Few things feel better than summiting an epic peak or racing down a mountain on skis, and few things feel worse than a bouncing blood sugar forming jagged peaks across your cgm monitor and ruining your epic day. Some of my favorite outdoor activities take place above sea level, but exercising at high altitude can be a glycemic nightmare for a type 1 diabetic.
Curious as to why this is, I set about to understanding what’s happening on the inside that could be impacting blood glucose at high altitude. The answers I found boil down to physiology and biochemistry, and I’ll do my best to lay it out for you. Please see the end of the article for more research on these topics.
Why do I tend to be hyperglycemic at high altitude?
Your body at High Altitude
There is less oxygen and air pressure at higher altitudes. The earth’s gravity holds oxygen close to the surface, so that half of the oxygen in the atmosphere is found below 18,000 feet. This means that as you climb above sea level atmospheric pressure and oxygen pressure fall.
Our bodies have multiple ways to adapt to existing at high altitude and having less oxygen, and there are short term adaptations as well as longer term adaptations. For the purpose of explaining altitude’s effect on blood glucose, I will focus primarily on the shorter term responses.
“Why am I always winded when working out at high altitude?”
Hyperventilation and Hypoxia:
If you’ve ever hiked at higher altitudes, you’ll recognize the first one-hyperventilation. As you ascend you often find yourself breathing heavily, and you may feel easily winded. For most of us living close to sea level, this is normal. With less oxygen available, you begin to hyperventilate as a means of releasing excess CO2 (carbon dioxide), and increasing the rate of fresh air through the lungs. As PaCO2 (the partial pressure of carbon dioxide in the blood) drops, the pH increases and you may experience respiratory alkalosis. This is reduced over time by renal bicarbonate excretion, meaning the kidneys excrete excess bicarbonate by decreasing hydrogen ion secretion.
It is thought that acute hypoxia (lack of oxygen) inhibits oxidative metabolism during exercise, which forces accelerated glycolysis to meet the high energy needs of contracting muscles. Physical exertion typically reduces blood glucose, but a build up of lactate can actually result in an increase in blood sugar.
Note: Acute mountain sickness (AMS) aka altitude sickness typically occurs at about 8,000 feet and can include dizziness, nausea, headaches, and shortness of breath. A majority of people living at or close to sea level who ascend to above 8,000 feet will experience at least mild symptoms of AMS.
Changes in Sympathetic Tone:
Research conducted at extreme high altitude suggests that the changes in arterial oxygen saturation (basically less oxygen in your arteries) during short-term exposure (<7days) to high altitude increases your sympathetic nervous system activity, which may increase hepatic glucose production (production of glucose in the liver).*
Your body also reacts by increasing hemoglobin per unit of blood. This results in greater oxygen (O2) carrying capacity. This is why many athletes choose to train and or sleep at altitude, a natural kind of doping.
Cardiac output = Heart rate x Stroke volume
It is thought that hemoconcentration elevates catecholamine levels, increasing heart rate and decreasing stroke volume. Elevated levels of catecholamines have been linked to hyperglycemia. Nonessential bodily functions are also suppressed such as digestion, which may also impact blood glucose control at altitude.
Your Blood Sugar at High Altitude
Now, here’s where it gets complicated, most diabetics experience hyperglycemia (high blood sugars), while others seem to experience hypoglycemia (low blood sugar) while exercising at high altitude. Research has suggested that exposure to high altitude increases glycemic response to carbohydrates and decreases insulin sensitivity, but exercise at high altitude appears to increase insulin sensitivity, so to sum it up and over simplify:
Hypoglycemia at high altitude:
Exercise + less oxygen -> higher energy needs -> Use more glucose-> hypoglycemia & all the fun symptoms that go along with it (dizziness rapid heart beat…) -> potentially release more glucose into the blood stream -> hyperglycemia
High altitude -> elevated catecholamines* + increased sympathetic tone + excess cortisol-> release of excess glucose into the blood + insulin resistance -> High blood sugar
- Elevated levels of catecholamines (including epinephrine, norepinephrine, and dopamine) at high altitude have been linked in multiple studies with increased insulin resistance.
- Cortisol seems to be the dominant theory regarding altitude and hyperglycemia. Our bodies also release cortisol at high altitudes , which you may know as the stress hormone. Cortisol helps your body deal with lower levels of oxygen at high altitude by stimulating the production of new red blood cells. Under stressful conditions cortisol will provide the body with excess glucose by stimulating gluconeogenesis, breaking down protein stores and converting it into glucose, which in turn increases blood glucose. Cortisol not only stimulates gluconeogenesis, but also increases insulin resistance, meaning not only are you releasing more glucose but you’re insulin is not acting as effectively, generally leading to hyperglycemia.
Advice for Exercising at altitude- Minimize the Peaks
- Test Regularly. Having a CGM can be very handy when exercising at higher altitudes, especially as it can be difficult to test in really cold climates.
- Always bring back up- Meters and other supplies don’t always work at low temperature, so keep things warm.
- Don’t let your insulin freeze or denature (depending on the climate).
- Contact your physician about changes to your insulin regimen.
- Consider using a temporary bolus if you’re on an insulin pump.
- Carry excess Glucose with you.
- Stay hydrated. It is easy to get dehydrated at high altitude, and dehydration is particularly dangerous for a type 1 diabetic at high altitude.
- Discuss the risks associated with medications for AMS with your physician. (Note: acetazolamide, a common medication taken for altitude sickness, increases risk of ketoacidosis!!!)
- Consider a slower ascent, allowing substantial time for acclimatizing.
- Carry glucagon with you.
- Stay safe and have fun!
For More information, Check out:
Diabetes at High Altitude
Physiology at High Altitude:
*Reeves JT, Wolfel EE, Green HJ, Mazzeo RS, Young AJ, Sutton JR, Brooks GA: Oxygen transport during exercise at altitude and the lactate paradox: lessons from Operation Everest II and Pikes Peak. Exerc Sport Sci Rev 20:275–296,1992
James PR Brown, Michael PW Grocott; Humans at altitude: physiology and pathophysiology, Continuing Education in Anaesthesia Critical Care & Pain, Volume 13, Issue 1, 1 February 2013, Pages 17–22, https://doi.org/10.1093/bjaceaccp/mks047
Studies that have linked Catecholamines to increased insulin resistance:
Boström M, Nie Z, Goertz G, Henriksson J, Wallberg-Henriksson H. Indirect effect of catecholamines on development of insulin resistance in skeletal muscle from diabetic rats.Diabetes. 1989 Jul;38(7):906-10.