Considering cutting on fat this year? A new mice study from UC Riverside researchers suggests one more reason to do so: a high-fat diet may harm the immune system and brain function.
In the study that appeared in Nature Scientific Reports, UCR researchers fed mice three different diets for 24 weeks, where 40% of the calories came from fat.
One group of mice ate a diet based on saturated fat from coconut oil, the type of fat that is associated with increased "bad" cholesterol levels and, as a result, a higher risk of heart disease and stroke.
Another group consumed a monounsaturated, modified soybean oil, and the rest of the mice were fed with an unmodified soybean oil high in polyunsaturated fat. Both types are considered "the good" fats when consumed in moderation.
After 24 weeks, the researchers looked at the microbiome and genetic changes in all four parts of the mice intestines.
They found that all three mice groups on a high-fat diet experienced changes in gene expression, the process that turns genetic information into a function.
As expected, high-fat diets led to intestinal changes, such as in genes related to fat metabolism and the composition of gut bacteria. For instance, the mice experienced an increase in E. coli and the suppression of Bacteroides that provide protection from pathogens and supply nutrients to other gut microorganisms.
Mice fed on high fat also experienced changes in genes regulating susceptibility to infectious diseases.
"We saw pattern recognition genes, ones that recognize infectious bacteria, take a hit. We saw cytokine signaling genes take a hit, which help the body control inflammation. So, it's a double whammy. These diets impair immune system genes in the host, and they also create an environment in which harmful gut bacteria can thrive," said Frances Sladek, a UCR cell biology professor and senior author of the new study, in a statement.
The mice on high-fat diets also experienced changes in the expression of several neurotransmitter genes, reinforcing the notion that diet can impact a gut-brain axis.
Increasing research on the bilateral communication between the gut and the brain has linked imbalanced gut microbiota — microorganisms living in the gut — to mood disorders and neurodegenerative diseases, such as Alzheimer's and Parkinson's.
The study authors emphasize that although plant-based diets are generally beneficial for health, this may not apply to plant-based diets high in fat.
The dangers of soybean oil
A diet high in coconut oil caused the most changes in gene expression, followed by unmodified soybean oil. The differences between the two types of soybean oil suggest that polyunsaturated fatty acids, primarily linoleic acid, in unmodified soybean oil, can play a role in altering gene expression.
In this study, negative effects on the microbiome were more pronounced in mice fed the soybean oil diet. Previous research by the same team has suggested that soybean oil induces obesity, diabetes, insulin resistance, and fatty liver in mice.
Moreover, high consumption of soybean oil was found to affect genes in the brain related to conditions like autism, Alzheimer's disease, anxiety, and depression.
The researchers note that the risks are only associated with soybean oil but not other and not to other soy products, tofu, or soybeans themselves. Nevertheless, the findings are concerning, as soybean oil is the most commonly consumed oil in the United States.
However, the study was conducted on mice, and the findings in mice studies do not necessarily translate to humans despite sharing 97.5% of their working DNA.
What are healthier fat options?
The Dietary Guidelines for Americans recommends that fat intake should not exceed 20-35% of total daily calories, with limiting saturated fat intake to 10% of total daily caloric intake.
Saturated fats come from fatty meats like beef, lamb, and pork, as well as dairy products, including butter, cheese, and ice cream.
Another common source of this type of fat is tropical oils, such as coconut, palm, and palm kernel oils, and some baked and fried foods.
The Centers for Disease Control and Prevention (CDC) recommends replacing saturated fats with healthier alternatives — lean meats, seafood, and fat-free or low-fat milk, cheese, avocados, nuts, and seeds.
When cooking, consider substituting meats with beans as the source of protein and use oils higher in polyunsaturated and monounsaturated fat, such as canola, corn, olive, peanut, safflower, soybean, and sunflower oil.
More research is needed to determine whether high-fat diets can harm humans' immune system and brain function. At the same time, the link between diets high in fats and conditions like obesity and coronary heart disease is already well established, emphasizing the need to look for healthier fat alternatives.
- Nature Scientific Reports. Impact of various high fat diets on gene expression and the microbiome across the mouse intestines.
- UC Riverside. New reasons eating less fat should be one of your resolutions.
- American Heart Association. Saturated Fat.
- CDC. Prevent High Cholesterol.
- U.S. Department of Human and Health Services. Dietary Guidelines for Americans 2020 - 2025.