Dietary Fat & Cognitive Performance: Can a High-Fat Diet Be Bad for the Brain?

By Bronwyn Storoschuk, ND

As human life expectancy continues to increase, there is also an increased risk for cognitive impairment over the course of a longer life.¹ Brain health and cognitive performance have received a lot of recent attention by researchers in order to understand, and develop, strategies that will reduce the risk for cognitive decline.² Furthermore, greater importance is being placed on “healthspan” versus “lifespan,” and there is an increased demand to find ways to optimize overall health, including brain health and cognitive performance.

In the past few years, more scientific interest on the influence of nutrition on brain health and function has emerged, especially as dietary fats have regained popularity among consumers.² It has been well-documented that a ketogenic diet can have profound benefits on the brain and cognitive function; however, there is also evidence that suggests consuming a high-fat diet increases the risk of cognitive decline and may impair brain performance.^2,3 To clear some of the confusion, it is important to differentiate between the different types of fats and the potential mechanisms that may explain impairment in cognitive function.  

As far back as 1990, animal studies showed that diets high in saturated fats caused significant impairments in learning and memory.⁴ The results from subsequent human studies showed similar findings. Research showed that high-fat diets, containing mostly omega-6 fatty acids and saturated fats, were associated with worse performance on cognitive tasks.⁵ In addition, diets that contained mostly saturated fats and transfats have been associated with an increased risk of brain disorders.⁶ It has also been determined that high-fat diets with elevated amounts of saturated fats and cholesterol may impair intellectual function, along with increased risk for other health concerns.⁷ As most Americans follow a “Standard American Diet,” which contains high amounts of omega-6 fatty acids, saturated fats, and trans fats and low omega-3 fatty acids, it is not surprising that rates of cognitive decline are increasing in the US.^2,8

In the United States, the major sources of saturated fats come from:⁹

• Fatty cuts of meat
• Pastries and baked goods
• Cheese
• Milk
• Margarine
• Salad dressings
• Mayonnaise
• Butter

The major sources of trans fats, which are primarily industrially produced from vegetable oils, are found in:¹⁰

• Baked goods like cakes and cookies
• Crackers
• Margarine
• Fried foods
• Potato chips
• Popcorn  
• Shortening

Omega-6 fatty acids are found in many commonly used plant oils including canola, safflower, sunflower, soybean, corn, and cotton seed oil.¹¹ In the Standard American Diet the consumption of vegetable oils is significant; thus omega-6 fatty acid consumption has become much higher than omega-3 fatty acid intake.¹¹ The suggested dietary intake of omega-6:omega-3 ratio is around 1-4:1; however, most Americans consume these fats within the range of a 10:1 to 20:1 ratio.¹¹ Although omega-6 fatty acids are essential, meaning the human body requires them for good health but cannot synthesize them, and they must be eaten to provide benefits, once consumed, they are metabolized into arachidonic acid, which at excessive levels is proinflammatory.¹¹ In recent years, research has attempted to determine the biological mechanisms behind the detrimental cognitive effects associated with a high-fat diet. The major proposed mechanisms include insulin resistance, oxidative stress, and inflammation.²

Insulin resistance

Although insulin is usually discussed in relation to carbohydrate intake, consumption of both saturated and trans fats have been studied to impair insulin sensitivity.¹² In addition, data have shown diets high in saturated fats are associated with increased total body weight and abdominal obesity, which also contribute to insulin resistance.¹³ Overall, it has been found that cognitive performance declines as whole body insulin resistance increases.¹⁰

It is important to consider that the Standard American Diet is also comprised of large amounts of refined sugars and refined grains.² Increased consumption of refined carbohydrates also leads to insulin resistance, the greatest effects of which are seen when high sugar intake is combined with excessive caloric intake—often found in conjunction with a high-fat diet.¹⁴ So although specific fats can induce insulin resistance, this combination is more detrimental and very common in the US population.²

Oxidative stress

It has been observed that a high-fat diet, primarily composed of increased intakes of saturated fats and omega-6 fats, raises the levels of free radicals in tissues and the brain.^11,15 Free radicals, or reactive oxygen species (ROS), contribute to oxidative stress and lead to cellular damage.¹⁶ Chronically high levels of oxidative stress are known to lead to cognitive decline.¹⁶ Research has shown that high-fat diet-induced oxidative stress also leads to reduced levels of brain-derived neurotrophic factor (BDNF), which plays an important role in the survival, and growth, of brain cells and may explain some of the impairment in cognitive performance.^2,17 Interestingly, data from preclinical studies indicate vitamin E, a potent antioxidant, is associated with better cognitive performance.^18,19 While these findings still need to be confirmed in human studies, this information suggests that oxidative stress is involved in cognitive impairment and may be an outcome of a high-fat diet.²

Moreover, high-fat diets, specifically the fats included in the Standard American Diet, commonly lack essential vitamins, minerals, and antioxidants, which may further limit the body’s ability to effectively combat the increased levels of oxidative stress resulting from this high-fat diet.²⁰

Inflammation

Studies show high-fat diets composed primarily of saturated fats and omega-6 fatty acids have been associated with significantly increased levels of inflammation both systemically and in the brain.² The brain is very sensitive to levels of inflammation, as inflammatory mediators can easily cross the blood-brain barrier.² In one animal study, a diet comprised of 60% saturated fat showed significantly increased levels of inflammatory mediators, reduced levels of brain-derived neurotrophic factors, and highly reactive cells in the brain. As inflammatory mediators increased, significant impairment in cognitive performance was observed.²¹

Fats & cognition  

It is clear that all fats are not created equally. For instance, a diet that is rich in omega-3 fatty acids has been found to support cognitive processes.¹¹ Accordingly, diets high in omega-3 fatty acids are associated with enhanced memory and learning and may play a role in supporting healthy cognition.^24-25 The most important omega-3 fatty acids for brain health are EPA and DHA.²⁶ However, it can be challenging to get the appropriate intake of EPA and DHA by diet alone, especially when looking to enhance cognitive performance.²⁶ Also, it is important to note that a low intake of total fat, less than 20% of caloric intake, has been studied to impair cognitive performance due to an inadequate intake of fat-soluble vitamins and essential fatty acids, all of which are necessary to support cognition and general health.¹⁰

Regardless of what diet is followed, when fat is consumed, it is very important to choose the right fats. Brain function is impacted by insulin resistance and is sensitive to oxidative stress and inflammation, all of which are increased on a high-fat diet.² However, this does not mean that all types of fats are bad, as it is well-documented that omega-3 fatty acids support  cognition, and fat, in general, is required for optimal brain health.²⁴

This content is not intended as a substitute for professional medical advice, diagnosis, or treatment. Individuals should always consult with their healthcare professional for advice on medical issues.

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Bronwyn Storoschuk, ND Bronwyn Storoschuk, ND is a board-certified naturopathic doctor trained at the Canadian College of Naturopathic Medicine. Prior to attaining her ND, Dr. Storoschuk completed her Bachelor of Science (Honours) in Kinesiology at Queen’s University in Kingston, Ontario. She currently works in private practice in Toronto, Ontario. One of her practices is located within an integrative fertility clinic, where she provides naturopathic care to individuals undergoing assisted reproductive technology (ART). Dr. Storoschuk integrates evidence-based medicine with the understanding of the body’s natural physiology and innate healing wisdom. She is passionate about empowering women to take control of their hormonal health and has a clinical focus in hormone balance, reproductive health, and fertility.

Dr. Storoschuk is a paid consultant and guest writer for Metagenics.