Low Fat Diets Not Healthy?--Not So Fast
It has recently been reported in the popular press that low fat diets do not
reduce the risk of breast cancer, colon cancer or cardiovascular disease. Nor do
low fat diets lead to significant weight loss. The headline in the Los Angeles
Times on Feb. 8, 2006 read "Eating Lean Doesn't Cut Risk" and the
article claims that this research overturns three decades of conventional
wisdom. This has led Dr. Walter Willett of the Harvard School of Public
Health, an opponent of low fat diets, to conclude that "this is ... the end
of the low-fat era." But is this the last word on low fat dietary
recommendations? I say no!
These reports were based on three studies that were published in the Feb. 8
issue of the Journal of the American Medical Association (JAMA) that have
examined the relationship between a low fat diet and the risks of cardiovascular
disease and cancer, and on weight loss, in the Jan. 4, 2006 JAMA issue.(1,2,3,4)
These studies are referred to as the Women's Health Initiative (WHI)
trials. But a careful reading of these studies shows a trend towards improved
health and problems with the studies that may have resulted in a lack of
significant positive findings.
Let's examine the cancer risk trials first, then the cardiovascular risk trial,
and finally the weight loss study:
Low Fat Diets and Cancer
To begin with, there were significant problems with these studies. The women
who, on average, were consuming 35% fat were told to reduce fat intake to 20%.
But this goal was never achieved. By the 6th year, these women were consuming
29% fat. Therefore, the conclusion is based on reducing fat intake from
35% to 29%, which is not a large enough change to see much difference. And
29% fat can hardly be considered low fat.
Secondly, no recommendation was made on the type of fat to be consumed, whether
it be saturated, polyunsaturated, monounsaturated, trans fats, or omega 3 fatty
acids. And we now know that the type of fat is as important as the amount of fat
on the health effects. Reducing saturated and trans fats, while increasing
levels of omega 3 and monounsaturated fats would have had a more significant
impact on improving health.
Thirdly, cancer is a disease that typically develops slowly over a long period
of time and it is doubtful that the eight years that this study lasted is a long
enough time to study it.
Fourth, with the breast cancer study, there was actually a 9% lower incidence of
breast cancer in the women who reduced their fat intake, though this did not
reach a level that was considered statistically significant. And
those women who started with a higher level of fat consumption than average and
who subsequently lowered their fat consumption the most, had a 22% decrease in
their breast cancer risk, which obviously is statistically significant.
Low Fat Diets and Cardiovascular Risk
How about the risk of cardiovascular disease with low fat diets? The
conclusion stated was that this low fat dietary intervention program did not
significantly reduce the the risk of heart attack or stroke. As mentioned above,
the goals of the study were not met as few of the women lowered their fat intake
to 20%. After 6 years, the average fat intake was 29% as opposed to 20%. There
are other reasons to doubt how well the recommendations were followed. Despite
the fact that the participants were asked to increase their intake of grains and
also of fruits and vegetables, by year 6, the amount of grains in their diets
actually went down--from 4.7 servings per day to 4.3 servings per day.
There were several positive trends towards lowering cardiovascular risk that
were observed in this study. For one thing, there was a small but
statistically significant reduction in cholesterol, low density lipoprotein
levels, diastolic blood pressure, and certain clotting factors (factor VIIc).
For a second thing, increasing carbohydrate intake did not increase
cardiovascular risk factors, as some have claimed.(5,6) There were no
negative effects on lipid profiles or levels of glucose or insulin. There
were no significant increases in triglycerides or decreases in HDL-C levels.
Finally, a third point to be made, is that trends toward greater reductions in
cardiovascular risk were seen in those with lower intakes of saturated fat or
trans fat or higher intakes of vegetables and fruits. What this means is that if
the recommendations of the study were actually followed, we probably would have
seen a positive effect in terms of cardiovascular risk.
It is worth comparing the results in these current low fat studies with 6
previous clinical trials that have examined the effect of dietary fat on
cardiovascular risk. In four of these studies, individuals started with
extremely high levels of saturated fat and decreased this significantly,
resulting in 12 to 15% reduction in total cholesterol and a significant
reduction in cardiovascular disease.(7,8,9,10) In two other studies, the
participants only achieved a much more modest reduction in cholesterol and thus
no reduction in cardiovascular disease risk (CVD) was seen.(11,12) The
authors of this current trial note that these previous trials "support the
implication that the lack of effect on CVD in the current study was attributable
to the small decrease in saturated fat and reduced intakes of monounsaturated
and polyunsaturated fat that yielded the subsequent minimal change in LDL-C
levels." They also note that the significant changes in fat reduction
and thus in reduction in CVD seen in the prior trials were easier to achieve due
to the generally higher levels of saturated fat intakes in the U.S..
As noted in an editorial on this trial in the same issue of JAMA, this trial
"was not optimally designed to test hypotheses related to CVD" for the
following reasons(13):
1. Most participants were overweight
and the trial did not involve weight reduction as a goal. Weight reduction more
likely would have resulted in a reduction in CVD.
2. It did not pay attention to the
type of fat consumed, such as whether it was saturated, polyunsaturated,
monounsaturated, or trans fats. Therefore, not only were fats that
increase CVD risk reduced, such as saturated fats, but so were fats that may reduce
CVD risk, such as monounsaturated fats like olive oil. Instead of just
reducing fat, if they had reduced saturated and trans fats and had maintained or
increased levels of monounsaturated and/or omega 3 fats, they more likely would
have seen a reduction in CVD.
3. This trial did not involve
dietary modifications that lower blood pressure, such as reduced salt intake,
increased potassium intake, the DASH diet, or weight loss.
Low Fat Diets and Weight Loss
Now let's examine the part of the WHI study that reported on weight loss
resulting from this low fat dietary approach.(4) The conclusion of this
study was that there was no weight gain from raising the intake of carbohydrates
(resulting from lowering intake of fat), as some low carbohydrate proponents
have argued. In fact, despite the fact that it was not one of the goals of the
study to reduce calorie intake, there was a trend towards weight loss--an
average of 4.5 lbs--that was maintained during the course of the study. This
should put to bed the idea that you can not lose weight on a low fat diet.
This correlates with a previous study that compared weight loss with low carb
(Atkins, Zone) vs low fat (Ornish) vs balanced (Weight Watchers) diets.(14)
In fact, those in the Atkins group actually lost the least amount of weight: 4.6
lbs compared to 7.3 for Ornish and 7.0 for Zone and 6.6 for weight watchers. The
conclusion that I feel should come out of these two studies is that you can lose
weight on any calorie restriction program and low fat diets are at least as
effective as low carbohydrate diets. And let's not forget, that if you really
want to lose bodyfat, not only do you need to lower your calorie intake, you
need to exercise regularly.
REFERENCES:
1. Prentice RL, Caan B, Chlebowski RT, et al. Low-Fat
Dietary Pattern and Risk of Invasive Breast Cancer: The Women's Health
Initiative Randomized Controlled Dietary Modification Trial. JAMA.
2006;295:629-642.
2. Beresford SAA, Johnson KC, Ritenbaugh C, et al.. Low-Fat Dietary
Pattern and Risk of Colorectal Cancer: The Women's Health Initiative Randomized
Controlled Dietary Modification Trial. JAMA. 2006;295:643-654.
3. Howard BV, Van Horn L, Hsia J, et al. Low-Fat Dietary Pattern and Risk
of Cardiovascular Disease: The Women's Health Initiative Randomized Controlled
Dietary Modification Trial. JAMA. 2006;295:655-666.
4. Howard BV, Manson JE, Stefanick ML, et al. Low-Fat Dietary
Pattern and Weight Change Over 7 Years: The Women's Health Initiative Dietary
Modification Trial. JAMA. 2006;295:39-49.
5. Pereira MA, Liu S. Types of carbohydrates and risk of cardiovascular
disease. J Womens Health. 2003; 12:115-122.
6. Hellerstein MK. Carbohydrate-induced hypertrigylceridemia: modifying
factors and implications for cardiovascular risk. Curr Opin Lipid. 2002;
13:33-40.
7. Dayton S, Pearce ML, Hashimoto S, et al. A controlled trial of a diet
high in unsaturated fat for preventing complications of atherosclerosis. Circulation.
1969; 60(suppl 2):111-163.
8. Leren P. The Oslo diet-heart study: eleven year report. Circulation.
1970; 42:935-942.
9. Turpeinen O, Karvonen MJ, Pekkarinen M, et al. Dietary prevention
of coronary heart disease: the Finnish mental hospital study. Int J Epidemiol.
1979; 8: 99-118.
10. Watts GF, Lewis B, Brunt JNH, et al. Effects on coronary artery
disease of lipid-lowering diet or diet plus cholestyaramine, in the St. Thomas'
Atherosclerosis Regression Study (STARS). Lancet. 1992; 339: 563-569.
11. Low fat diet in myocardial infarction: a controlled trial. Lancet.
1965; 2:501-504.
12. Controlled trial of soya-bean oil in myocardial infarction. Lancet.
1968; 2:693-700.
13. Anderson CA, Appe, LJ. Dietary modification and CVD prevention. JAMA.
2006;295:693-695.
14. Dansinger ML, Gleason JA, Griffith JL, et al. Comparison of the
Atkins, Ornish, Weight Watchers, and Zone diets for weight loss and heart
disease risk reduction. JAMA. 2005;293:43-53.
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