At the end of 2015, Geoffrey Woo was preparing to embark on his first experiment with radical intermittent fasting: a 60-hour stint, from Sunday night to Wednesday morning.

“At first I thought, is this even possible?” says Woo, co-founder of a San Francisco-based company called HVMN, which sells nootropic supplements the company says can “keep your brain sharp for the long term.”

“But I saw some of my co-workers doing it successfully, saying they were more clear and more productive.”

Woo didn’t begin fasting only for the reasons science tentatively recommends: to live longer or to lose weight. He was just as thin several years ago as he is today at age 30. What he was gunning for was a cognitive hack. “The plan was longevity and the cognitive benefit,” he says. Woo wanted to clear the “brain fog,” as it’s colloquially called, and optimize his mental agility.

“The first two times I tried to fast it was terrible. It was hard,” he says. “By the third time, it felt pretty refreshing. I felt pretty clear.”

Woo now cuts out all food for 24 hours once or twice a week. Every three months, he goes 36 to 72 hours without a meal. He justifies his cleanses based on several years of tracking his own blood sugar with a continuous glucose monitor he wears on his arm. While fasting, Woo says, his blood sugar remains steady throughout the day, instead of spiking and crashing after a big lunch.

As a Silicon Valley entrepreneur, Woo is perhaps the ideal guinea pig for this sort of diet experimentation. He’s part of an ever-growing movement of biohackers finagling with nutrition in service of the one part of our anatomy that no one ever sees: the brain.

Eating for brain energy and focus has become one of the central underpinnings of a series of diet trends growing in popularity. There’s intermittent fasting; the low-carb and high-fat ketogenic diet; the MIND diet, which combines the Mediterranean diet and the heart-healthy Dietary Approaches to Stop Hypertension (DASH) diet; and the Bulletproof Coffee diet, founded by biohacker Dave Asprey, which prescribes dunking a pat of butter into your morning brew for better energy and cognitive function. The diets have an especially notable following among technocrats like Woo. Twitter CEO Jack Dorsey says he does a daily 22-hour fast, and Kevin Rose, the co-founder of Digg, launched an app called Zero to help people track daily fasting.

“If somebody really feels like it works for them, honestly, you can’t argue with that.”

Data suggests that lifestyle behaviors, including diet, can play an important role in keeping the brain healthy and preventing cognitive decline, though dietitians and researchers caution against using small studies of unique eating trends to inform an entirely new food regimen. “More now than ever, people are looking for a quick fix,” says Marjorie Cohn, a registered dietician and spokesperson for the Academy of Nutrition and Dietetics. “I think a lot of what explains it is our access to information and social media. A lot of these diet hacks are being promoted and circulated not from professionals, but from nonprofessionals promoting nonprofessionals.”

Even so, eating patterns like occasional fasting or eating mostly fats and protein—while perhaps unnecessarily strict—may demonstrably improve a person’s reported energy and focus. Whether they actually change the brain is still being worked out. Meanwhile, the Woos of the world are eagerly adapting to new ways of eating before the results are clear. “Who did the randomized controlled trial on the FDA food pyramid?” says Woo.

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.

“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.”

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.”

While there’s compelling evidence to suggest that eating does have an impact on the brain, there’s little evidence that extreme diet patterns are better than the other, more low-key methods. “I don’t believe something until I see the scientific evidence,” says Martha Clare Morris, professor of epidemiology at Chicago’s Rush University Medical Center (RUMC). “It’s amazing to me how many people say they’re on the keto diet when we don’t know the long-term effects.”

In her quest for evidence on a diet that does nourish the brain, Morris teamed up with other researchers at RUMC and the Harvard School of Public Health and arrived at one answer in 2015: the MIND diet. It pulls together foods from the Mediterranean diet, like whole grains, legumes, and fish, and the plant-based low-fat DASH diet, which is designed to combat high blood pressure. Traditionally, both diets are recommended to prevent heart disease.

“These are two great diets that have been shown in randomized trials to have beneficial effects on chronic diseases of aging, lowering inflammation, and preventing diabetes in people with high glucose levels,” says Morris, who has spent 25 years researching how diets affect brain aging.

Antioxidant nutrients, such as vitamins C, E, and B, as well as omega-3 fatty acids, are found to have positive effects on our cognitive capabilities by counteracting two of the factors thought to cause neurodegenerative diseases: oxidative stress and inflammation.

In a study of the MIND diet that lasted almost five years and included more than 900 people ages 58 to 98, Morris and her team demonstrated that even moderate adherence to the MIND diet led to a 35% reduction in developing Alzheimer’s disease. Now they’re conducting a follow-up randomized trial of the MIND diet to measure cognition over a three-year period. Morris says research like this is necessary to truly gauge the effects of a diet for brain health.

What’s notable about the MIND diet is that the eating plan is not revolutionary. It tracks with what’s long been recommended as part of any healthy diet. Meal plans with foods high in antioxidants, like berries, nuts, seeds, and leafy greens, as well as nutrient-dense foods like beans and foods packed with omega-3s, like fish, all appear to help the brain.

“Honestly, the solid research is just swinging back around to what our grandparents and great-grandparents ate,” says the Academy of Nutrition and Dietetics’ Cohn. “We should be eating fat; we should be eating eggs. More whole food, less processed food.”

If the best way to eat for the brain is to follow the usual advice—eat more fruits and vegetables—then why are restrictive diets so enduring? After politics and religion, the virtues of respective diets are perhaps the next easiest subject to conjure up vehement disagreements.

“A lot of this comes down to subjective experience,” says Cohn. “If somebody really feels like it works for them, honestly, you can’t argue with that.”

Pojednic of Simmons University says there may be a psychological component that makes mental clarity diets feel more effective for the people who follow them than standard healthy eating. “Is simply having this feeling of control enough to help your brain?” she says.

“It gives you confidence to be able to do something that’s impossible.”

Woo agrees that fasting is morale-boosting. The longest fast he ever did was for seven days. “It gives you confidence to be able to do something that seems impossible,” he says. “That’s a psychological side effect of being able to control your animal instincts and channel them in a productive way.”

The bottom line is that these various hacks for uncorking brainpower share a strong redeemable characteristic: They all call for cutting out the crud that makes you feel foggy, groggy, or lethargic. They also lean into fatty foods that contain omega-3s, which are believed to be important for brain health by preserving cell membranes and aiding communication between brain cells. “These diets are wonderful in that they’re getting people to be more aware and self-conscious about what they’re putting into their body,” says Selhub, the doctor and wellness consultant. “The key is: What am I learning from this diet about how my body feels?”

It’s a matter of testing out what works best for you. Not a single person I spoke to advocated die-hard adherence to one of these diets—not even Woo. “It’s less of a religious thing,” he says.