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Introduction to brain metabolism

In this video from the University of Torino we introduce further notions about the brain, focusing on its metabolic requirements and characteristics.
The brain is one of the largest organs of the body. This is true but what’s also true is that the brain only accounts for about two percent of the body weight and, despite that, it is responsible for about 20% of the resting metabolic rate. That is one-fifth of the energy consumption of the entire body when it is not involved in any activity. To make a comparison, the energy consumed by one gram of brain during a minute of an average day is the same as a gram of leg muscle when we are running a marathon. How does the brain consume all of this energy and how does it get it? Two-thirds to three-quarters go into signaling, what we call neural activity.
The rest is used for in, layman terms housekeeping: Recycling or expelling waste products maintaining homeostasis and keeping the neurons in good health. We know these thanks to studies that measure the brain metabolic rate in different conditions such as anesthesia and coma and found a similar rate of energy consumption. We also know that our brain metabolism doesn’t slow down that much when we are sleeping and during rem sleep can even increase. Therefore the brain needs a lot of energy and it needs it all the time. Can the brain store it, for instance in the form of glycogen, like the muscles and the liver do? Well, not much.
In fact, the brain stores only one-hundredth glycogen of what a liver weighing the same would do. So, to function properly, it needs a constant supply of nutrients such as glucose. In popular culture, it’s often said that the brain needs carbohydrates to work. Is that true? Can the brain be refueled with any admissible source of energy or is it specific in its requirements? It turns out that the correct answer is the latter. The adult brain uses almost exclusively glucose as fuel, Though in the first months of life and during prolonged starvation it can use other sources of energy to such as lactate or ketone bodies.
As the name suggests lactate is formed by lactic acid, the same substance that accumulates in muscles during a prolonged effort while ketone bodies are molecules produced by the liver during periods of starvation or fasting. What about fatty acids, the molecules that constitute fats? Interestingly, the brain does not really use them as a source of energy. This is a puzzling fact, as fatty acids are a very dense source of energy and they are used by the body to store it. It was thought that the brain doesn’t use fatty acids as their molecules cross slowly the blood-brain barrier, but more recent research points out that fat metabolism has distinct
disadvantages in respect to glucose metabolism: It requires more oxygen, it is slower and it generates oxidative substances that can damage neurons and glial cells. As the metabolism of ketone bodies and lactate is not associated with the same disadvantages, this might explain why evolution disfavored the use of fatty acids in the brain selecting instead alternative energy sources in the absence of glucose. In the end, we can notice a curious situation. The organ that regulates feeding is also one of the greatest consumers of energy. It cannot store large amounts of fuel and it is very specific when it comes to what it can use.
It is therefore of no surprise that there is a specific communication pathway between the brain and the gut called the gut-brain axis. We will discuss in more detail the gut-brain axis in the next lectures along with other important structures axis such as the hypothalamic-pituitary-adrenal that connects the brain to the endocrine system.

What fuel does the brain use?

In the previous steps, we learned some crucial notions about the brain. We now end with a brief analysis of brain metabolism and its characteristics.

In this video, you will learn some surprising facts about the brain – for instance, it consumes as much energy as a leg muscle when we are running a marathon but it stores very little energy.

In the next steps, we will explore emotions and cognitions, before starting with the main part of the course and our first topic: food and the reward systems.

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Food for Thought: The Relationship Between Food, Gut and Brain

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