New research from Uppsala University shows that saturated
fat builds more fat and less muscle than polyunsaturated fat. This is the first
study on humans to show that the fat composition of food not only influences
cholesterol levels in the blood and the risk of cardiovascular disease but also
determines where the fat will be stored in the body. The findings have recently
been published in the American journal Diabetes.
The study involved 39 young adult men and women of normal
weight, who ate 750 extra calories per day for seven weeks. The goal was for
them to gain three per cent of their starting weight. The project received
considerable attention when it started in 2011, partly because the extra
calories were ingested in the form of muffins with high fat content, baked in
the lab by Fredrik Rosqvist, a doctoral candidate and first author of the
study.
One half of the subjects were random to eat surplus calories
from polyunsaturated fat (sunflower oil), while the other half got their
surplus calories from saturated fat (palm oil). Both diets contained the same
amount of sugar, carbohydrates, fat, and protein; the only difference between
muffins was the type of fat.
The increase in body fat and the distribution of fat in the
body was measured using a magnetic resonance imaging (MRI scans) before and
after the weight gain, as was the muscle mass in the body. Gene activity was
measured in the abdominal visceral fat before and after the weight gain with
the help of a gene chip that studies several thousand genes at a time.
Despite comparable weight gains between the two diet groups,
the surplus consumption of saturated fat caused a markedly greater increase in
the amount of fat in the liver and abdomen (especially the fat surrounding the
internal organs, visceral fat) in comparison with the surplus consumption of
polyunsaturated fat. Moreover the total amount of body fat was greater in the
saturated fat group, while, on the other hand, the increase in muscle mass was
three times less for those who ate saturated fat compared with those who ate
polyunsaturated fat. Thus, gaining weight on excess calories from
polyunsaturated fat caused more gain in muscle mass, and less body fat than
overeating a similar amount of saturated fat. Since most of us are in positive
energy balance, and consequently gain weight slowly but gradually over time,
the present results are highly relevant for most Western populations.
"Liver fat and visceral fat seems to contribute to a
number of disturbances in metabolism. These findings can therefore be important
for individuals with metabolic diseases such as diabetes. If the results
regarding increased muscle mass following consumption of polyunsaturated fat
can be confirmed in our coming studies, it will potentially be interesting for
many elderly people, for whom maintaining muscle mass is of great importance in
preventing morbidity," says Ulf Risérus, associate professor at the
Department of Public Health and Caring Science and director of the study.
When it comes to the risk of developing diabetes and
cardiovascular diseases, it seems more important where in the body the fat is
stored than how much fat the body has. Visceral fat, along with a high
proportion of fat in the liver, is closely associated with increased risk for
developing type-2 diabetes. These fat depots are therefore important targets
for new drugs and dietary strategies. A number of studies have indicated that a
higher intake of polyunsaturated fats from plant oils and nuts is associated
with a decreased risk of type-2 diabetes, but the reasons for this remain
unclear.
The present study proposes a potential explanation for such
an association, showing that polyunsaturated fatty acids can affect fat
distribution in the body more favorably than saturated fats, probably by
regulating increased energy combustion or decreased storage of visceral fat in
connection with calorie-rich diets.
The researchers were also able to see that over-consumption
of saturated fats seems to be able to "turn on" certain genes in
fatty tissue that increase the storage of fat in the abdomen and at the same
time hamper insulin regulation. Polyunsaturated fats, instead, can "turn
on" genes in visceral fat that in turn are linked to reduced storage of
fat and improved sugar metabolism in the body. However, more research is
required to understand how this occurs in humans.
The discovery may also be a contributing factor regarding
the tendency of some individuals to accumulate fat in the liver and abdomen.
The new findings suggest that the fat composition of the diet, in the long
term, might play a role in preventing obesity-related disorders, like type-2
diabetes, at an early stage, before overweight develops.
"This is of great interest, as we lack preventive
treatments for fatty liver and visceral fat today. The new findings also
support international dietary recommendations including the new Nordic
nutritional recommendations, which, among other things, recommend replacing
some saturated fat from meat, butter, and palm oil, for example, with
unsaturated fats from plant oils and fatty fish," says Ulf Risérus.
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