From the Research
During the third day of a water-only fast, your brain's preferred fuel source shifts from glucose to ketones. Initially, your brain primarily uses glucose for energy, but after about 2-3 days of fasting, your liver begins converting fat into ketone bodies (primarily beta-hydroxybutyrate and acetoacetate) which can cross the blood-brain barrier 1. At this point, ketones can provide up to 70% of the brain's energy needs, with the remaining energy coming from the small amount of glucose produced through gluconeogenesis from amino acids and glycerol.
Key Points to Consider
- The brain's ability to adapt to different fuel sources is crucial for its survival during periods of food scarcity.
- Ketones are an efficient fuel source for the brain and may have neuroprotective effects, which is why many people report improved mental clarity after the initial difficult days of fasting.
- The transition to ketones as the primary fuel source for the brain is an evolutionary survival mechanism that allows the brain to function during periods of food scarcity.
Relevant Evidence
- A study published in Experimental Neurology in 2018 found that the brain is highly dependent on glucose for energy, but also highlighted the importance of lactate as a preferred energy metabolite in the nervous system 1.
- Another study published in The Proceedings of the Nutrition Society in 1995 found that the brain's first choice for a metabolic fuel is glucose, but it can also metabolize ketone bodies and lactate if they are available 2.
Clinical Implications
- The shift from glucose to ketones as the primary fuel source for the brain during fasting has important implications for our understanding of brain metabolism and function.
- This knowledge can be used to inform the development of new treatments for neurological disorders, such as epilepsy and Alzheimer's disease, which may benefit from the use of ketone bodies as an alternative fuel source.