HEALTH UPDATES
The Wrong Message?
Results from an ongoing national study may mislead patients about dietary fat consumption
Earlier this month, some surprising
health news regarding dietary fat, hit the headlines. The unexpected message
was that reducing dietary fat intake does not decrease the risk of cardiovascular
disease, breast cancer or colorectal cancer in postmenopausal women. This
headline was based on the results of three studies published in the February
8, 2006 issue of The Journal of the American Medical Association (JAMA).
Unfortunately, an important message lost in the translation was that the
type, and not the amount of fat consumed, may play a significant role
in the prevention of certain diseases that affect women in later years.
The Women’s Health Initiative (WHI)
The results were based on three studies conducted as part of the Women’s
Health Initiative (WHI). The WHI, a long-term clinical study funded by
the National Heart, Lung and Blood Institute (NHLBI) of the National Institutes
of Health (NIH), is designed to investigate strategies for the prevention
and control of cancer, cardiovascular disease and osteoporosis in postmenopausal
women. The WHI was initiated in 1992 and follows over 160,000 women ranging
in age from 50 to 79 in over 40 U.S. clinical centers.
During the study, women were assigned to either a clinical trial group
or an observational study group. In the clinical trial group, women were
randomized to one of three interventions: 1) a low-fat eating pattern,
2) hormone replacement therapy or 3) calcium and vitamin D supplementation.
The Low-Fat Dietary Study
The results from the three recently published studies observed nearly
49,000 healthy women in the Dietary Modification Trial. In the intervention
group, 19,541 postmenopausal women received intensive behavior modification
sessions along with the goal of reducing fat intake to 20% of energy and
increasing intake of fruits and vegetables (5 or more servings daily)
and grains (6 or more servings daily). The low-fat dietary pattern was
not designed for weight loss and the participants were not asked to lose
weight. The comparison group included 29,294 women who were not asked
to make any dietary modifications. The average follow-up time for both
groups was 8.1 years.
Cardiovascular Disease Findings
In one study, Barbara V. Howard, PhD. and researchers at Howard University
in Washington, D.C. examined the effect of the intervention diet on cardiovascular
disease (CVD) and coronary heart disease (CHD). Researchers found that
levels of low-density lipoprotein (LDL) cholesterol and diastolic blood
pressure were significantly reduced in the intervention group. However,
levels of high-density lipoprotein (HDL), triglycerides, glucose and insulin
were not significantly different between the two groups. Researchers found
that diet had no significant effects on the incidence of CHD, stroke,
CVD or heart attack.
The most likely reason for these results is that the study was not designed
specifically to reduce heart disease. The study was designed to reduce
all types of fat, to determine if reduced fat intake prevents breast cancer.
It was assumed that if fat intake was reduced then saturated fat, a known
contributor to heart disease intake, would also be reduced. However, a
diet designed to reduce heart disease would specifically reduce saturated
and trans fat intake and would not reduce polyunsaturated and monounsaturated
fat intake. Studies have shown that such a diet leads to lower blood cholesterol
and reduces the risk of heart disease. Unfortunately, in 1992, when the
study was initiated much less was know about the importance of modifying
specific types of fats but instead the emphasis was placed on the total
amount of fat in the diet. Importantly, a trend toward greater reductions
in CHD risk were observed in study participants with lower intakes of
saturated fat or transfat intake or higher intakes of fruits and vegetables.
Breast Cancer Findings
In a second study, researchers wanted to test the hypothesis that a low-fat
diet can reduce breast cancer risk in postmenopausal women. The study
results, however, did not show a statistically significant difference
between the intervention and the observational group for invasive breast
cancer. A secondary analysis of the data did show some benefits of the
low-fat diet for women who had the highest fat intake at the beginning
of the study (>76 grams of daily fat intake). The low-fat diet also
lowered the risk of tumors that were positive for estrogen receptors and
negative for progesterone receptors.
Colorectal Cancer Findings
In a third study, Shirley A. A. Beresford, PhD. of the University of Washington,
Seattle, and her colleagues, examined the effects of a low-fat dietary
pattern on the incidence of colorectal cancer in postmenopausal women.
The study results, however, did not show a statistically significant difference
between the intervention and the observational group for invasive colorectal
cancers. The results were similar regardless of the location of the cancer
– the entire colon, upper or lower colon or rectum. However, the
number of women reporting polyps was 9% lower in the intervention group
than the comparison group.
What Next?
The WHI is the most comprehensive study to date, of the causes and prevention
of the major diseases affecting postmenopausal women. And, given the study’s
size and design, the findings are important. However, a number of questions
remain unanswered. For example, while the clinical trial group was directed
to reduce fat intake from 35% to 20% of energy, by the end of the study,
fat intake had only been reduced to 29%. In addition, direction was not
given on the types of fats to eat and specifically no direction was given
to avoid saturated and trans-fats. While the women were followed for 8
years, from a disease progression standpoint, more years of follow-up
might be required to reveal causal relationships. The average age of study
participants was 62. Perhaps more benefit would be gained if dietary modifications
were made earlier in life. Finally, the WHI focused on healthy women.
Patients with pre-existing cardiovascular disease or breast cancer may
obtain greater benefit from making modifications to their diet.
Based on the WHI findings along with the evidence from other studies,
it still makes sense for patients to adopt a plant-based diet that is
high in fruits and vegetables. In addition, sources of protein should
be low in saturated fat (e.g., turkey breast) and consumption of unhealthy
fats including saturated and trans-fats should be reduced while consumption
of healthy monounsaturated and Omega-3 polyunsaturated fats should be
increased.
Sources:
"Low-Fat Dietary Pattern and Risk of Invasive Breast Cancer: The
Women's Health Initiative Randomized Controlled Dietary Modification Trial."
Published in the Feb. 8, 2006 Journal of the American Medical Association
(Vol. 295, No. 6: 629-642). First author: Ross L. Prentice, PhD, Fred
Hutchinson Cancer Research Center.
"Low-Fat Dietary Pattern and Risk of Colorectal Cancer: The Women's
Health Initiative Randomized Controlled Dietary Modification Trial."
Published in the Feb. 8, 2006 Journal of the American Medical Association
(Vol. 295, No. 6: 643-654). First author: Shirley A. A. Beresford, PhD,
University of Washington.
"Low-Fat Dietary Pattern and Risk of Cardiovascular Disease: The
Women's Health Initiative Randomized Controlled Dietary Modification Trial."
Published in the Feb. 8, 2006 Journal of the American Medical Association
(Vol. 295, No. 6: 655-666). First author: Barbara V. Howard, PhD, Medstar
Research Institute.
Women’s Health Initiative, www.whi.org/findings/dm/dm.php
American Cancer Society; www.cancer.org
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