“Bulletproof Coffee is not your average latte,” declares Bulletproof’s website. “It’s a high-performance drink that has a massive impact on your energy and cognitive function.”

What is butter coffee?

Bulletproof coffee (also called butter coffee) is meant to replace breakfast. The official Bulletproof recipe transforms a simple cup of joe into a nearly 500-calorie fat-laden drink by blending up to two tablespoons each of unsalted butter and medium-​chain triglyceride (MCT) oil into 8 to 12 ounces of coffee. (Proponents recommend starting with one teaspoon of MCT oil and working your way up, since some people report that the oil leads to bloating, gas, and diarrhea.)

That means no protein, fiber, healthy fat, carbs, fruits, or vegetables for breakfast. Just roughly two days’ worth of saturated fat. The sat fats in MCTs raise LDL, or “bad,” cholesterol, but not as much as the sat fats in butter do.

Ketones for concentration?

Enthusiasts claim that it’s the ketones—which the liver makes when it breaks down MCTs—that keep you focused.

“Pair ketones with the slowly releasing caffeine and you can literally feel your brain turning on,” claims Bulletproof.

The brain usually runs on glucose, or blood sugar. “But when blood glucose levels are low, the brain can use ketones as an alternative fuel source,” says Jason Brandt, a neuropsychologist at Johns Hopkins University School of Medicine.

For example, when researchers made 11 people with type 1 diabetes score poorly on tests of attention and memory by lowering their blood sugar, MCT oil reversed the decline.

But are brains sharper when they burn ketones instead of glucose? No human studies have looked.

“The evidence to support the butter coffee trend is all anecdotal,” says Brandt.

The bottom line

There’s no evidence that replacing your breakfast with butter coffee “has a massive impact...on cognitive function,” as Bulletproof promises.