Franziska Spritzler, RD, CDEIt’s a common belief that the brain needs carbohydrates, but this isn’t entirely accurate. For one thing, there’s a difference between the carbohydrates you eat and the glucose in your blood. While the brain does rely on glucose for certain functions, it can also use other energy sources, like ketones—an alternative fuel source produced in the liver from fat—as a primary energy source. In fact, during times of prolonged fasting, the brain can derive between 60 to 70% of its energy needs from ketones.(1) This ability to switch between different fuel sources is deeply rooted in our ancestral past.
When you consume very few carbohydrates, your body’s glucose levels fall, prompting your liver to tap into its reserves of glycogen (the storage form of glucose) for energy. These glycogen stores typically last 24 to 48 hours, depending on factors like activity level. As glycogen becomes low, the liver shifts to converting fatty acids into ketones, which provide energy for the brain and other organs in your body. Once ketones reach a certain level, you are said to be in nutritional ketosis–– also known by the new term euketonemia––which spans a range of beta-hydroxybutyrate levels between 0.5 to 5.0 mmol/L.(2)
From an evolutionary perspective, this ability to rely on ketones was crucial for survival. Our ancestors didn’t have a steady supply of high-carb foods. Instead, they ate all parts of the animals they hunted or fished, which provided mainly fat and protein. This allowed them to stay sharp and active without relying on carbs. Even when they went for extended periods of time without food, ketosis helped keep their brains and bodies fueled effectively without any carbs at all.
If ketones only provide up to 70% of the brain’s energy requirement, doesn’t this mean that the brain needs some glucose? Yes, it does, but you don’t need to consume carbs to supply it. Your body can generate glucose from non-carbohydrate sources like amino acids from protein, the glycerol portion of fat, and lactate that is produced during metabolism. This process, known as gluconeogenesis, provides the small amount of glucose your brain needs during fasting or very low-carb intake.(3) Together with ketones, this internally produced glucose keeps the brain fully fueled without the need for carbohydrates.
Interestingly, the Food and Nutrition Board of the US National Academy of Sciences acknowledged that carbohydrates are not an essential requirement almost 20 years ago. Their 2005 textbook Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids states: “The lower limit of dietary carbohydrate compatible with life apparently is zero, provided that adequate amounts of protein and fat are consumed.”(4)
More recently, researchers are finding that not only does the brain function without carbs, but that maintaining euketonemia may actually benefit brain health. Studies suggest that ketones may offer neuroprotective benefits by helping to reduce oxidative stress and inflammation.(5, 6) Beyond their well-known effectiveness for treating drug-resistant epilepsy, growing research demonstrates that ketogenic diets may also help manage certain neurological conditions–– such as Alzheimer’s disease, Parkinson’s disease, and migraines(7)–– and mood disorders.(8, 9) The stable energy supply from ketones, coupled with reduced inflammation, may support cognitive resilience and performance over time.
In summary, we don’t need to eat carbs to fuel our brains. In fact, research shows that our brains may be better off with minimal carbohydrate intake. In our modern-day environment, where carb-rich foods are readily available, achieving nutritional ketosis requires a deliberate shift in eating habits. However, this metabolic state aligns with our evolutionary roots and can promote cognitive function, steady energy, and overall metabolic well-being.
REFERENCES
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Brain Metabolism During Fasting, The Journal of Clinical Investigation 1967
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Nutritional Considerations During Major Weight Loss Therapy: Focus on Optimal Protein and a Low-Carbohydrate Dietary Pattern, Current Nutrition Reports 2024
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Unraveling the Regulation of Hepatic Gluconeogenesis, Frontiers in Endocrinology 2018
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Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids, US Institutes of Medicine 2005
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Effects of Ketone Bodies on Brain Metabolism and Function in Neurodegenerative Diseases, International Journal of Molecular Sciences 2020
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Impact of fasting & ketogenic interventions on the NLRP3 inflammasome: A narrative review, Biomedical Journal 2024
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Efficacy of low carbohydrate and ketogenic diets in treating mood and anxiety disorders: systematic review and implications for clinical practice, British Journal of Psychiatry Open 2023
