Carbohydrates are often thought to be unhealthy. Carbohydrates are made up of sugars, and these are fatteners. This idea is partly unjustified. Sugars are the energy source for an indispensable organ in our body, the brain. Why are sugars so important to keep our brain running? And can we use all kinds of sugar for this?

Author: Renee Clausing

Energy consumer

It is only a small part of our body, but it is very important: the brain. Although this organ takes up only two percent of our total body weight, it is one of the largest energy consumers in the body. The brain uses up to twenty percent of our energy, which is equivalent to about 14.6 watts in an adult man. This is almost double of what the average energy saving bulb consumes. To keep the brain running, this energy must be supplied continuously. A lamp can be connected to electricity to get energy, but where do we get energy from? Right, food. To be precise, there are three types of nutrients that can provide us with energy: carbohydrates, fats and proteins. Of these three, carbohydrates are the main source of energy for our body. Odd, you may think, since fats have the most calories per gram.

As with many things, there is a reason for our body to take carbohydrates as the main source of energy. Carbohydrates can simply consist of one sugar molecule – glucose and fructose are common in our diet – but they can also be long chains of ten or more sugar molecules, such as starch. The breakdown of carbohydrates is much easier for our body than the breakdown of fats. As a result, they deliver energy much faster and more efficiently. Sugar molecules, unlike fats, are also soluble in water. This ensures that sugar molecules are more easily transported via the blood to their final destination, such as the brain.

Glucose versus fructose

It sounds too good to be true that all the sugars we consume can be used by our brain. And indeed, glucose is the only sugar molecule that can directly supply energy to the brain. When the brain is satisfied, glucose is converted and stored as glycogen, which can serve as a long-term energy storage. Fructose cannot directly supply the brain with energy. Fructose must first be converted to glucose in the liver. The disadvantage of this, is that when the brain is already supplied with enough glucose, fructose is not converted into glucose and forms an attack on your liver.

Photosynthesis

Why can one sugar molecule supply energy directly to our brain, while the other cannot? Why did our body once choose to use glucose as the primary fuel, not fructose? To find an answer to this, we go back in time, about 3.5 million years. Jan van Maarseveen, Professor of Bio-Inspired Organic Synthesis, explains that evolution can help us find an answer to the question of why glucose is so important to our energy. “The first organisms capable of photosynthesis appeared on earth 3.5 million years ago. Photosynthesis is the process of converting CO2 and light into oxygen and glucose. Before that time, oxygen was toxic to the microorganisms that lived on Earth”.

Many years after photosynthesis started, organisms that can withstand oxygen appear. As a primary product of photosynthesis, glucose was also up for grabs for these organisms. At that time, it was important to have food that was readily available, i.e. glucose. “Suppose that fructose is the primary energy source for our body, then the available glucose would first have to be converted into fructose, which in turn costs energy. In times when you are hungry and have to be very careful with your energy, your brain is the organ that is kept alive the longest. In that case, if glucose first had to be converted into fructose in order to continue to function, this would only cost energy”, explains van Maarseveen. “Life is founded on photosynthesis. The closer you stay to photosynthesis, the less energy it costs you.”

No AH around the corner

As you read this article, you may have just finished your breakfast, are planning a lunch date with a friend, and are already thinking about what you will be cooking tonight. It is difficult for us to imagine not eating three meals a day. There were times when this was different. “We are descended from primary life forms that did not have an Albert Heijn around the corner and sometimes had to survive for several days without food,” continues van Maarseveen. “Our body is therefore set up to function from food that can be stored, such as glycogen. Glycogen is actually nothing but glucose molecules that are linked together. So, it makes sense that glucose was chosen as the primary energy source.”

Simple and complex carbohydrates

For the bread lovers among us, it is good news that carbohydrates keep our brains running. Bread is full of it. Note that not all carbohydrates provide the same benefits. In order to get the most out of a carbohydrate-rich diet, it is important to distinguish between two types of carbohydrates. First, you have simple, fast carbohydrates. These come in white bread, table sugar and biscuits. As the name implies, these types of carbohydrates are quickly broken down in the body and converted into glucose. The effect of this is a rapid rise in sugars in your blood. You may have noticed this: the sugar rush you get after devouring a bar of chocolate.

Another type of carbohydrates are complex carbohydrates. You can find these in your whole grain sandwich and other whole grain products. Complex carbohydrates are more difficult to break down in the body. This allows for a more gradual release of glucose into the blood. This gradual and stable release of glucose into the blood comes in handy for the brain, which can be supplied with energy continuously. An additional advantage is the avoidance of the sugar rush and its subsequent energy dip. In short, while simple carbohydrates can provide you with energy quickly, complex carbohydrates are generally more nutritious because they can supply your body and brain with their energy needs for an extended period of time.

Sugar, a friend from the past

Glucose being the most important source of energy for our brain can therefore be traced back thousands of years. The human body was set up a long time ago to deal with food scarcity. With glucose as the main source of energy, the brain can continue to function even when a meal is not served three times a day. It is still a strong property of our body to be able to obtain energy from stored glycogen. Think about that next time when you feel hangry feelings coming on.

About the author:
Renee works at Baking Lab and as an intern in science communication at the Netherlands Brain Institute. She previously completed a master’s degree in neurobiology (UvA) with a specialisation in science communication (VU).