Addiction is a medical condition in which the person has uncontrollable desire to take a substance or engage in an activity, despite knowing that it may lead to adverse effects. Taking that substance or engaging in that activity will make the person feel good. Does sugar fit the bill? Clearly yes, as so many of us cannot control cravings for something sweet.
Sugar is used daily by most of us. Modern food is very rich in sugar and this abundance of sugar is viewed as one of the main reasons for growing obesity epidemics. Overconsumption of sugar does not only results in a flood of extra calories – it can also lead to addiction. Sugar can interact with different substances in the brain, influencing and changing their normal levels. Most notably, it affects the level of dopamine. It can also change the concentration of some receptors in the brain.
The most common form of sugar in our food is sucrose. When ingested, this sugar is split in the digestive system into its two constituents, glucose and fructose. Insulin and glucagon are two enzymes most important for the metabolism of glucose. They both regulate the level of glucose in the human body.
After ingestion and degradation of sugar, glucose molecules are absorbed and distributed to all organs and cells in the body. A group of proteins called GLUCs are responsible for transportation of glucose in the blood. GLUT1 is the main transporter of glucose to the brain.
When glucose reaches destination cells, it has to go inside the cells where it is consumed. There are different mechanisms to achieve this aim. Some cells, such as red blood cells, use passive transport, also known as diffusion, to get the glucose from the blood plasma. Many other cells use active transport mechanisms to deliver glucose inside the cells.
One of the human tissues which cannot tolerate low levels of glucose is brain tissue. The main reason for that is the inability of neurons to store glucose and use that stored glucose when levels decrease. That is the reason why the human brain is the first in line for the glucose supply. The brain is also the biggest burner of glucose in the human body.
Some people might say that they eat candies to feel happy. And they are not wrong. Sugar increases the release of neurotransmitter serotonin, which gives a person the happy feeling. The catch is that sugar also causes the release of insulin which eventually normalizes the glucose level, and when glucose is back to relatively low levels, we will again strive to take sugar just to feel happy again. This may lead to a vicious circle of constantly eating sweets just to feel good. The result is overeating and possible addiction.
We all know how much children love sweets and sugar. That love is not a consequence of habits and upbringing, though. Recently, researchers have found out that childrens’ love for candies is caused by the biology of their brain. The concentration of neurotransmitters and their receptors is different in children compared to adults. That difference slowly reduces while we grow older. The problem is, sugar addiction can be formed early in childhood and stay for the rest of life.
Another problem with sugar addiction is the fact that the human brain reacts differently to the different types of sugar that we ingest. There are big differences between brain reactions to glucose and fructose. For example, our body will need much less glucose to feel good and to trigger the impulses that tell us to stop eating. With fructose the situation is rather different. The human body needs much more fructose to suppress eating.
Researchers from Yale School of Medicine discovered this phenomenon using functional magnetic resonance imaging analysis. They conducted their study on healthy non-obese subjects. The scientists used fMRI to detect different brain reactions on glucose and fructose. After taking glucose, there was a reduction of blood flow in brain areas responsible for appetite, the reward system, and motivation. It also caused immediate satisfaction. Fructose ingestion did not cause these changes in blood flow.
The problem is that fructose is often used in modern food and drinks. Since the human brain can’t regulate fructose ingestion properly, it may lead to food-seeking behavior, overeating, and eventually obesity.
There is a number of different cells in the human brain, each with a different set of functions. Glial cells surround neurons and provide them with support. One type of glia cells are astrocytes, which play an important role in creating the blood-brain barrier. The blood-brain barrier controls the movement of substances between brain tissue and blood in both directions. New research studies show that the functions of astrocytes can be controlled by enzymes such as insulin and leptin.
Researchers at the Technical University of Munich found that astrocytes play an important role in glucose intake. They have insulin receptors on their surface that react to glucose in blood. PET scans indicated that insulin can interact with astrocytes and regulate their permeability to glucose that will result in differences in the brain levels of glucose. When astrocytes in the parts of the brain responsible for appetite get activated, this leads to the feeling of satisfaction. However, when these astrocytes are not reached by glucose, they do not get activated and the person will continue to strive for glucose.
Despite resent discoveries, sugar addiction and especially its mechanisms of action in the brain remain poorly studied. A better understanding of this phenomenon may pave the way to more effective therapeutic interventions aimed at preventing obesity.
García-Cáceres, C., Quarta, C., Varela, L., Gao, L., Gruber, T., et al. (2016) Astrocytic Insulin Signaling Couples Brain Glucose Uptake with Nutrient Availability. Cell, 166 (4): 867-880. DOI: 10.1016/j.cell.2016.07.02
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