Is the Brain Fueled by Fat, Protein, or Carbs?

The human brain consumes up to 20% of the energy used by the entire human body which is more than any other single organ. The brain represents only 2% of body weight yet it receives 15% of the cardiac output and 20% of the total body oxygen consumption. (source)

Our brains create major nutrition demands on our bodies in order to function optimally. So is it best to fuel the brain with fat, protein, or carbohydrates?

The answer is none of these. Even though the brain is composed of 60% fat, it is designed to be fueled by glucose. The brain accounts for 25% of the total body glucose utilization.

How do we get glucose to the brain? There are two ways.

1. Glucose is the human body’s key source of energy. The breakdown of carbohydrates (eg: starch) yields mono- and disaccharides, most of which is glucose. (source) If glucose is available, the body will use it first since it is easiest and quickest to metabolize.

Whole simple carbohydrates like raw fruit and whole complex carbohydrates like grains, legumes, and tubers are excellent sources of glucose for the brain. Refined carbohydrates can deprive the brain of glucose.

Click: Know Your Complex, Simple, and Refined Carbs

Glucose is virtually the sole fuel for the human brain, except during prolonged starvation. In starvation, ketone bodies generated by the liver partly replace glucose as fuel for the brain. (source)

2. If insufficient carbohydrates are consumed to meet our fuel needs, then fats and proteins can be converted into sugars. The human body has little capacity to store excess carbohydrates or protein, but can convert both to fat stores for later use as fuel when converted to glucose via gluconeogenesis.

When fats are converted to sugar in the absence of carbohydrates, ketones are produced. These molecules are very similar to acetone in their structure. They affect brain function in a fashion that is similar to alcohol, impairing our decision-making abilities, as well as our awareness and judgment.

Protein, fat, and even complex carbohydrates must be broken down to simple carbohydrates in order to be used as fuel. While this breakdown is often an energy-intensive operation, sometimes requiring almost as much fuel as the consumed food supplies, simple carbohydrates such as glucose and fructose are absorbed without needing any digestion whatsoever.

This energy-conserving quality is the primary reason that where physical performance is concerned, the simple carbohydrates found in whole, raw fruit are a better source of fuel than protein and fat. (source)

Is ketosis (the formation of ketones) safe?

With insufficient intake of the body’s primary fuel, carbohydrate, the body turns to fats from foods and from body fat for fuel. Byproducts of this metabolism are acidic substances called ketones (acetacetic acid, B-hydroxybuteric acid, and acetone). The metabolic condition is known as ketosis. Ketosis is associated with loss of appetite, nausea, fatigue, and hypotension (lower blood pressure). The result is a decrease in food (calorie) intake. Ketosis allows the body to starve while reducing the suffering of severe hunger pangs.

With little carbohydrate in the diet the body resorts to using its glycogen stores of glucose. Glycogen, stored in the liver and muscles, can meet the average person’s glucose needs for about 12 to 18 hours. With each gram of glycogen is stored 2.7 grams of water. The average body stores 300 grams of glycogen. Depletion of the body’s glycogen would result in an almost overnight weight loss of 1110 grams (37 ounces or over 3 pounds). The ketones also cause a strong diuretic effect on the kidneys, resulting in losses of large amounts of fluid. (source)

Ketosis is metabolically very similar to starvation. If you’re trying to increase your fertility or if you’re currently pregnant, ketosis can actually be counterproductive. Fertility is greatest when the body feels well-nourished (which makes perfect sense: evolutionarily, conceiving a child during a time of food scarcity could be very dangerous), so a very low carbohydrate diet that imitates starvation is not ideal for conception. Similarly, the healthiest nutritional state during pregnancy is being consistently well-nourished; ketosis can be dangerous for both mother and baby.

People who do a lot of high-intensity metabolic conditioning should also avoid ketosis. This kind of activity demands glucose for fuel. Your body can make its own glucose from fat and protein, but not at the rate that you need it for regular sprint workouts or Crossfit metcons. If you regularly try to push yourself through this kind of workout on a low-carb diet, you’ll burn through all your stored muscle glycogen right away, and then see your performance start to decrease.

Kidney stones are a well-known side effect of prolonged ketosis. Some studies also indicate a risk of bone density loss, a problem that could lead to osteoporosis or further complications down the road. Children on a ketogenic diet grow more slowly than their peers – not surprisingly, given that ketosis is so similar to starvation. A less serious but irritating side effect is constipation [from lack of dietary fiber]. Other risks of very low carbohydrate diets in general include thyroid problems, Vitamin C deficiency, low energy, mood disorders, and bad breath. (source)

Ketosis is an emergency, survival state of the body. Is it safe? It is safe when it is needed to survive. It can’t be considered safe to voluntary force a body into a state of unnecessary and prolonged ketosis. Overnight ketosis (fasting while sleeping) is natural, normal, and within safe limits. When dealing with diabetes, epilepsy, or other brain disorders, a plant-based ketonic diet can prove beneficial.

Dietary Options

With three macro nutrients, these are the possible combinations of dietary approach:

Low Carb – High Protein – High Fat
Low Carb – High Protein – Low Fat
Low Carb – Low Protein – High Fat
Low Carb – Low Protein – Low Fat

High Carb – High Protein – High Fat
High Carb – High Protein – Low Fat
High Carb – Low Protein – High Fat
High Carb – Low Protein – Low Fat

The human body’s ability to metabolize protein ends at around 35%. Thus “high protein” is a relative term: even advocates of higher protein consumption are not claiming that it should account for the majority of calories. (source)

Taking out the high protein options, this is what remains:

Low Carb – Low Protein – High Fat
Low Carb – Low Protein – Low Fat

High Carb – Low Protein – High Fat
High Carb – Low Protein – Low Fat

Simple carbs and fat don’t mix. Because of the easy and quick metabolism of simple carbs, dietary fat slows the process causing a back-log of sugar in the bloodstream.

Click: The Problem with Fruit

Removing the high carb – high fat option, this is what remains:

Low Carb – Low Protein – High Fat
Low Carb – Low Protein – Low Fat

High Carb – Low Protein – Low Fat

The only way to get the low carb – low protein –  low fat option is to fast, which is a viable option at times for specific healing situations, but for adequately fueling the brain, let’s remove it. This is what remains:

Low Carb – Low Protein – High Fat

High Carb – Low Protein – Low Fat

Basically, there are two main options for fueling the brain. A person will either get the bulk of their needed calories from carbohydrates (simple or complex, not refined) or from fat.

A high fat diet will put the body in a state of chronic ketosis due to lack of carbohydrates. A high fat diet built on animal-based consumables will negatively impact the environment and exploit animals.

It is possible to eat a high-fat plant-based diet which shows benefit to human health compared to high-fat animal-based diet without creating a negative environmental impact or exploiting animals, yet ketosis will still be experienced.

When you realize the negative impacts of high-fat on the human body you realize that whole plant fats need to be kept to a minimum. The brain is oxygen dependent and high-fat diets reduce the oxygen carrying capacity of red blood cells.

Click: 6 Health-Preserving Reasons to Stop Consuming Oil (and reduce all fat)

So if we remove high fat as a dietary option, all that is left is:

High Carb – Low Protein – Low Fat

Whether you take your carbohydrates in fast simple carbs (raw fruit) or slow complex carbs (cooked starches) or a combination of both is up to you. You will avoid ketosis, fuel your brain adequately, and intake ample amounts of the macro- and micro-nutrients if sourced from a variety of whole foods all while maintaining a low impact on the environment and avoid needlessly killing any animals.

An ideal ratio of daily macro-nutrient calories is 80% carb, 10% protein, and 10% fat. A tad more dietary fat can be safely consumed with complex carbs than can be consumed with simple carbs. Sourcing these calories and nutrients from whole fruits, vegetables, grains, legumes, tubers, seeds, nuts, and fungi is responsible, possible, and healthful to not only the brain but the entire human body, not to mention the planet and her animals.

Additional Resources

Nutrition Only in Animal-Based Foods

Choosing the Safest Fats, Carbs, and Proteins

Is Alzheiemer’s Disease Cardiovascular Disease?

Carla Golden is a vegan nutritionist and a massage therapist in private practice specializing in therapeutic essential oils. With a Bachelor of Science degree in Holistic Health & Healing, she enjoys helping others discover the benefits and liberation inherent in a whole food, plant-based vegan diet. The Vegan Key™ is her newest online nutrition program based on tried and true methods which foster performance, vitality, and purpose. Join Carla in person at a Palmetto Plant Eaters Club meeting!

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