Health: The Myth of the Low-fat Diet ; For Years, We've Been Advised to Eat a
Low-fat Diet in Order to Help Prevent Heart Attacks And Promote Weight Loss.
But, Says Jerome Burne, the Latest Research Suggests That Such a Diet May
Actually Do More Harm Than Good
By JEROME BURNE
Looking for something healthy and non-fattening for your evening meal?
How about a nice porterhouse steak, which is 50-50 fat and protein? It's a
suggestion that comes about as close to heresy as we get these days, but there
is increasing evidence that a low-fat diet is not the panacea we have been
promised. For the past 30 years such a diet has been officially promoted,
on both sides of the Atlantic, as the route to plaque-free arteries and a slim
figure. A message that has propelled 15,000 low-fat products on to
American supermarket shelves.
However, the campaign has had no obvious effect on the incidence of heart
disease, nor have the pounds been falling off the national waists and hips.
In fact, according to a recent report, we are getting fatter. Not only is
a low-fat diet largely irrelevant to reducing heart disease but it may be
responsible for the worrying rise of diabetes.
Praise for the fat-laden porterhouse steak came in an award-winning
investigative article, published last year, on just how little evidence there is
supporting the low-fat dogma (Science, 30 March 2001). Virtually ignored in the
UK at the time, it should be required reading for anyone interested in diet. The
problem with the low-fat message is that it is far too simple.
For instance, we've all been told to avoid animal fats because they are
saturated and that saturated fat raises cholesterol levels in the blood. But
half the fat in a steak is actually "monounsaturated" - the same type
as found in "good for the heart" olive oil. The other half is, indeed,
saturated but about a third of it is a type called stearic acid, which, like
olive oil, raises the "good" HDL cholesterol in the blood. So just 30
per cent of the fat in a steak is the sort of saturated fat that can raise
"bad" LDL cholesterol. However, even this demonised fat will
simultaneously raise the "good" HDL. "All of this suggests,"
writes science journalist Gary Taubes, author of the Science article, "that
eating a porterhouse steak rather than carbohydrates might actually improve
heart disease risk".
The recommendation that dietary fat be reduced to 30 per cent of the total
calorie intake is contained in a 1976 Senate report. Written by a journalist,
who had only previously reported on labor relations, it drew on just two days
of testimony, most from an eccentric Harvard nutritionist Mark Hegstead, who
regarded dietary fat as the nutritional equivalent of cigarettes.
That would not have mattered had the evidence come in to support his
recommendation - but, beyond a certain point, it hasn't. Undoubtedly if you are
at high risk of having a heart attack - overweight, high blood pressure, no
exercise, etcetera - and you have very high levels of cholesterol, then reducing
them with diet or drugs can significantly reduce your chance of a heart attack.
What has not been shown convincingly, however, is that someone who is not at
risk will have their life cut short as a result of regularly eating more than
the recommended level of dietary fat. As Taubes reports in his article, at least
four large trials between 1980 and 1984 comparing disease rates and diet
"showed no evidence that men who ate less fat lived longer or had fewer
Since the early Seventies Americans' fat consumption has dropped from an
average of 40 per cent of the diet to 34 per cent, but the incidence of heart
disease hasn't fallen too. In fact, between 1979 and 1996, largely reflecting
the range of new developments, the number of medical procedures for heart
disease increased from 1.2 million to 5.4 million. At the same time the
proportion of obese Americans has soared from 14 per cent to 22 per cent.
A low-fat diet may be actively harmful. In the late Eighties, David Jacobs,
from the University of Minnesota, did a study in Japan on the effects of
cholesterol and, interestingly, found a link between low blood cholesterol
levels and an increase in non-heart disease related deaths. He reported to the
National Heart, Lung and Blood Institute, which hosted the American Heart
Association conference in 1990. At that conference the results of 19 studies
from around the world on the links between cholesterol levels and disease were
pooled. Taubes writes: "The data were consistent. When investigators
tracked all deaths instead of just heart disease, the cholesterol curves were
U-shaped for men (both high and low increased the risk) and flat for
women." He adds: "As for women, if anything, the higher their
cholesterol the longer they lived."
Meanwhile, the link between low-fat diets and weight loss hasn't fared well
either. The ongoing Women's Health Initiative - a $100m study on women's health
- enrolled 50,000 women in a randomized trial, putting half of them on a
draconian diet that provided only 20 per cent of their calories from fat. After
three years they had lost, on average, just one kilogram.
Critics of the low-fat hypothesis, such as Peter Ahrens of Rockefeller University in New York City, have always been concerned that simply lowering fat
intakes could have a range of unforeseen effects. Fat is a major component of
cell membranes, the brain is 70 per cent fat, and changing fat ratios could
affect all sorts of processes, from immune responses to hormone levels.
Just how much else is involved in determining the effect of fat levels in the
diet was illustrated by the Lyons Diet Heart Study (16 February 1999). This
involved two groups of heart attack survivors, one getting a typical low-fat
diet and the other a Mediterranean diet with more bread, cereals, beans,
vegetables, olive oil, fruit and fish. The total amount of fat and the type of
fat type that each group ate were very different. Intriguingly, however, since
high fat is supposed directly to affect cholesterol, the cholesterol levels in
the blood of the two groups were very similar. After four years the
Mediterranean group had had 14 heart attacks, compared with 44 for those on the
"low-fat" diet. This suggests that reducing blood cholesterol is not
simply a matter of reducing dietary fat. What is crucial, it turns out, is the
type of fat and what you eat along with that fat.
A danger of the low-fat advice may be that it is encouraging us to eat too
much of the wrong sort of food. Given the chance, people tend to eat about the
same amount of calories, however varied their composition, and those who eat
lots of meat and dairy products, like the Finns or Americans, tend not to eat
lots of vegetables and fruits. So if you reduce fat, it is likely to be replaced
with refined carbohydrates, and that seems to be the problem.
Troublingly, the evidence has been growing that diets high in carbohydrate
can increase the blood level of dangerous fats called triglycerides and reduce
the "good" or HDL cholesterol. Diets high in sugar and other
carbohydrates may also lead to a condition called insulin resistance - the extra
carbohydrates are turned into extra glucose, which makes the body produce extra
insulin and after a while the body becomes less sensitive to insulin. This
combination produces something that Stanford endocrinologist Gerald Reaven has
called "syndrome X" (New Scientist, 1 September 2001).
In the United States an estimated 30 per cent of males and 10 per cent to 15
per cent of post-menopausal women have insulin resistance, which commonly leads
to diabetes and is linked with a raised risk of heart disease. High-energy
snacks are one way to expose the liver to damaging levels of insulin, although
exercise can keep the level of harmful triglycerides down. Another element of
the high-carbohydrate diet that has been linked with syndrome X is a high
consumption of a type of sugar known as fructose. Fructose makes up half of
ordinary sugar but corn syrup, now used to sweeten a vast range of foods -
breakfast cereals, many low-fat snacks and fizzy drinks - is almost pure
fructose. Rather than a high-fat diet, a major contributor to our creeping
obesity epidemic could be increased consumption of carbohydrates, especially
those coming from sugars.
What's very interesting is that researchers who are concerned about syndrome
X come up with the same sort of dietary advice to avoid it as those who are
studying fats and heart disease. One again, olive oil, fish oils, plenty of
fruits and fresh vegetables and slow release carbohydrates like lentils, beans,
brown rice and oats are recommended as a way of avoiding insulin resistance, as
well as rendering saturated fats safe.
One reason for the survival of low fat as a recommended diet, which really
only seems relevant to people at risk of a heart attack due to high cholesterol,
is the difficulty of giving general dietary advice. Not only do fats and
cholesterol levels interact in a variety of complicated ways but so also do the
ways we lay down fat. The billions that a successful anti-fat pill would
generate, means huge amounts of research effort are being put into research to
uncover the multiple pathways controlling appetite and fat storage. The
complexity of this emerging system suggests why the simple low- fat mantra has
proved so unsuccessful as a weight loss program, too.
A glimpse of just how complicated this system is came from a study by
researchers at Rockefeller University, using an extraordinary technique to
trace the brain areas involved in appetite. A pseudo-rabies virus, which infects
linked brain cells, genetically modified to produce green glowing jellyfish
protein, was injected into rats' brains. Most hunger research concentrates on
the appetite center in the hypothalamus but tracing the green lines left by the
virus revealed that brain centers that control the emotions, smell and the
higher centers had all been infected. Our appetite seems very much a part of who
A similar program to begin to tease out what is involved in the
relationship between dietary fats and cholesterol should eventually yield more
sophisticated and useful advice than the low-fat diet.
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January 28, 2013