Dietary advice about fats and the risk of heart
disease is called into question as a clinical trial
shows that replacing saturated animal fats with
omega-6 polyunsaturated vegetable fats is linked to
an increased risk of death among patients with heart
disease.
Oil extracted from safflower seeds are used
mainly in margarine and cooking oil. Image source:
USDA.
The researchers say their findings could have
important implications for worldwide dietary
recommendations.
Advice to substitute vegetable oils rich in
polyunsaturated fatty acids (PUFAs) for animal fats
rich in saturated fats to help reduce the risk of
heart disease has been a cornerstone of dietary
guidelines for the past half century. The most
common dietary PUFA in Western diets is omega-6
linoleic acid; concentrated sources of these fats
are safflower oil, corn oil, soybean oil, sunflower
oil, and cottonseed oil.
UK dietary recommendations are cautious about high
intakes of omega 6 PUFAs, but some other health
authorities, including the American Heart
Association, have recently repeated advice to
maintain, and even to increase, intake of omega-6
PUFAs. This has caused some controversy, because
evidence that linoleic acid lowers the risk of
cardiovascular disease is limited.
An in-depth analysis of the effects of linoleic acid
on deaths from coronary heart disease and
cardiovascular disease has not previously been
possible because data from the Sydney Diet Heart
Study - a randomised controlled trial conducted from
1966 to 1973 - was missing.
But now, a team of researchers from the US and
Australia have recovered and analysed the original
data from this trial, using modern statistical
methods to compare death rates from all causes,
cardiovascular, and coronary heart disease.
Their analysis involved 458 men aged 30-59 years who
had recently had a coronary event, such as a heart
attack or an episode of angina.
Participants were randomly divided into two groups.
The intervention group was instructed to reduce
saturated fats (from animal fats, common margarines
and shortenings) to less than 10% of energy intake
and to increase linoleic acid (from safflower oil
and safflower oil polyunsaturated margarine) to 15%
of energy intake.
Safflower oil, a concentrated source of omega-6
linoleic acid, provides no omega-3 PUFAs, typically
associated with fish oils. The control group
received no specific dietary advice.
Both groups had regular assessments and completed
food diaries for an average of 39 months. All
non-dietary aspects of the study were designed to be
equal in both groups.
The results show that the omega-6 linoleic acid
group had a higher risk of death from all causes, as
well as from cardiovascular disease and coronary
heart disease, compared with the control group.
The authors then used the new data to update an
earlier meta-analysis (a review of all the
evidence). This also showed no evidence of benefit,
and suggested a possible increased risk of
cardiovascular disease, emphasizing the need to
rethink mechanisms linking diet to heart disease.
The current best estimate is that half of all the
clinical trials that are conducted and completed are
never published. Even when they are, the underlying
data that the results are based on is rarely open to
external analysis - which is a cornerstone of proper
scientific scrutiny.
This means doctors cannot be certain that the drugs
they are prescribing daily are properly evaluated
for safety and efficacy.
For more information
New look at old study casts doubt on value of
vegetable fats for heart health
(MDN)
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