent approximately 30,000 premature coronary deaths per year, and

epidemiologic evidence suggests this number is closer to 100,0000 premature deaths annually.

These reductions are higher than what could be achieved with realistic reductions in saturated fat

intake.

What alternatives exist to trans fats?

In Europe, producers have responded rapidly to the evidence on effects of trans fats by

developing trans-free margarines that are also low in saturated fats.

48

More recently, these

products have also become available in the U.S., although a large share of the market is still

heavily hydrogenated stick margarine.

5

It is thus evident that trans-free products are feasible, and that the technical constraints

often invoked by the food industry can be overcome. However, out of the trans fatty acids

provided by hydrogenated vegetable oil in the U.S., only 25%

5

to 37%

49

comes from margarines,

4

the remainder comes from baked goods, fast foods and other prepared foods. Replacement of

trans in such products by healthier fats may be more difficult than in margarines, but can be

achieved.

In spite of this, many products including most baked goods and fried fast foods still are

made with partially hydrogenated fat both in Europe and in the U.S. and are high in trans fatty

acids. It is unlikely that this situation will change without strong federal regulations.

• How important are label changes?

Current regulations in the U.S. require food labels to include the amount of saturated fat,

but not the amount of trans, thereby providing an incentive to manufacturers to increase the trans

content while decreasing the amount of saturated fat.

Although changes in labeling are extremely important, many products, including fast

food, which often contain extremely high levels of trans isomers, are exempt from labeling

regulations and can carry deceptive labels such as “cholesterol-free” and “cooked in vegetable

oil.”

For example, a person eating one doughnut for breakfast (3.2 g)

50

and a large order of

french fries for lunch (6.8 g)

50

would ingest 10 g of trans fatty acids, or 5 percent of the total

energy of an 1,800-calorie diet. Thus, simple labeling changes alone will not be sufficient.

The following is a more detailed review of the scientific studies behind these conclusions:

Effects on plasma lipids

Studies in the 1960's compared the effects of partially hydrogenated fat with those of

5

unhydrogenated vegetable oils or saturated fats on the concentrations of total serum cholesterol.

Overall, these earlier studies suggested that the cholesterol raising effect of hydrogenated fat was

somewhat lower than that of saturated fats.

1, 6

Only in 1990 was attention given to the fact that

although trans fatty acids increase LDL cholesterol to a similar degree as saturated fat, they

decrease HDL cholesterol relative to both cis unsaturated or saturated fats.

3

In a rigorous

metabolic study, Mensink and Katan demonstrated that replacement of 10% of energy from oleic

acid (the primary monounsaturated fat in diets) with trans 18:1 fatty acids caused a 0.34 mmol/L

increase in LDL cholesterol and a 0.17 mmol/L decrease in HDL cholesterol; whereas

replacement of oleic acid with saturated fat caused a similar increase in LDL cholesterol, but

virtually no change in HDL cholesterol. As a result, the LDL/HDL cholesterol ratio was

significantly higher on the trans (2.58) than on the saturated (2.34) or oleic (2.02) diets. These

findings were soon confirmed in several investigations, including the study by Lichtenstein et al.

that appears in this issue of the Journal, using lower levels of trans fatty acids and different

mixtures of trans isomers.

7-10

Figure 1 summarizes the randomized trials that allow a direct

comparison of trans fatty acids with isocaloric amounts of cis unsaturated fat;

3, 7-15

also included

is the study of Aro et al.

13

that used stearic (which is usually said to have a neutral effect on

blood lipids) rather than oleic acid as the control diet. Overall, trans fatty acids increased LDL

cholesterol similarly to saturated fat, but, unlike saturated fat, they also decreased HDL

cholesterol. As a result, the net effect of trans fat on the LDL/HDL cholesterol ratio is

approximately double that of saturated fat. The difference between the effect of trans fat and that

of saturated fat on the LDL/HDL ratio was significant in each of the six studies that allowed a

direct comparison. The corresponding P values were: <0.0001,

3

<0.001,

7

<0.001,

8

<0.009,

9

6

<0.01,

10

<0.05.

14

Thus, the probability that these results were due to chance is vanishingly low;

taken together, these studies provide definitive evidence that trans fats raise the LDL/HDL ratio

more than saturated fats. Moreover, these effects of trans fat on the LDL/HDL cholesterol ratio

are remarkably constant across studies. The only somewhat discordant result was obtained by

Sundram et al. among 27 men and women who were staff of the Palm Oil Research Institute in

Malaysia.

14

In that study, the effect of trans fat on the LDL/HDL cholesterol ratio was

considerably stronger than in the other investigations (see Figure 1), whereas little effect was

seen for saturated fat (palmitic acid). The more marked response has been attributed to the

habitual Malaysian diet consumed by participants at baseline, which is lower in total fat (26%

energy) than typical western diets.

14

Although the possibility that the adverse effects of trans fat

are more marked in populations with a lower percent of energy from fat cannot be excluded, we

have conservatively excluded the Malaysian study in estimating the regression lines in Figure 1.

Its inclusion would make the association between trans fat and LDL/HDL ratio even stronger.

Also not included was the study by Almendingen at al. that compared the effects of diets

containing partially hydrogenated fish oil (8.4% energy from trans fat), partially hydrogenated

soy bean oil (6.6% energy from trans fat), or butter. The LDL/HDL cholesterol ratio was

significantly higher on the partially hydrogenated fish oil diet (4.20) compared to both the

partially hydrogenated soy bean oil diet (3.65) or the butter diet (3.85), that were not significantly

different from each other. Thus, the Almendingen Study did not find an increase in the

LDL/HDL ratio on partially hydrogenated soybean oil compared with butter. The lack of a high

oleic or polyunsaturated diet prevented its inclusion in Figure 1, but the inclusion of these data

does not materially change the estimated effect of trans fatty acids on the LDL/HDL ratio.

7

In addition to the studies summarized above, other dietary trials have compared the

effects of butter and margarine on blood lipids. Because margarines are usually higher in cis

polyunsaturated fat than butter, the specific effects of trans fat cannot be accurately estimated

from these trials. A meta-analysis of these investigations, however, showed that replacement of

butter with hard stick margarines -- that typically have contained 20-25% trans fat -- does not

affect the total/HDL cholesterol ratio, whereas a reduction was obtained with low-trans tub

margarine.

16

These results confirm the deleterious effects of trans fat on blood lipids and

indicate that these may offset the beneficial effects of polyunsaturated fat. Thus individuals who

are replacing butter with margarine high in trans fat to reduce their risks of coronary disease may

obtain no benefit or -- if trans fat has deleterious effects beyond those on LDL and HDL -- may

even increase their risk.

In addition to increasing the LDL/HDL cholesterol ratio, trans fatty acids increase Lp(a)

when substituted for saturated fat. A significant increase was reported in nine

3, 8, 10, 12-15, 17

of ten

trials. The null result was from an investigation that included only 14 subjects who consumed a

diet with 3.7% energy from trans fat; the power of this study may have been to low to

demonstrate an effect.

11

High blood levels of Lp(a) have been associated in some studies with

increased risk of CHD, independently of LDL or HDL cholesterol concentrations. However diet-

induced variations in blood concentrations of Lp(a) are modest relative to the genetic differences,

and their quantitative impact on risk of CHD remains to be established.

Yet another effect of trans fatty acids on blood lipids is that on fasting triglyceride levels.

Already in 1961, the group of Ancel Keys noted that hydrogenated corn oil resulted in higher

triglyceride levels than natural oils or butter.

6

A triglyceride-raising effect was also consistently

8

seen in seven recent studies that directly compared trans fatty acids with cis-unsaturated fatty

acids;

3, 7-9, 11, 14, 15

the increases ranged from 0.5 to 12 mg/dL, with an average of 1.5 mg/dL per

1% of energy intake. The effect on triglyceride levels of substituting saturated fatty acids for cis-

unsaturated fatty acids is about zero.

18

Thus, trans fatty acids increase triglyceride levels when

compared with other fatty acids. Eliminating 2% of energy trans fatty acid from the diet would

lower triglyceride levels by about 3 mg/dL; the relation between triglycerides and risk of CHD is

still uncertain, but the resulting benefit is probably modest.

Potential effects of trans fat on LDL oxidation

8, 19

and coagulation and fibrinolytic

factors

20-22

have also been investigated, but so far there is no conclusive evidence of adverse

effects.

Epidemiological studies

One of the most influential studies on diet and CHD was the work of Keys

23

who related

the incidence of heart disease in 16 defined populations in seven countries to their intake of fat

and cholesterol. The clear association that he found between percent of energy as saturated fat

and CHD incidence and mortality has often been quoted as strong evidence that saturated fat

increases the risk of CHD. The original investigations have now been complemented by

collection and analyses of food composites representing the average intake of each cohort at

baseline, so that the relation of CHD incidence and mortality to intake of trans fat and other

specific nutrients could be examined.

24

Whereas saturated fat intake was strongly correlated to

CHD mortality (r=0.88; P < 0.0001), confirming the original results, a similar positive

9

correlation was found between CHD mortality and trans fat intake (r=0.78; P < 0.0001).

Interpretation of such comparisons of populations with widely different lifestyles is hazardous,

but at the very least these data leave room for a major effect of trans fat on CHD risk.

Several case-control or cross-sectional studies have also been conducted. In a case-

control study in the Boston area, we found a strong and significant positive association between

trans fat intake assessed using a FFQ and risk of acute myocardial infarction.

25

The relative risk

comparing the highest to the lowest quintile of trans fat intake was 2.4 (P for trend <0.0001);

this association was entirely explained by trans intake from hydrogenated vegetable oil.

Previously, Bolton-Smith et al. examined cross-sectionally the association between trans intake

and undiagnosed CHD among participants in the Scottish Heart Study.

26

Subjects were

considered to have CHD if they had angina or possible MI according to the Rose chest pain

questionnaire, or an electrocardiogram recording indication of ischemia.

26

Trans intake,

estimated by a FFQ, was positively correlated with the (LDL+VLDL)/HDL cholesterol ratio.

The odds ratio for risk of CHD comparing the highest versus the lowest quintile of intake were

elevated but not statistically significant (1.26 in women, and 1.08 in men). Positive associations

between consumption of margarine and risk of acute myocardial infarction were found in a case-

control study in Italy

27

and one in Greece.

28

Investigations in which tissue or plasma fatty acids

composition was used as a biomarker of trans intake

29-36

gave conflicting results. With one

exception,

33

however, these investigations were small, and the power too low to reliably detect

an association. The only large study, the EURAMIC study,

33

included 671 men with acute

myocardial infarction in eight European countries. The overall analyses revealed no association

10

between trans intake and risk of myocardial infarction (multivariate odds ratio comparing the top

vs. the bottom quartile was 0.97). The two centers in Spain, where CHD rates are very low,

however, had extremely low trans levels and little between-person variation compared to those

from other countries, and thus provided little or no information on the relation between trans and

CHD. After appropriately excluding these centers, the odds ratio in the third and fourth quartile

increased to 1.53 (95% confidence interval: 1.02, 2.28) and 1.44 (0.94, 2.20) respectively. In

addition, there was significant heterogeneity in the odds ratios between countries, from 0.2 in

Spain and Moscow, to 5.0 in Finland and 5.4 in Norway. This heterogeneity may be due to

different amounts, and sources of trans -- and therefore different isomers -- in different countries

(for example in Spain, unlike in the other countries, most dietary trans fatty acids are from

animal sources), or, more likely, to interaction with other dietary factors, or confounding by

unmeasured or poorly measured covariates. Interpretation of the EURAMIC results is

controversial, but in any case they do not provide strong evidence against the hypothesis that

trans fatty acids increase the risk of CHD, and if anything add weight to the existence of a

positive association.

The strongest epidemiological evidence relating dietary factors to risk of CHD is

provided by prospective investigations. The relation between trans fatty acids intake and risk of

coronary disease has now been reported from three large cohort studies, the Health Professionals

Follow-up Study (HPFS),

37

the Alpha-Tocopherol Beta-Carotene study (ATBC)

38

and the

Nurses’ Health Study (NHS)

39

. In these studies, trans fat consumption was assessed using

detailed food frequency questionnaires (FFQ) that were validated by comparison with adipose

11

composition

40, 41

or several days of diet records.

42

In addition, the relation between margarine

intake and risk of CHD has been reported from the Framingham cohort.

43

The results of each of

these investigations support an adverse effect of trans fatty acids. The relative risk of coronary

heart disease for a 2% increase in trans fatty acids intake was 1.36 (95% confidence interval:

1.03, 1.81) in the HPFS, 1.14 ( 0.96, 1.35) in the ATBC, and 1.93 (1.43, 2.61) in the NHS. The

higher relative risk in the NHS may be related to the fact that this investigation took advantage of

up to four repeated dietary measurements during the follow-up, thereby reducing the error in

assessing trans consumption; in analyses using only the baseline dietary measure, the

corresponding relative risk was 1.62. In all cohorts, these relative risks were considerably higher

than those for saturated fat. For example, in the NHS replacing 5 percent of energy from

saturated fat with energy from unsaturated fat was associated with a 42 percent lower risk,

whereas replacing 2 percent of energy from trans unsaturated fat with energy from

unhydrogenated, unsaturated fats was associated with a 53 percent lower risk. These studies

have been criticized on the grounds that measurements of trans intake were unreliable;

5

however,

errors in measuring trans fatty acids intake can only have led to underestimation of the

association with CHD risk.

44

Also, it has been suggested that the observed associations resulted

from a shift from butter to margarine among subjects at high risk of CHD.

45

If so, the association

between trans intake and risk of CHD should be weaker among subjects with stable margarine

consumption, and should be stronger during the first few years of follow-up. In fact, the opposite

was true in the Nurses’ Health Study,

46

where exclusion of women who changed their diet before

the beginning of the study strengthened the association,

46

and in the Framingham cohort, where

the positive association between margarine consumption and CHD risk was strengthened after

12

excluding the first ten years of follow-up.

43

Moreover, high consumption of trans (or margarine)

was not related to other dietary behaviors perceived as healthy for the heart, such as a preference

for skim rather than whole milk

43

and high-trans foods that are hardly perceived as healthy, such

as cookies, were also positively associated with risk of CHD in the Nurses’ Health Study.

46

Thus

there appear to be no likely alternative to the hypothesis that high trans intake increases the risk

of CHD. Although confounding by unmeasured or poorly measured risk factors cannot be

excluded, as is usually the case in observational studies, we lack a credible hypothesis of what

such confounder(s) could be, as these associations were controlled for an extensive number of

other dietary and lifestyle risk factors. In the Health Professionals Follow-up Study, adjustment

for dietary fiber attenuated the relation of trans to risk of CHD, however no attenuation occurred

in the other two cohorts

38

(Hu, personal communication). In summary, prospective studies

provide strong evidence that trans fatty acids consumption increases substantially the risk of

CHD.

Quantitative estimates of risk

Independent estimates of the effect of trans fat can be obtained by combining the effects

of trans on blood lipids and the relationship between lipids and coronary heart disease risk, or

from the results of cohort studies. We have used these different methods to estimate the number

of deaths that may be due to consumption of trans fatty acids from partially hydrogenated fat in

the amount of 2% energy (approximately the U.S. average).

The first approach uses only the effect of trans fats on blood lipids, and ignores the

13

associations observed in epidemiological studies. As shown above (Figure 1), replacing 2%

energy from cis unsaturated fat with an isocaloric amount of trans fat causes a 0.13 increase in

the LDL/HDL cholesterol ratio (we used this because the effect of trans on the total/HDL

cholesterol ratio that we used in the past was not provided in one of the new studies). Since a

change in one unit in the total/HDL cholesterol ratio has been associated with a 53% change in

the CHD risk,

47

we estimated a relative risk of 1.07 (1 + 0.53 x 0.13) for a 2% increase in trans

fat (the use of LDL/HDL ratio instead of total/HDL cholesterol ratio has trivial effects),

corresponding to an attributable risk of 6.5% (0.07 divided by 1.07). The effect of saturated fat

on the LDL/HDL ratio is about half that of trans fat, so that the same attributable risk would be

estimated for a 4% of energy increase from saturated fat. These are likely to be underestimates of

the true effect, because the lipid-CHD relation that we used has not been corrected for the

attenuation caused by within-person variation in lipid measurements. Also, the estimate for trans

fat does not take into account adverse effects on triglycerides or Lp(a). In the metabolic studies,

cis unsaturated fats replaced trans fat as would be the case if the original oils were simply not

partially hydrogenated. Because unsaturated fats themselves have beneficial effects on blood

lipids, the benefits of eliminating trans or saturated fats would be less if they were replaced by

carbohydrate.

The second approach calculates risk directly from the strength of the association between

trans fat and CHD as observed in epidemiological studies. A pooled estimate of the results

reported in the prospective studies (ATBC, HPFS, and NHS) gives a relative risk of 1.31 (1.15,

1.49) for an increase in trans consumption of 2% of energy. Assuming that this relation is

causal, the attributable risk would be 24%, or over 100,000 coronary deaths per year. Moreover,

14

according to the results of the HPFS and NHS, it would require a 10% of energy reduction in

saturated fat intake to obtain a benefit comparable to that of eliminating trans fat from the U.S.

diet. No benefit of reducing saturated fat intake would be predicted by the results of the ATBC.

Our first approach, using data from metabolic studies obtained above, assumes that the

adverse effects of trans are entirely mediated by their effects on blood levels of LDL and HDL;

whereas, the second approach, using results of epidemiological studies, suggests that the increase

in risk of CHD caused by trans fat is higher than predicted by effects on blood lipids alone.

Ignoring this possibility could cause a substantial underestimation of the adverse effects of trans

fat.

Conclusion

Five years ago evidence was strong that trans fat had deleterious impacts on blood lipids;

ensuing studies have confirmed these metabolic findings and strengthened epidemiologic support

for an important adverse effect on risk of coronary heart disease. These data highlight the need

for rapid implementation of labeling requirements that include fast foods. Because partially

hydrogenated fats can be eliminated from the food supply by changes in processing that do not

require major efforts in education and behavioral modification, these changes would be an

extremely efficient and rapid method for substantially reducing rates of coronary disease.

Acknowledgment

15

We would like to thank Jill Arnold for her expert assistance with this review.

16

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