The Brain’s Fuel of Choice Isn’t Sugar — It’s Ketones
Yes, the brain can use glucose. In fact, it must use a small amount of it at all times. But the idea that the brain prefers glucose isn’t entirely accurate. In reality, when the body has both glucose and ketones available, the brain consistently shifts its energy metabolism toward ketones. These molecules — produced by the liver during fasting, carbohydrate restriction, or endurance exercise — are not just backup fuel. They are clean, efficient, and neurologically protective.
So where did this myth come from?
In the 20th century, most research on brain energy metabolism was conducted on subjects following high-carbohydrate diets. Naturally, under these conditions, glucose appeared to be the brain’s primary fuel source. This led early scientists to conclude that the brain needed about 120 grams of glucose per day — a number that still shows up in outdated textbooks. Clinical observations supported this too: people with dangerously low blood sugar (hypoglycemia) often became confused, tired, or even lost consciousness, further reinforcing the idea that sugar equals brainpower.
Metabolic Flexibility and the Brain’s Selective Fuel Use
But what was overlooked is that in a healthy, metabolically flexible body, the liver quickly compensates for falling glucose levels by producing ketones. These ketones can cross the blood–brain barrier and efficiently fuel the brain. In fact, the more ketones are available, the more the brain uses them — and in many regions, even prefers them over glucose.
This has been confirmed by PET scan studies using labeled tracers, which allow researchers to observe how different parts of the brain absorb and utilize fuel. Research published in Frontiers in Aging Neuroscience in 2016, for example, found that in older adults, ketone uptake by the brain was nine times more efficient than glucose. Other studies have shown that in conditions like Alzheimer’s disease, where glucose metabolism in the brain is impaired, ketones remain a viable and highly effective source
of energy.
Why Sugar Sabotages Focus and Ketones Fuel It
Ketones also have several advantages: they provide stable, long-lasting energy without the spikes and crashes associated with sugar. They reduce oxidative stress, support mitochondrial function, and may even promote the expression of genes linked to longevity and brain repair.
Yet, despite this growing body of evidence, the belief that sugar boosts concentration remains widespread. It's not hard to see why: sugar creates a short-lived rush of dopamine, along with a quick surge in blood glucose. This gives the illusion of increased energy and focus — but what follows is usually a crash in blood sugar, a drop in attention, and often a craving for more sugar. It's a rollercoaster of false productivity.
Skip a Meal, Sharpen Your Mind: The Cognitive Power of Ketones
Meanwhile, ketones offer steady cognitive performance without the crash. They support working memory, mental clarity, and sustained attention — not just in people on ketogenic diets, but also during intermittent fasting or moderate carbohydrate restriction. Even short-term ketosis, achieved by skipping a meal or exercising in a fasted state, can significantly increase ketone levels and improve cognitive outcomes.
So the next time you reach for chocolate while studying, ask yourself: are you feeding your brain, or feeding a myth? True mental performance doesn’t come from a sugar spike — it comes from metabolic stability. And when given the choice, the brain doesn’t just use ketones — it prefers them.