There are plenty of anecdotal observations about the effects of extreme diets and, in some cases, short-term randomized trials sussing out the effects of various diets. But long-term, randomized trials—the gold standard of scientific inquiry, where people are followed for more than 20 years to see how certain diets might influence diseases or conditions later in life—are in short supply.
A salient feature that can be observed across mental clarity diets is that they eliminate some type of food or food group. All of them have barely any room for processed foods. Some explicitly outlaw sugar and white-flour products, like pizza and cereal. Experts say that it’s likely the removal of these food groups that creates the clearheadedness and that counteracts so-called brain fog.
Last September, I spent an entire month following the all-meat-all-the-time carnivore diet. I could only eat foods like steak, chicken, eggs, venison, salmon, brisket, and absolutely nothing that could be classified as a fruit, grain, or vegetable. It was in the service of journalism—a piece I wrote for Outside magazine, although I was personally curious how protein-packed meals with no carbs, sugar, or vegetables would make me feel.
My mother was certain I would become gravely ill, but after eating a diet heavy on ribeye steak, I felt less groggy and more energetic. Other people on the same diet told me their brain fog had also lifted and that they were less sleepy. People who follow the ketogenic diet—which is basically a serving of carbohydrates away from the full-on carnivore lifestyle—report similar effects.
Nutrition experts like to point out why people on low-carb or no-carb diets might feel more awake throughout the day: Glucose is the brain’s chosen fuel, and it typically gets it from carbs and sugar. The Western diet tends to be heavy on simple carbohydrates. If you eat a slice of pizza and wash it down with a soda for lunch, your glucose will spike, which will cause an overcompensation in the amount of insulin your body produces to process that blood sugar. Around the middle of the afternoon, you will begin to feel sluggish—because your glucose levels have plummeted.
When you eat heavily processed foods, such as potato chips or a fast-food hamburger, your body responds to them as though they are foreign objects invading its turf, which causes inflammation. Inflammation itself is linked to many chronic diseases that appear over time, such as diabetes, arthritis, and coronary heart disease.
“The best thing is to understand that for your brain to feel clear, you need to cut down on the sugars, your gut needs to be happy, and you need to have a nutrient-rich diet,” says Dr. Eva Selhub, a former instructor at Harvard Medical School who now runs her own wellness consultancy. “The reason we’re a sick nation is because everything has carbohydrates in it, and that’s why people feel better when they do these diets.”
In the keto and carnivore diets—as in Woo’s fasting diet—the body uses fat as its energy source instead, converting it into liver-produced chemicals called ketones, which the brain can also use as fuel. In a small study published in 2012, researchers put 23 adults on a high-carb or very low-carb diet and found that the men and women following the low-carb diet demonstrated better memory function.
“Your brain prefers glucose to everything else,” says Rachele Pojednic, an assistant professor of nutrition at Simmons University. “If you’re on the ketogenic diet, it could be that ketones make you feel better, but we don’t know that yet because we haven’t studied this pathway. Most of the ketogenic data we have is on weight loss.”
Bulletproof Coffee devotees usually say the fat from butter makes it so that your body absorbs caffeine at a slower pace, creating a lasting alertness instead of a sudden energy jolt. While there’s no research that says butter in your coffee is harmful, there’s no conclusive research that says buttered coffee leads to better brain focus.
“It might just be that you’re not hungry because you just ate a ton of calories in the morning and so when you get to the afternoon you’re not tired,” says Pojednic. “What the data is showing overall is that it’s not one nutrient in isolation, which is what a lot of these diets really focus on.”