How Exercise Impacts Blood Sugar and Diabetes Control
Diabetes & Exercise 101: Understanding How Exercise Affects Blood Sugar Levels
There are many variables that can affect your blood sugar response to exercise. Factors such as the type of exercise/energy system being used, blood sugar at the start of exercise, your overall training status (just starting or seasoned pro), prior exercise, type of insulin used, and timing of your last meal are all important considerations. Additionally, external factors such as the temperature and other environmental conditions can affect your blood glucose levels. Furthermore, if you are a female athlete special considerations need to be considered to phase of menstrual cycle, and pregnancy.
Individually, each of these factors can be a long discussion. The purpose of this blog post is to focus on how the type of exercise you perform affects your blood sugar levels.
Before you can begin to tackle proper diabetes management as an athlete with diabetes, you first must understand how exercise affects your body, specifically your blood sugar levels. As you have probably found out in your athletic career, not all exercise is created equally. Likewise, not all exercise affects your blood sugar levels equally as well. Let's explore this further below.
Types of Energy Systems Used in Exercise
Your body has three distinct energy systems that supply your muscles with energy in the form of adenosine triphosphate (ATP). ATP is a high energy compound found in cells that directly fuels muscle contractions/work. It is the currency by which your muscles use to contract. Without ATP, your muscles cannot contract and you cannot exercise.
The three energy systems work on a continuum to allow your body to move. These systems are the ATP-CP system, lactic acid system, and aerobic system.
1. ATP-CP System: Short & Intense
The ATP-CP is an energy system your body uses for very short lived activities. It uses ATP that is already stored in muscle and creatine phosphate (CP) which rapidly restores ATP. It is anerobic, meaning it does not require oxygen for ATP production. It is important to note the creatine phosphate does not directly fuel activity, but the energy released from its rapid breakdown is used to synthesize ATP. This system can supply energy for activity for ~ 10 seconds before being depleted. Activities such as power lifting, short sprints, explosive manuevers in basketball/football/soccer.
--> Glucose Effects: Since this energy system does not rely on glucose, it has no direct effect on your blood sugar levels. However, it is important to note that the type of activities that use this energy system produce stress hormones (cortisol, adrenaline, etc) which can result in elevating blood sugar levels. When performing all out activities such as power lifting, sprinting, or other related short burst activities it is important to monitor how these exercises affect your glucose levels and to monitor for high blood sugar levels.
2. Lactic Acid System
After you have used the fuel from the ATP-CP energy system the next energy system your body uses to fuel activity is the lactic acid system also termed anaerobic glycolysis. The lactic acid system supplies fuel for activities lasting 10 seconds to approximately 2 minutes. Similarly to the ATP-CP energy system, the lactic acid system produces energy without the need for oxygen; therefore, it is also considered anerobic. Gylcogen, the storage form of glucose, is stored in your muscles. The lactic acid system produces energy through a process called glycogenolysis. Your body breaks down glycogen through a metabolic process called glycolysis which forms ATP and a byproduct called lactic acid.
Why does the lactic acid system only provide energy for ~ 2 minutes? Great question! The amount of ATP the lactic acid system is relatively small, only 3 ATP from each glucose molecule. This is a relatively small amount compared to aerobic respiration (the third energy system) which produces 37-39 ATP per glucose molecule. Consequently, this system cannot supply energy for prolonged exercise like the aerobic system can. Activities that commonly use the lactic acid system include: high interval intensity training (HITT), 800 m sprints, 200m swimming events, and start/stop sports such as basketball, lacrosse, field hockey, and ice hockey.
--> Glucose Effects: This energy system does utilize glucose, but again can result in the release of stress hormones which can initially cause an increase in blood sugar levels. Longer term effect on your blood sugar include lower blood glucose levels for up to 24 hours after exercise. Furthermore, as your body adapts to this type of exercise it becomes better at processing lactic acid and glucose which increases your insulin sensitivity.
Aerobic System
After 2 minutes of exercise, your body relies on your main energy system, aerobic respiration. This energy system relies on oxygen and can create ATP from a seemingly unlimited supply of carbohydrates, fats, and proteins. The main macronutrient used for aerobic respiration is carbohydrates; however, fats and protein can also contribute as an energy source via various biochemical reactions in the body. Muscle glycogen supplies more of this carbohydrate compared to circulating blood sugar levels (the reading you see on your continuous glucose monitor). Hormones like adrenaline can mobilize fats from fat cells to use as fuel for aerobic respiration during less intense activities such as walking. It is important to note that insulin has a negative feedback loop on the release of fats. This means that higher levels of insulin in your body will decrease/impair the release of stored fat from fat tissue limiting its use for aerobic respiration. Activities that commonly use the aerobic system including: running, soccer, cycling, backpacking, rowing, cross country skiing, etc.
--> Glucose Effects: Since this system heavily relies on glycogen and carbohydrates, it will result in a decrease in blood sugar levels over time; therefore, it is important to monitor your blood sugar during prolonged activities for low blood sugars. Refer to the athlete toolkit for some tips/tricks to prevent low blood sugars during prolonged activities.
Concluding remarks
Understanding normal exercise physiology is imperative to understand how exercise and your body's natural response to exercise affects your blood sugar levels. You will use all three of these energy systems at varying degrees in your sport. Taking note of which energy system you predominately use can give you a heads up on how to proactively control your diabetes and maximize your performance.
~ Harness the power of your journey, and never stop pushing boundaries. ~
Dr. Richard Edgar
***Disclaimer: Always consult with your healthcare provider before making any changes to your treatment or management plan.