Photo of fried onion rings via Kaba/Flickr.
Dietary fat is having a moment. From Paleo diet converts who hail the virtues of saturated fat to nutritionists who want to do away with upper limits on total fat consumption, Americans are getting a strong message: Fill up on fat and stay away from refined carbohydrates and added sugar.
Here to throw a wrench into this fashionable mode of thought is a small but illuminating rodent experiment presented on July 10 at the Society for the Study of Ingestive Behavior in Denver. Neuroscientist Krzysztof Czaja of the University of Georgia College of Veterinary Medicine made the case that a high-fat diet changes the composition of gut bacteria in the body, causing the release of toxins that damage the connection between the brain and the gut. The result? Inflammation in the parts of the brain that are in charge of making rodents feel full, which made them overeat and become obese.
The mechanisms that Czaja was able to demonstrate in his series of experiments may offer an intriguing explanation for why human beings who successfully lose weight often gain it back when they try to moderate their restrictive diets.
The Background
The stomach signals to the brain when it is either hungry or full via the vagus nerve. Among many other functions, this nerve carries chemical signals from the stomach to the brain once we've eaten enough. For people who struggle with weight control, the question of satiety isn't quite so straightforward. Those who overeat and gain weight don't seem to get the same satiety signal as those of a normal weight and, so far, scientists aren't quite sure why. Czaja was able to demonstrate at least one possible explanation in a series of experiments conducted on 50 rodents.
The Setup
For two weeks, he fed them all the same balanced, healthy chow that all lab rats need to stay at a normal weight. Then he took half of them and fed them high-fat rat food, made with a mixture of saturated fats and trans fats, like the fats that are found in our highly processed foods such as fast food, frozen pizza and pastries made with vegetable shortening.
The Findings
No surprise: After two months, the rats on the high-fat diet were obese. But in an analysis of their gut bacteria, Czaja also saw that a certain bacteria that produced neurotoxins had overpopulated in the rats who ate the high-fat diet, while the rats eating normal chow had a more balanced microbiome.
In the final stage of the experiment, Czaja dissected the rats' brains to see how the connection between the vagus nerve and the rat’s satiety centers fared. In rats with high-fat diets, the parts of the brain responsible for feelings of fullness had inflammation damage. He also saw that the brain circuitry was wired differently than the rats who ate the normal food.
The Takeaway
Of course, rats aren’t humans. But rats often stand in for human subjects because their bodies function similarly. These similarities are why Czaja hypothesizes that his experiments indicate a mechanism by which overweight and obese people might have inflammation damage in parts of their brain that handle satiety cues, which leads to an inability to keep weight off long-term.
“The communication is broken,” he explained. “It’s like running a car with partially functioning brakes.”
Czaja said that his research doesn't suggest people should avoid all fat as a rule — only that if you were to indulge in, say, fried chicken or a cinnamon roll, it would be best to follow up with vegetables and other healthy foods.
“What I’m trying to say is, don’t eat only fried foods,” he added. "The composition of bacteria in our gut is huge, and delivering balanced nutrients will help to maintain a healthy micro biome in the gut.”
His next steps are to prepare his findings for publication (note that conference presentations are not yet peer-reviewed) and, in further research, to examine if the damage to the vagus nerve and brain centers can be reversed with diet.
The Catch
While trans fats are already eschewed for their role in contributing to cardiovascular disease, the research on saturated fat is more nuanced. In their 2015 report, the Dietary Guidelines Advisory Committee (a group of independent nutrition experts tasked with advising the federal government on food policy) said that replacing saturated fats with unsaturated fats would help someone lower their total and LDL cholesterol levels, which in turn lowers their risk of cardiovascular disesase. However, they also cautioned that people shouldn't replace total fat with carbohydrates (i.e., go on a low-fat diet), as this does not help lower disease risk and in fact might cause other types of dysfunction.
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