“When your thinks have run dry, In the blink of an eye;
There’s another think there! If you open your mind.”
This wonderful line from the Dr. Seuss book, ‘The Thinks you can Think’, reminds us how amazing the human brain is, and how important it is that we look after our ‘thinks’. Of course, brains do a lot more than just allow us to think – they help control our voluntary movements (e.g. running) and involuntary (e.g. breathing) movements, and to do so they use a large amount of energy .
The average human brain weighs around 1.4 kg, and uses about 5.6 mg (0.0056 grams) of glucose per 100 g per minute : that is, a daily energy requirement of 100–130 grams of glucose per day.
But is this really true?
Many people have made the assumption that we must supply our brains with this much glucose every day – and that this must come from our food intake – or we will suffer from impaired brain function. However, this is incorrect.
In our bodies, cellular energy (in the form of adenosine triphosphate, or ATP) can be produced from all three macronutrients – carbohydrate, protein and fat – with different types of cells utilizing different sources of energy. Most cells in our body have a store of fat and glucose (the latter in its storage form called glycogen) that can be mobilized when energy is needed.
Brain cells are unique because they cannot use fat, and they don’t have a store of glucose (or glycogen). This means that the brain’s fuel needs must be met by a continuous supply from the bloodstream. Most people eating the modern high carb diet will fuel their brains with just glucose, provided through the consumption of a large amount of carbohydrate-based foods. However, when carbohydrate intake is reduced, insulin (the hormone that facilitates the movement of glucose out of the bloodstream and into the cells) levels decrease, stimulating the liver to produce molecules called ketones. To make ketones, first the liver breaks down fat (either consumed in the diet or from stored body fat) into its constituent parts: glycerol and fatty acids. The fatty acids are then broken down further in a process called ketogenesis to produce ketones.
When the level of ketones in the blood increases, the brain uses them as an additional fuel source, reducing the need for glucose. Through this mechanism, ketones can supply up to 75% of the brain’s energy needs.
However, even when ketones are being used, the brain still requires a small amount of glucose to function. (Using the calculations above, this could be around 25–50 grams of glucose per day). So where does this come from if you consume less than this amount in your diet?
When carbohydrate from the diet is decreased, glucose can be supplied through the breakdown of glycogen via a process called glycogenolysis, or through an alternative pathway known as gluconeogenesis: the making of glucose from other (non-carbohydrate) sources. During gluconeogenesis, glucose can be manufactured in the liver from the glycerol molecules released during the production of ketones. Our bodies use glycogenolysis and gluconeogenesis to maintain blood glucose levels within a narrow range, facilitated mainly by the action of hormones (insulin and glucagon) produced in the pancreas (the obvious exception to this is people with diabetes, who have to actively manage their own blood glucose levels, usually with the use of exogenous (injected) insulin and ingested glucose). Blood glucose levels are kept at around 80-85 , regardless of carbohydrate intake; thus ensuring that the brain always has a sufficient supply of glucose.
In this way, your body (and especially your liver) ensures that your brain has all the energy it needs, even if you don’t consume any carbohydrate at all. So even without eating carbohydrates you can think all the thinks you can think:
“Think left and think right, and think low and think high.
Oh the thinks you can think up if only you try!”
 Volek, JS and Phinney, SD (2011) The art and science of low carbohydrate living. p. 67. (Beyond Obestiy, LCC)
 Erbsloh F, et al. (1958) The glucose consumption of the brain and its dependence on the liver. Arch Psychiatr Nervenkr Z Gesamte Neurol Psychiatr. 196: 611–626.
 Bernstein, RK (2011) Dr Bernstein’s diabetes solution. The complete guide to achieving normal blood sugars. p. 57. (Little Brown & Co.: New York)