By: Michael Callahan
According to Schmidt (2015), 47 pounds of cane sugar and 35 pounds of high-fructose corn syrup is consumed by the average American per year. The U.S. Department of Agriculture (USDA) claims the average American consumes 156 pounds of added sugar per year. It is no secret that sugar consumption is way up. Added sugar is found in all of the packaged and processed foods we nibble on today. This is often being consumed in the form of fructose, which is absorbed much more rapidly than glucose. It is essential to understand that sugar, or glucose, is also necessary for proper brain functioning. According to Mergenthaler (2014), the human brain consumes 20% of glucose-derived energy.
At 5.6 mg glucose per 100 g human brain tissue per minute, the brain is considered the main consumer of glucose, and yet weighs only 2% of the body weight. Metabolism of glucose generates ATP, which provides the fuel for proper brain functioning, as well as provides a foundation for the generation of neurotransmitters and cellular maintenance. This exemplifies the importance of proper regulation of glucose metabolism within our bodies. Considering how sugar affects the physiology of our brain and body can give us insight into the possible impacts of sugar overconsumption.
To better understand how it is processed in the body, let’s first define sugar. All sugars are carbohydrates, or compounds containing Carbon, Hydrogen, and Oxygen (C, H, and O). Sugar, in its simplest form, is considered a monosaccharide. Glucose and Fructose are examples of monosaccharides. Together, Glucose and Fructose combine equally to create the polysaccharide sucrose. That wonderful white sugar we put into our coffee is known as sucrose (C12H22O11). Other forms of sugar include lactose, dextrose and starch.
Sensations from sugar can be experienced from pizza or refined starch, as it is converted to sugar in the body. The faster the starch is converted to sugar, the quicker the brain gets the reward for it. Highly refined foods and fructose are absorbed faster, and are therefore more desirable to us because they bring immediate pleasure. This often leads to overconsumption, leading to the body being flooded with more sugar than it can handle. Whole grains are digested more gradually and in an orderly fashion. According to an Australian Psychologist Robert Mcbride, there is an optimum concentration in food and breaks, known as the bliss point, at which sensory pleasure is maximal. Sugar has the highest bliss point, which can be seen by the overconsumption of sugar as opposed to protein or fat.
Energy homeostasis and food uptake is regulated by a complex network communication between the peripheral (ex: stomach, gut, liver, pancreas) and central (brain and spine) nervous system. This regulation directly affects the endocrine and metabolic systems of the peripheral and central nervous system. Sugar affects both the peripheral and central nervous system which both work simultaneously to process the consumption of sugar.
Energy homeostasis and how and when we decide to eat is regulated by our peripheral sweet taste and sugar detectors on our tongue. Within the oral epithelium and in the gut are taste-signaling mechanisms. These mechanisms play a role in sugar detection as well as regulation of intestinal and pancreatic hormone secretion (for example, insulin when sugar is digested) (Ochoa, 2014). It is important to understand how both the peripheral and central nervous system digest, absorb, and process sugar separately to understand its effect on the body as a whole.
Sugar consumption provides our body with glucose, the most important energy source in our body. Digestion begins in the mouth, typically before the food has even entered. The sight of sugar alone typically elicits the release of salivary amylase from our taste buds, which creates the mouth-watering sensation. During the digestion process, sugar is broken down from its complex form into monosaccharides such as glucose, fructose and galactose. After just minimal digestion in the mouth, the sugars travel down the esophagus to the stomach where hydrochloric acid (HCl) neutralizes salivary amylase and continues to break down the sugars. It is then passed on to the small intestine, where most of the digestion occurs. Various enzymes lie in the intestine and target specific sugar molecules. Enzyme Lactase, for example functions to break down lactose, whereas enzyme sucrase functions to act on sucrose. These monosaccharides are then passed on and stored in the liver for processing and distribution. As sugar is being digested, the pancreas detects this surge of sugar and releases insulin. The role of insulin is to regulate blood sugar levels in the body. Insulin’s role is to remove glucose from the bloodstream in order to maintain homeostasis, and glucose is stored as glycogen, our reserve energy fuel.
Glycogen is stored in the liver and in muscle and is our primary source of energy. This exemplifies the importance of sugars (carbohydrates) in our diet. Rapid overconsumption of sugar however, signals to the pancreas to release excess insulin resulting in hypoglycemia. This is also known as the “sugar crash”, or having low blood sugar from excess glucose uptake in the bloodstream by insulin. This is where the brain comes into play. The low blood pressure following the sugar crash communicates to the brain to consume more sugar. This causes a positive feedback cycle to even more consumption of sugar and a drop in blood sugar all over again.
But how is the digestion of sugar processed in the brain? Glucose is essential to brain function. Thinking, memory and learning are all dependent on the consumption of carbohydrates to provide the fuel for our body. It is important to understand the role of glucose in brain functioning to better understand how it processes sugar in our body. Before sugar is even consumed, salivary amylase is being released. This results in the mouth-watering sensation that happens before eating delicious food. Perception of sugar begins immediately on the apical surface of taste receptor cells, located within taste buds in the tongue. There are two primary groups of sugar detectors: G-protein coupled receptors (GPCR), and sugar transporters. Enteroendocrine cells sense sugars through GPCR, which includes the sweet taste receptors type 1, T1R. As sugar is tasted, sucrose binds to the sweet taste receptors T1R2 and T1R3, which promote a dopamine release. These sweet taste buds fire up and send dopamine signals up to the brain stem to the cerebral cortex. This is where our taste is processed. Chemical and electrical signals will then stimulate the brain’s reward system. Food is considered a “natural reward” in the brain. Eating, having sex, and nurturing others are all behaviors that contribute to our survival as a species. It is therefore important to reinforce these behaviors in our brain as pleasurable. This will reinforce us doing these behaviors, and encourage us to repeat. Because the brain requires sugar as its main fuel source, it makes sense that sugar stimulates the brain’s reward system. Simply put: our brain wants sugar. Our brain feels happy when we consume sugar. The brain’s reward system reinforces our consumption of sugar with pleasurable feelings. Sugar provides us with the energy necessary to survive and reproduce. It is innate within us as human beings to want sugar.
Let’s take a further look into the neuroscience behind the consumption of sugar and the reward system of the brain. Two systems in our brain regulate our intake of sugar, including energy homeostasis/the regulation of feeding, and our reward system. The hypothalamus, and the dorsal vagal complex, along with certain structures that are a part of the limbic system are what control our hunger (Ochoa, 2014). This is called the hedonic drive to eat, and works directly with our motivation-reward system. Within the arcuate nucleus of the hypothalamus of the brain, glucose levels and energy homeostasis signal to the peripheral nervous system. Hormones (signals) such as insulin, are sent and received to and from the gut and the brain in an attempt to regulate glucose in the blood. Recent research within the past few decades has indicated there are neuromodulators that regulate these energy and reward pathways. These substances are often orexigenic agents (increase feeding) or anorexic agents (decrease feeding). Within our brain are also glucose-sensing neurons which play a role in neuroendocrine function, nutrient metabolism, and energy homeostasis. Glucose-sensing neurons in the brain contain receptors and respond to peripheral hormones such as insulin (regulates sugar), and leptin (regulates fat), indicating our energy reserve levels.
Our reward system is supported by the mesolimbic dopamine (DA) system in the brain. Dopamine is considered the primary neurotransmitter involved with this reward system pathway. An evolutionary trait known as the mesolimbic pathway is a system within the brain that deciphers our natural rewards for us. Pretend that you just ate a freshly baked cookie out of the oven. This pleasurable sensation creates the release of the neurotransmitter dopamine, by a bundle of neurons called the ventral tegmental area. An area of the brain known as the nucleus accumbens receives this dopamine signal. The nucleus accumbens functions alongside the prefrontal cortex, which dictates motor movement. These areas of the brain work together to tell us how great that chocolate chip cookie was, and that maybe it’s time to eat another one, or two…or three. The prefrontal cortex also activates hormones that processed these pleasurable feelings telling us to remember that for the future.
So far we have learned that sugar is not necessarily a bad thing; it actually contributes to proper brain functioning and homeostasis. Our brain receives sugar as a pleasurable sensation, remembers that sensation, and encourages us to do it again. In a balanced, healthy body these signals are working properly and will communicate to us when it is time to eat, and more importantly, when it is time to stop eating. Overconsumption of sugar, however causes a disruption in our electrical and chemical systems ultimately throwing our bodies out of balance. At this point, you may still be okay with the occasional sugar crash. The entire tray of Christmas cookies is sometimes just worth it. But let’s take a further look at what it means to listen to our brains positive feedback signals to consume more sugar. As mentioned previously, sugar spikes the release of dopamine in the nucleus accumbens. Just like sugar, drugs are involved in the same pathway involving dopamine release. Long term exposure to excess sugar intake changes the gene expression in the midbrain and frontal cortex. An excitatory receptor called D1 experiences an increase in concentration from sugar, whereas inhibitory receptor type D2 experiences a decrease in concentration. The dopamine transporter, a protein pump which pumps dopamine out of the synapse and back in the neuron afterward, is also inhibited by repeat sugar consumption. This experience can be explained very similarly to how drugs are processed. Regular sugar intake causes prolonged dopamine signaling. This means that we ultimately need more sugar to activate the midbrain dopamine receptors like before. Basically, the brain develops a tolerance to sugar and more is needed to experience a dopamine release similar to before.
Data supporting sugar and its effect on the brain is currently a hot topic. The correlation between the rise of obesity rates and high fructose corn syrup consumption, and sugar in general, poses an interesting hypothesis on sugar’s effect on the brain and the body. Studies are becoming more common in an attempt to use data to support these claims. Looking at how sugar is processed and digested in our body gives us a new perspective of how it can potentially affect us on a daily basis. From the beloved first cookie, to the crazy sugar rush, to falling asleep upside down on the couch. Regardless of the data to support sugar intake and body function, one of the best (and free) ways to test this is on yourself. Try giving up sugar and take notice of your daily symptoms throughout the day, and then try reintroducing it (hopefully not in the form of an entire pack of Oreos) and see how it makes you feel. As mentioned earlier, carbohydrates and glucose play an extremely important role in energy and brain function. Our memory, focus, alertness are all dependent on glucose to work properly. Within the broad category of carbohydrates, fruits and vegetables and whole grains will provide the body with proper nourishment. It is best to become more aware of what is being consumed in ingredients lists to avoid excess sugar intake. Paying better attention to food labels and added sugar can be very eye opening. Studies on dietary sugars such as sucrose, glucose and fructose and their effects on the peripheral and central systems are becoming much more common. Understanding how each system is affected can help us to better understand how our body is affected as a whole. More data will continue to come out to support claims of sugar’s effect on the brain and body.
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