The brain is the most energy-demanding organ in the body. It is composed of a network of nerve cells (neurons) that is constantly working. Just as the muscles need energy to perform, the brain also relies on a constant supply of fuel in order to sustain a high level of function. During athletics, the fuel your muscles use depends upon the type of activity. The energy used by the muscles can be from glucose, but there are also other options for fuel, like fat oxidation.
Just as muscles can use different fuel sources, so can the brain. The brain can use glucose through the generation of adenosine triphosphate (ATP). However, the brain lacks the ability to store glucose. Thus, when glucose is not available in circulation the brain is unable to use it for fuel. This is problematic for individuals that are not metabolically flexible and rely on a continuous supply of exogenous glucose for fuel. For these individuals, when the level of circulating blood glucose falls, the rate of cerebral glucose metabolism will decline as well. However, when individuals are metabolically flexible, they have the ability to adapt to changing conditions and utilize a different fuel source for the brain when glucose is not available. This alternative fuel source is ketones. Ketones are produced by the liver and do not require an exogenous fuel source so they can be used to power cognitive function while maintaining a low and stable blood sugar (which is metabolically healthy). The ability to switch back and forth between utilizing glucose or ketones at different times improves metabolic health and increases the number of available substraights to power cognitive function. This article will detail the pros and cons of each substrate (glucose and ketones). It will also feature how to manage blood sugar to improve metabolic flexibility and build intuition to help inform fueling choices for health and performance.
Fuel Option 1: Glucose and the Brain
How much glucose does the brain use?
The amount of fuel needed for the brain depends upon the individual and their activities. Having blood glucose that is too low (hypoglycemia) or too high (hyperglycemia) will hinder athletic and cognitive performance in individuals that are reliant upon glucose as fuel. Just as physically complex activities require more fuel than effortless physical tasks, more demanding mental tasks require a greater amount of glucose than simple mental tasks. This is due, in part, to an increase in glucose demands when an individual is under mild stress.
Low blood glucose leads to a loss of mental focus and feelings of weakness. Signs of low blood glucose (hypoglycemia) are dizziness, shakiness, headache, and irritability. Clearly, none of these symptoms are desirable when trying to think clearly, maintain focus, and execute physical tasks, such as sinking a putt on the 15th hole of a golf course. In addition, it is problematic for athletes to run out of available energy (glucose) during a sporting event. This can happen because an athlete is not metabolically flexible and did not consume enough glucose during the athletic event. The deleterious symptoms, and subsequent performance detriments, that accompany hypoglycemia can be avoided by maintaining steady blood sugar levels. This maintenance can be achieved in one of two ways: 1) by consuming foods that sustain blood glucose, 2) by improving metabolic flexibility so that your brain can use ketones as fuel when glucose is not available. The first option involves constantly supplying the brain with glucose in order to keep your focus. The second option enables you to power cognitive functioning when exogenous fuel is not available.
High blood glucose (hyperglycemia) can be detrimental to athletic performance due to reactive hypoglycemia. Reactive hypoglycemia occurs a few hours after a high-carbohydrate meal. The sudden rise in blood glucose caused by carbohydrates triggers the body’s release of insulin. Insulin release is a healthy response that initiates rapid glucose uptake by tissues, which either stores the glucose as glycogen or uses it for energy production. An overproduction of insulin, however, causes a rapid decrease in blood sugar below normal levels (reactive hypoglycemia). The symptoms of reactive hypoglycemia include difficulty concentrating, dizziness, anxiety, weakness, and irritation. Clearly, these symptoms are not conducive to optimal athletic or cognitive performance.
Chronically, high blood sugar spikes can be problematic as well. A study in 2008 investigated healthy, non-diabetic individuals and found that women with higher fasting glucose levels and post-meal glucose levels had a significant decrease in performance on episodic memory tests (recall and recognition testing). An example of a common episodic memory test is visually recreating a picture you saw earlier, or verbally repeating a list of words you were previously told. This is an important trait for athletes in many sports. Whether you need to recall plays, remember the layout of a course, or recall the green’s speed, accurately remembering these traits can allow you to prepare for the next part of a course. The link between glucose and episodic memory test performance found in the 2008 study may be explained by the hippocampus. Episodic memory is mainly affected by the hippocampus (a region of the brain), and the hippocampus is highly susceptible to metabolic changes. Thus, changes in blood glucose influence the hippocampus, which, in turn, reduces recall and recognition.
Fuel Option 2: Ketones and the Brain
In the preceding paragraphs it was mentioned that cognitive and physical decline are associated with low blood glucose, so how are keto athletes able to cognitively function? When you reach nutritional ketosis (circulating ketone levels >0.5 mmol/L) you set off a cascade of metabolic adaptations. Under normal (non-ketogenic) conditions, the liver will produce small amounts of ketones and the brain will rely primarily on glucose for fuel (because it is available). In a state of ketosis, however, when the body’s primary fuel source is fat, the production of ketones is increased. In this case, the brain can utilize ketones to function. Ketones are water-soluble compounds produced by the liver. There are three different types of ketones (beta-hydroxybutyrate, acetoacetate, and acetone) but beta-hydroxybutyrate and acetoacetate are the most abundant.
The use of ketones for brain power allows keto athletes to adequately fuel their cognitive function while keeping their blood sugar levels low and stable (which is metabolically healthy). Athletes on a ketogenic diet have the unique advantage of non-competing fuel sources, as opposed to athletes that rely on exogenous glucose for fuel. Prolonged high-intensity exercise depletes glucose and glycogen stores, which is problematic for athletes that rely on glucose because it depletes the fuel source for both the muscles and the brain. Keto athletes do not have this problem. During prolonged exercise, the blood ketone concentration is not depleted, instead, it rises. This ensures the brain a stable fuel supply.
Luckily, people do not have to choose between using exclusively glucose or ketones to power the brain. Metabolically flexible individuals have the ability to switch back and forth between fuel sources. In this way, athletes have the ability to adapt to changes and still meet the metabolic demands of their activity. Becoming metabolically flexible takes time and is achieved by keeping glucose low and stable over time while reducing insulin excess. Tools like GOOD IDEA and continuous glucose monitors (CGMs) can be valuable resources to help regulate blood sugar and provide biofeedback in order to help keep glucose levels low and stable.
Regulating Blood Sugar
When trying to maintain stable blood sugar for enhanced metabolic flexibility and athletic performance, the duration and type of exercise needs to be taken into account. This means having enough fuel to power the physical and cognitive demands of your sport while also not overshooting and then causing a spike and crash (which is not healthy for the brain or body). Each sport’s energy requirements are different. For sports like golf, which require sustained, low intensity (as defined by <60% VO2 max), significant spikes and dives in blood sugar can lead to performance-altering consequences. Consuming foods that maintain a stable blood sugar can help to power the physical and cognitive functions needed to sink puts on the 15th hole. While everyone has a unique glucose response to food, some foods are notorious for producing blood sugar spikes. Swapping highly processed foods, sugary drinks, or candy for high-protein options can help to regulate blood glucose. Additionally, using a functional beverage like GOOD IDEA has been scientifically shown to reduce post-prandial blood glucose excursions by 20-30%. By reducing glucose spikes we help to keep our blood sugar steady, allowing us to avoid the subsequent energy crash during a time when performance and alertness are needed.
A CGM can also help athletes become more aware of changes in their blood glucose levels. The feedback provided by a CGM can work as a monitor to avoid potential hypoglycemia. A CGM allows an athlete to see when/if their glucose goes too low during competition. By monitoring these decreases in glucose an athlete can learn appropriate times when they might need a little exogenous fuel to maximize performance and avoid the detrimental symptoms that accompany hypoglycemia (difficulty concentrating, dizziness, irritability, shakiness). This knowledge can help you make educated decisions and help maximize health and performance.
Author: Dr. Colleen Gulick