The fat composition of food not only influences cholesterol levels and the risk of cardiovascular disease but also governs where the fat will be stored in the body, say researchers.
The human study investigated how overfeeding people with saturated or polyunsaturated fats affects fat accumulation and body composition – finding that the overfeeding of these fats has distinctly different effects on liver and visceral fat accumulation.
Led by Ulf Risérus from Uppsala University, Sweden, the study involved feeding healthy adults an additional 750 calories per day in the form of muffins that were made with either polyunsaturated fat (sunflower oil) or 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, said the researchers.
Writing in Diabetes, the Swedish team found those consuming the saturated fat muffins accumulated more abdominal fat and less muscle than polyunsaturated fat, which was linked to an increased accumulation of lean tissue.
“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,” said Risérus.
“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."
Risérus and his team recruited 39 young adult men and women of normal weight to the study. The participants ate 750 extra calories per day for seven weeks – all of which came in the form of the muffins made with either polyunsaturated or saturated fat.
The team measured body fat and the distribution of fat in the body using magnetic resonance imaging (MRI scans) before and after the seven week weight gain period, in addition to monitoring lean muscle.
Gene activity was also measured in the abdominal visceral fat before and after the seven week study with the help of a gene chip that studies several thousand genes at a time, said the authors.
Despite comparable weight gain in the two groups, Risérus and his colleagues found the surplus consumption of saturated fat caused a markedly greater increase in the amount of fat in the liver and abdomen when compared to those consuming polyunsaturated fat.
Total body fat was also found to be greater in the saturated fat group, while increases in muscle mass were three times less in participants who ate saturated fat when compared with those consumed polyunsaturated fat.
Risérus and his team therefore concluded that gaining weight via excess calories from polyunsaturated fat caused more gain in muscle mass, and less body fat than over eating a similar amount of saturated fat.
They added that since most of people are in positive energy balance, and consequently gain weight slowly but gradually over time, the results are ‘highly relevant’ for most Western populations.
The team’s data also hinted at a potential explanation for these associations, showing that polyunsaturated fatty acids can affect fat distribution in the body more favourably than saturated fats - probably by regulating increased energy burning and decreased storage of visceral fat.
They were also able to see that over-consumption of saturated fats turned on certain genes in fatty (adipose) tissue, which in turn increased the storage of fat in the abdomen and interfered with insulin regulation.
In contrast, polyunsaturated fats were found to up-regulate genes that are linked to reduced storage of fat and improved sugar metabolism in the body.
Published online ahead of print, doi: 10.2337/db13-1622
“Overfeeding Polyunsaturated and Saturated Fat Causes Distinct Effects on Liver and Visceral Fat Accumulation in Humans”
Authors: Fredrik Rosqvist, David Iggman, et al