It seems like every day now I see a magazine, blog, or advertisement for a product or menu item touting the benefits of eating lots of “healthy fats,” avoiding carbohydrates (even in the form of whole foods like potatoes, beans, and fruit), and boosting cholesterol levels with “clean protein” sources like egg yolks and grass-fed beef. The popularity of fruit juice “cleansing” has faded, as it’s now more in vogue to do a ketogenic juice “cleanse,” feast on “fat bombs,” and drink buttered coffee for breakfast in lieu of calories that can be chewed.

One of the claimed benefits of a ketogenic or high fat, low-carb diet is a boost in brain function. The reasoning goes that since the brain is primarily made of fat and cholesterol, eating more fat and cholesterol will help improve its functioning. Furthermore, it’s often said that fat provides the brain with a clean source of fuel, unlike sugar, which is said to be toxic (especially if it comes from grains and sweet fruits).

This belief system is really coming from a misunderstanding of how the brain works, and a confusion between ketones and fats as fuel sources for body and brain. Many of the people who claim a high fat diet is good for the brain are well-intentioned, while others are simply just trying to make money riding the wave of the current popularity of ketogenic and high fat, low carb diets. What these people have in common is that they’re just parroting a claim they heard from somebody they thought was reputable, without seeking out any primary sources (peer-reviewed, published research) to verify whether this claim is actually true.

If you want to build a better brain, prevent age-related cognitive decline and dementia, and maybe even boost your IQ, keep reading this. I’m going to give you a brief overview on what the science actually says about this topic. You can click on any words highlighted in blue in order to read the primary research yourself and verify that I’m not making any of this stuff up. This is not about my own personal beliefs. This is about an evidence-based approach to brain health.

The brain’s primary source of fuel is glucose (sugar), and the evidence is clear that hypoglycemia (blood glucose <70 mg/dL) and reduced glucose metabolism in the brain precedes brain conditions such as depression and Alzheimer’s disease. Chronic hypoglycemia (which is common when consuming a ketogenic diet) and recurrent episodes of hypoglycemia (such as what occurs when people living with type 1 and type 2 diabetes  take too much insulin or glucose-lowering medications) cause the brain to switch to its emergency fuel source — ketones. Eventually the brain becomes less efficient at using glucose for fuel, and this is referred to as “glucose hypometabolism” in the scientific literature.

When the brain has “forgotten” how to use glucose for fuel, it has to continue using its emergency fuel source — ketones. (Basically, this condition is often referred to as insulin resistance of the brain or type 3 diabetes.) There is promising research around the use of intermittent fasting and calorie restricted diets combined with the use of exogenous ketones (ketone supplements), or the use of a ketogenic diet with supplemental medium chain triglycerides (MCTs) to help supply the brain with an emergency fuel source that it can use if it is no longer able to use glucose efficiently. The use of intermittent fasting, calorie restriction, and exogenous ketones is likely to be the safer and more effective strategy than the ketogenic diet with MCTs because of the side effects of the latter, such as heart disease, fatigue, and electrolyte abnormalities.

This does NOT mean that using ketones instead of glucose for fuel will prevent brain abnormalities or boost brain function in a normal brain that knows how to use glucose for fuel. It’s like when your electricity goes out due to a bad storm that has knocked down a power line. You then have to use your battery-operated generator in order to have power to charge your cell phone, keep your refrigerator cool, and turn your lights on. It does not mean that using your battery-operated generator will prevent the power line from going down in a storm or that you should use your battery-powered generator instead of electricity for powering up all your appliances and keeping your lights on.

There’s actually a growing amount of research showing that a high fat diet causes brain shrinkage. Additionally, evidence shows that saturated fat, cholesterol, animal proteins, and advanced glycation end products (AGEs) in the diet each independently impair brain function in children and adults. See here, here, here, here, here, and here for just a handful of studies. Animal studies suggest that a ketogenic diet may impair learning and memory and accelerate neurodegeneration (it shrinks the brain). This shouldn’t be surprising since high fat, ketogenic diets tend to be high in saturated fat (butter, coconut oil, cream) and are known to cause high cholesterol, which is an independent risk factor for cognitive decline, brain fog, and dementia, as well as heart disease. (Yes, high cholesterol and saturated fat intake are still risk factors for heart disease!)

In contrast, a plant-based diet high in fruits, vegetables, whole grains, and legumes (which are all very low in AGEs and fat and high in carbohydrate sources of glucose) has been shown to boost cognitive function, alleviate depression, and prevent Alzheimer’s, Parkinson’s, mild cognitive impairment, and stroke. You may have heard of the MIND diet, which is essentially a low-fat plant-based diet that emphasizes leafy greens, berries, vegetables, legumes, whole grains, and nuts & seeds. There’s also the well-known Seventh Day Adventist study which found that Seventh Day Adventists who ate poultry and fish were twice as likely to develop dementia than their vegetarian counterparts. There are too many studies to list that support a plant-based diet for preventing chronic brain diseases, so I’m linking to this book which goes into more depth on this.

Given the general public’s current fear of carbohydrates (especially sugars), it’s interesting to note that one of the most beneficial categories of food for brain health is fruit (which delivers most of its calories as sugar). Mangos — which are eschewed by the low carb/keto set due to their high content of sugars such as fructose — are a well-studied fruit that contain components that can help prevent and reverse diabetes, heart disease, anxiety, depression, and cognitive impairment. (To discover even more about the benefits of mangos, see my article on foods that heal the gut microbiome.) There’s also research on bananas suggesting that they may help alleviate anxiety and depression as well as improve memory. The highly underrated papaya has may be able to reduce oxidative stress in the brain to help those with cognitive impairment. Red raspberries are also highly beneficial to neurons in the brain and have been recognized for their medicinal benefits for nearly 2000 years. Blue and purple berries such as blueberries, blackberries, grapes, and jaboticaba have actually proven to improve learning and memory in animals suffering from cognitive impairment as a result of being fed a high fat diet.

This is not a comprehensive list of all the foods that help benefit brain health, but rather just a handful of high carbohydrate foods rich in natural, intact sugars (including fructose) that can prevent and possibly even reverse brain conditions such as brain fog, memory loss, anxiety, and depression. If sugar found naturally in whole plant foods were really the cause of cognitive decline, then each of these foods would have research showing that they damage the brain rather than help heal it. And if consuming high fat, high cholesterol diets or ketogenic diets were really the key to prevention of brain diseases, we would expect the research to actually support this hypothesis. Instead, it appears that such diets are indeed risk factors for poor brain health.

If you missed my previous article about the difference between ketones and the ketogenic diet, make sure you check it out here. And if you aren’t already a subscriber to my free email newsletter, click here to sign up for it today for exclusive content that’s available only to subscribers.