Food laden with saturated fat does not drive up levels of
the 'junk' nutrient in the blood, contrary to popular opinion, research
suggests.
Higher consumption of carbohydrates, not saturated fat, was
associated with changes linked to diabetes and heart disease, scientists found.
The discovery turns on its head the widely-held view of the
harmful effects of eating too much saturated fat.
Butter, cheese, fatty cuts of meat, processed meat products
such as sausages and bacon, cakes and biscuits are all examples of 'unhealthy'
foods high in saturated fat.
Starchy carbohydrates such as potatoes, bread, cereals, rice
and pasta are considered important for health and should make up about a third
of the diet, according to expert advice.
US lead scientist Professor Jeff Volek, from Ohio State
University, said: 'There is a widespread misunderstanding about saturated fat.
'In population studies, there is clearly no association of
dietary saturated fat and heart disease, yet dietary guidelines continue to
advocate restriction of saturated fat.
'That's not scientific and not smart. But studies measuring
saturated fat in the blood and risk for heart disease show there is an
association.
'Having a lot of saturated fat in your body is not a good
thing. The question is, what causes people to store more saturated fat in their
blood, or membranes, or tissues?'
Prof Volek's team set out to answer this question by
studying 16 adults, all of whom had metabolic syndrome - a condition marked by
at least three risk factors that increase the chances of developing heart
disease or diabetes.
All the participants were fed the same diet, which changed
every three weeks for a total of 18 weeks as levels of carbohydrates were
progressively increased and those of saturated fat reduced.
The diets started with 47 grams of carbohydrates and 84
grams of saturated fat per day, and ended with 346 grams of carbs and 32 grams
of saturated fat.
Prior to the series of diets, everyone taking part in the
study was put on the same reduced-carb 'baseline' diet for three weeks.
Each day's meals provided the same energy intake, adding up
to 2,500 calories, and included about 130 grams of protein.
The highest carb level represented 55% of daily calories,
roughly matching the daily percentage of energy supplied by carbohydrates in
the average American diet.
By the end of the trial, participants had lost almost 22
pounds on average and experienced significant improvements in blood sugar and
insulin levels, and blood pressure, that were similar across all the diets.
Throughout the study, blood levels of total saturated fat in
the blood remained relatively stable in all the volunteers, and even fell at
the start when baseline fat consumption was doubled, the research showed.
In contrast, blood levels of palmitoleic acid - a fatty acid
associated with an unhealthy effect on carbohydrates that can promote disease -
went up as carbohydrate intake increased and fat consumption fell.
Higher amounts of palmitoleic acid in the blood indicate
that more carbohydrates are being converted to fat instead of being burned as
fuel, Prof Volek pointed out.
He said: 'It's unusual for a marker to track so closely with
carbohydrate intake, making this a unique and clinically significant finding.
As you increase carbs, this marker predictably goes up.'
Reducing carbohydrate consumption and adding controlled
amounts of fat to the diet ensured that the body burned saturated fat rather
than storing it, he added.
'When you consume a very low-carb diet your body preferentially
burns saturated fat,' Prof Volek said.
'We had people eat two times more saturated fat than they
had been eating before entering the study, yet when we measured saturated fat
in their blood, it went down in the majority of people. Other traditional risk
markers improved, as well.'
The findings are published in the online journal Public
Library of Science ONE.
Although blood levels of palmitoleic acid increased in all
participants as carbohydrate intake was raised, the amounts differed greatly
between individuals.
This was consistent with the idea that people's carbohydrate
tolerance varied widely, said Prof Volek.
He added: 'People believe 'you are what you eat,' but, in
reality, you are what you save from what you eat.
'The point is you don't necessarily save the saturated fat
that you eat. And the primary regulator of what you save in terms of fat is the
carbohydrate in your diet.'
Palmitoleic acid could provide a potential way of tracking
when the body is converting carbs to fat, thereby contributing to 'metabolic
mayhem', he believed.
'There is no magical carb level, no cookie-cutter approach
to diet, that works for everyone,' said Prof Volek.
'There's a lot of interest in personalised nutrition and
using a dynamically changing biomarker could provide some index as to how the
body is processing carbohydrates.'
ends
Raised levels of palmitoleic acid in the blood or fat tissue
are known to be associated with 'a myriad of undesirable outcomes', said the
researchers in their paper.
These included elevated triglycerides - a harmful type of
blood fat - high blood sugar, inflammation, metabolic syndrome, Type-2
diabetes, heart disease, heart failure and aggressive prostate cancer.
Victoria Taylor, senior dietician at the British Heart Foundation,
said: 'Only 16 people took part in this short term study which is not enough to
be conclusive. More research is needed to understand the effects of different
amounts of types of carbohydrates on our risk of coronary heart disease.
'In the UK it is recommended that saturated fat is reduced
and instead we should be switching to getting fats from unsaturated sources,
for example olive, rapeseed or sunflower oils, oily fish and nuts.
'This is because eating too much saturated fat is linked to
an increase in LDL (low-density lipoprotein) cholesterol levels, a known risk
factor for coronary heart disease.
'It's important to eat a healthy balanced diet and not focus
on individual nutrients or foods. The Mediterranean style diet includes plenty
of fruits and vegetables, pulses, fish, nuts and seeds and has been shown to be
beneficial in preventing coronary heart disease.'
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