A favorite argument of detractors of low-carb diets is that they don’t provide sufficient glucose for the brain to function. You’ve probably heard some well-meaning person say, “You need to eat 130 grams of carbohydrates a day for proper brain function.”
Fortunately, this is not true. Just think how much trouble our hunter-gatherer ancestors would have been in if brain function was compromised every time carbohydrates were scarce. A daily intake of 130 grams of carbs is no small feat when you don’t have a local grocery store stocked with packaged grains, bread, cereal, and chips for your convenience.
This widespread misconception is based on a misunderstanding of the brain’s fuel needs.
The brain does require about 50 grams of glucose a day, but this doesn’t need to come from dietary carbs, and the brain can get the rest of the energy it requires from ketones, a byproduct of fat metabolism. It works like this: the body can get glucose from three sources other than dietary carbs:
1) Glycerol, produced from dietary fat or fat tissue in the body
2) Lactate and pyruvate, produced from glycolysis
3) Amino acids, produced from protein
These substrates go to the liver where it turns them into glucose via the process known as gluconeogenesis. The glucose then travels via the blood to the brain where it uses it for energy.
The rest of the energy the brain requires is derived from ketones. Ketones are produced when insulin levels are low due to carb restriction or low calorie intake. Fatty acids that are released due to low insulin travel to the liver, which turns them into ketones that can be used for energy in cells to produce ATP.
Having the brain get energy from ketones improves brain function and is the reason that ketogenic diets are used to treat brain disorders. Having the brain run on ketones may have an anti-aging effect as well, protecting the brain from cognitive impairment and Alzheimer’s.
An adult’s brain will only use ketones for energy if glucose is not sufficiently available. This doesn’t mean that using ketones is “bad.” In fact, there is evidence that ketones are the preferred energy source for the brain, however, it is uncommon for both ketones and glucose to be available because the liver won’t produce ketones unless blood sugar (and insulin) levels are low.
Specially designed studies show that when both ketones and glucose are present in the body, ketones enter the brain in proportion to the plasma concentration and brain glucose uptake decreases accordingly. This decrease in brain glucose uptake when both ketones and glucose are available supports the notion that ketones are the brain’s preferred fuel.
Another indicator of the benefit of ketosis is its vital role in normal human development: Well-fed infants maintain a constant state of elevated blood ketone levels, suggesting that there are brain benefits from having ketones present.
The ketogenic diet—a very low carb diet—was first designed as a therapeutic approach to treat epilepsy—a brain disorder that can be managed when carbs are severely restricted to less than 20 grams a day with a moderate protein intake of 1 g/kg/bw and the rest of the calories coming from fat. More recent research has shown the ketogenic diet can be beneficial in treating cancer, obesity, diabetes, brain injuries, and neurodegeneration.
A ketogenic diet, even over the short-term, appears to have anti-aging benefits. As people age, a couple of things happen to the brain that contribute to cognitive decline and brain disorders like Alzheimer’s:
First, we experience a decrease in blood flow to the brain.
Second, there is a decrease in glucose uptake to the brain—in simple terms, the brain is less effective at using glucose for energy.
Finally, inflammation develops, neurons die off, and brain size decreases.
Scientists believe that ketogenic diets may reduce these negative effects while supporting brain function. Ketogenic diets improve mitochondrial function (how the brain makes and uses energy), while lowering oxidative stress that leads to inflammation. Additionally, when the brain uses ketones as the principal source of energy (instead of glucose), it bypasses the need for glucose uptake and the brain becomes more efficient.
If you’re interested in trying a ketogenic diet to protect your brain, here a few things to consider:
Ketogenic diets require an adaptation period that lasts 1 to 2 weeks. Depending on your metabolic state going in, this phase can be uncomfortable. People report a foggy brain, flu-like symptoms, and low energy during the adaptation process but these will go away once the body gets its metabolic machinery up to par.
You don’t necessarily have to adopt a ketogenic diet for the rest of your life. Trying if for a short time may be beneficial. That said, it’s not something you can use intermittently—such as day to day—in the same way as fasting because it takes a bit of time for ketone levels to increase sufficiently so that the body will start burning them.
The general ratio for a ketogenic diet is 75 percent fat, 20 percent protein, and 5 percent carbohydrates. However, these can be modified based on individual needs and goals. For the initial adaptation phase, carbs likely need to be below 50 grams a day, preferably from low-glycemic vegetables and select fruits, such as berries or other low-carb fruit.
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