What do high-fat diets do to the brain?

By Naveed Saleh, MD, MS, for MDLinx
Published December 6, 2019

Key Takeaways

In recent years, there has been a paradigm shift in how we view fats. At one point, experts viewed fats as the enemy and endeavored to cut it from diets at all costs. Now, we understand that monounsaturated and polyunsaturated fats are “good fats” that promote overall health, and low-fat diets eliminate both bad and good fats. We also understand that trans fats are “bad fats,” and saturated fats are “OK fats” in terms of general health.

Effects of fats on the brain, however, veer from this general understanding. Although it’s true that the brain is about 60% fat and needs fatty acids for fuel, a high-fat diet—no matter which type of fat it consists of—may not only fail to benefit memory, learning, and other cognitive functions, but actually impair them, at least according to current research.

How diet is a factor

Evidence is mounting to support the role of lifestyle changes in maintaining cognitive function in the elderly. Based on the research, experts hypothesize that cognitive decline, which tends to occur between 45 and 70 years of age, may be the result of diet and other lifestyle variables, as well as genetic susceptibility.

Some researchers have shown that higher lipid levels are linked to poorer cognition. Although the negative effects of trans and saturated fats on cognition are evident in the research, the effects of monounsaturated fatty acids on cognition are unclear, with findings mixed. Nevertheless, polyunsaturated fats might boost cognition, according to some studies.

Based on preclinical studies, changes to the hippocampus could underlie impairments to learning and memory due to a high-fat diet. These effects include decreased hippocampal sensitivity to insulin, decreased neurogenesis, increased inflammation, decreased neuronal plasticity, and altered blood-brain barrier (BBB) permeability.

“Despite some intriguing evidence suggesting the importance of dietary fat type for cognitive health, there are inconsistencies, and even opposing effects, in the literature,” wrote the authors of a study published in Frontiers in Human Neuroscience.

Effects in the elderly

Most studies assessing the effects of diet on cognition involve elderly participants who are administered a mini-mental status exam (MMSE), as well as a food-frequency questionnaire, which is not an accurate measure of intake. Another limitation of these studies is that it’s unclear which type of fat or how much is consumed.

Any negative effects of a high-fat diet on cognition are likely exacerbated by various factors, including older age. According to the authors of a review article published in Nutritional Neuroscience:

“[A] number of factors have been proposed to cause high-fat diet-induced damage to the brain, especially with aging, including oxidative stress, insulin resistance, inflammation, and changes to vascularization/BBB integrity. The contribution of insulin resistance, essential fatty acid consumption, and oxidative stress may be coordinated with inflammatory and vascular alterations to cause overall changes in brain function with consumption of high-fat and high-glycemic index-type diets.”

Effects in younger people

In the low-powered study in Frontiers in Human Neuroscience, investigators claimed to gather reliable dietary data using 7-day diet diaries. They also assessed cognitive function via more sensitive tests for memory and learning, and collected robust physiologic measures.

According to the authors, “We found evidence that high intake of saturated and trans fats in particular, and possibly [monounsaturated fats], may adversely affect memory and learning even in relatively young women.”

Importantly, the authors point out that cognitive decline is hypothesized to take place during a period of between 20 and 30 years. So, chronic dietary exposure could start affecting the cognition of young adults in their 20s.

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