| The research, funded by
the British Heart Foundation and published in the FASEB Journal,
may have implications not only for those eating lots of high-fat foods,
but also athletes looking for the optimal diet for training and patients
with metabolic disorders.
'We found that rats, when switched to a high-fat diet
from their standard low-fat feed, showed a surprisingly quick reduction
in their physical performance,' says Dr Andrew Murray, who led the work
at Oxford University and has now moved to the University of Cambridge.
'After just nine days, they were only able to run 50 per cent as far on
a treadmill as those that remained on the low-fat feed.'
High-fat diets, such as those that are prevalent in
Western countries, are known to be harmful in the long term and can lead
to problems such as obesity, diabetes and heart failure. They are also
known to be associated with a decline in cognitive ability over long
time spans. But little attention has been paid to the effect of high-fat
diets in the short term.
Physical endurance – how long we can keep exercising
–depends on how much oxygen can be supplied to our muscles and how
efficiently our muscles release energy by burning up the fuel we get
from the food we eat. In particular, using fat as a fuel is less
efficient than using glucose from carbohydrates, but the metabolic
changes induced by different diets are complex and it has been
controversial whether high-fat feeding for a short time would increase
or decrease physical performance.
The Oxford team set out to investigate whether rats
fed a high-fat diet for just a few days showed any change in their
physical and cognitive abilities.
All 42 rats were initially fed a standard feed with a
low fat content of 7.5 per cent. Their physical endurance was measured
by how long they could run on a treadmill and their short-term or
'working' memory was measured in a maze task. Half of the rats were then
switched to a high-fat diet where 55 per cent of the calories came from
fat. After four days of getting used to the new diet, the endurance and
cognitive performance of the rats on the low- and high-fat diets was
compared for another five days.
'With the standard feed, 7.5 per cent of the calories
come from fat. That's a pretty low-fat diet, much like humans eating
nothing but muesli,' says Dr Murray. 'The high-fat diet, in which 55 per
cent of the calories came from fat, sounds high but it's actually not
extraordinarily high by human standards. A junk food diet would come
close to that.
'Some high-fat, low-carb diets for weight loss can
even have fat contents as high as 60 per cent. However, it's not clear
how many direct conclusions can be drawn from our work for these diets,
as the high-fat diet we used was not particularly low in carbs,' he
adds.
On the fifth day of the high-fat diet (the first day
back on the treadmill), the rats were already running 30 per cent less
far than those remaining on the low-fat diet. By the ninth day, the last
of the experiment, they were running 50 per cent less far.
The rats on the high-fat diet were also making
mistakes sooner in the maze task, suggesting that their cognitive
abilities were also being affected by their diet. The number of correct
decisions before making a mistake dropped from over six to an average of
5 to 5.5.
The researchers also investigated what metabolic
changes the high-fat diet was inducing in the rats. They found increased
levels of a specific protein called the 'uncoupling protein' in the
muscle and heart cells of rats on the high-fat diet. This protein
'uncouples' the process of burning food stuffs for energy in the cells,
reducing the efficiency of the heart and muscles. This could at least
partly explain the reduction in treadmill running seen in the rats.
The rats that were fed a high fat diet and had to run
on the treadmill also had a significantly bigger heart after nine days,
suggesting the heart had to increase in size to pump more blood around
the body and get more oxygen to the muscles.
While this research has been done in rats, the Oxford
team and Andrew Murray's new group in Cambridge are now carrying out
similar studies in humans, looking at the effect of a short term
high-fat diet on exercise and cognitive ability.
The results will be important not only in informing
athletes of the best diets to help their training routine, but also in
developing ideal diets for patients with metabolic disorders such as
diabetes, insulin resistance or obesity. People with such conditions can
have high levels of fat in the blood and show poor exercise tolerance,
some cognitive decline, and can even develop dementia over time.
'These are startling results,' says Professor Kieran
Clarke, head of the research team at Oxford University. 'It shows that
high-fat feeding even over short periods of time can markedly affect
gene expression, metabolism and physical performance. By optimising
diets appropriately we should be able to increase athletes' endurance
and help patients with metabolic abnormalities improve their ability to
exercise and do more.'
'In little more than a week, a change in diet appears
to have made the rats' hearts much less efficient,' says Professor
Jeremy Pearson, Associate Medical Director of the British Heart
Foundation, who funded the research. 'We look forward to the results of
the equivalent studies in human volunteers, which should tell us more
about the short-term effects of high-fat foods on our hearts. We already
know that to protect our heart health in the long-term, we should cut
down on foods high in saturated fat.'
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