I come from a family of skiers, and I’ve been skiing pretty much since I was diagnosed with type 1 diabetes mellitus in 1992. Although I love skiing, it can be a giant pain for a diabetic. The tight constricting ski boots can be extra painful and I never understood why I was constantly hyperglycemic when I was exercising six hours a day. Why wouldn’t my blood sugar budge below 200 mg/dl for large periods of time, and then suddenly plunge to 60 mg/dl? Diabetes was again threatening to take some of the fun out of skiing.
The scientist in me set about to understanding why I am so insulin resistant at altitude. The answer of course boils down to basic physiology, and I’ll do my best to lay it out for you.
Your body at 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 roughly linearly.
Now, our bodies have ways to adapt to being at high altitude and having less oxygen, and there’s short term adaptation and longer term adaptation. For the purpose of explaining altitude’s effect on blood glucose, I will focus on the shorter term responses.
Hyperventilation and Hypoxia:
If you’ve ever hiked at altitude, you’ll recognize the first one-hyperventilation. With less oxygen you begin to hyperventilate as a means of releasing excess CO2 and increasing the rate of fresh air through the lungs. As PaCO2 drops, 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.
Your body also reacts by increasing hemoglobin per unit of blood. This results in greater 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. Nonessential bodily functions are suppressed such as digestion.*
Altitude and Blood Glucose
Now, here’s where it gets complicated, some diabetics experience hyperglycemia (high blood sugars) at altitude while others experience hypoglycemia (low blood sugar).
Hypoglycemia is a bit easier to comprehend. At high altitude our bodies have to work harder with low levels of oxygen, meaning we theoretically need more energy which means we use more glucose.
There are a couple explanations for this:
- Catecholamines (including epinephrine, norepinephrine, and dopamine) 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.*** Any type 1 diabetic can tell you that stress can make you more insulin resistant. 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
- Test Regularly or wear a CGM
- 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
- Increase your basal rate if your blood sugars are consistently high
- Carry excess Glucose with you
- Drink a lot of water
* “The Physiology of High Altitude,” 2012 Rn Ceus Interactive. http://www.rnceus.com/altitude/phys.html
** Some studies that have linked Catecholamines to increased insulin resistance:
Barth E, Albuszies G, Baumgart K, Matejovic M, Wachter U, Vogt J, Radermacher P, Calzia E.
Crit Care Med. 2007 Sep;35(9 Suppl):S508-18. Review
Boström M, Nie Z, Goertz G, Henriksson J, Wallberg-Henriksson H.
Diabetes. 1989 Jul;38(7):906-10.
*** Brubaker, Patricia L. “Adventure Travel and Type 1 Diabetes: The complicating effects of high altitude.” Diabetes Care 28.10 (2005): 2563.