Is Exercise Actually Good For You? Exploring the HPA-Axis and Cortisol.
For those of you who hate exercising, I’m sure you’re hoping to hear you don’t need ANY exercise! I promise I’m not here to bring entirely bad news for you so read on (and if you don’t like details just scroll to the very bottom under the heading “Then How Much Should I Exercise?”). Let me first start by explaining what the HPA-axis is. In the simplest of terms, it’s the system that helps your body manage the physical and mental stressors of life. HPA stands for the hypothalamic-pituitary-adrenal axis and the primary hormone which is excreted from the adrenals (the “A” part of the HPA system) is cortisol, best known as the stress hormone. This hormone has been demonized over the years but it’s actually crucial for our survival when it’s at appropriate levels. It should be at its highest level in the morning, just as you’re waking up, and continue to get lower as the day goes on, and very low when you’re sleeping. Cortisol’s main function is to modulate blood sugar levels, decrease the body’s immune response, to control salt and water balance, and enhance memory formation. When it is secreted in appropriate circumstances such as the “fight or flight response,” cortisol is GOOD as explained below.
Acute stress reaction=Good
Imagine a pack of llamas are attacking you, your eyes dilate, your heart races and you have more strength and agility than you ever thought possible. All thanks to the HPA system. The adrenals secrete cortisol which suppresses insulin thereby allowing more glucose availability for the muscles and increases blood pressure to get blood to the appropriate areas as quickly as possible.
Chronic Stress reaction = Bad
When cortisol is secreted several times a day, everyday, it’s NOT so good. The classic scenario producing chronic stress is the desk job with piles of boring paperwork and a hounding boss that won’t lay off. The adrenals pump out cortisol all day long because the stress is constant. (Picture Peter Gibbons in the movie Office Space, losing his mind in traffic on his way to work, or just watch it here for a comedic escape). The deleterious effects of cortisol kick so you may start noticing high blood pressure, afternoon sugar cravings, insomnia, fatigue, acne, excess abdominal fat, decreased libido and more specifically for women, excess hair growth called hirsutism, irregular menstrual cycles and infertility.
Why exercise is good (AKA: why you should get off your butt!):
Low intensity exercise (such as walking at comfortable pace), decreases cortisol levels but moderate intensity doesn’t seem to increase or decrease cortisol levels(1). Examples of moderate activity can be found here. Moderate activity also has been shown to have an affect on the HPA-axis helping to blunt the body’s response to cortisol in stressful situations (this means it helps the body to not produce as much cortisol, which is good if you’re chronically exposed to stress)(2,3). Daytime moderate exercise decreases nighttime cortisol levels helping to support the body’s natural healthy rhythm of cortisol which should be highest in the morning and lowest at night, preventing that “wired but tired” feeling right when your head hits the pillow (4).
How exercise may be bad
The most comprehensive study on this issue I’ve read so far talks about the potential deleterious effects of strenuous exercise in some populations.High intensity and long durations of exercise have been shown to increase cortisol levelsin the blood and also decreases the immune system(5,6,7,8). This study shows that high intensity exercise is primarily what increases cortisol compared to the actual duration of exercise.
This study shows an increase is abnormal heart rhythms (arrhythmias) following strenuous exercise in cross country skiiers. This one shows abnormal thickening of the right ventricle of the heart with multiple extreme endurance events over several years, which predisposes people to getting the arrhythmia as mentioned above. This one shows accelerated coronary artery calcifications (meaning increased risk of having a heart attack) in marathon runners. Specifically the report states that the amount of miles a subject ran every week was not associated with increased coronary calcifications but the amount of completed marathons was related.Therefore running a marathon here and there in your lifetime is unlikely to put you at risk, but it’s possible that the endurance sport addict completing several races a year may be doing more harm than good.
Then how much should I exercise?
I think the main point here is that every person is so different in how much exercise they can tolerate and this will differ depending on the season of life. For those of you who hate exercising, think about the phrase I left with on the last blog, “It’s not about how much exercise you can possibly get, but getting the least amount possible to obtain benefits.” That’s ok! Take a 30 minute stress relieving walk everyday. (You’ll learn to crave this rather than dread it) and try to get at least 20 min of moderate activity four times a week (getting your heart rate high enough where it’s difficult to have a conversation). This type of exercise decreases cortisol and helps to support a healthy HPA-axis by:
Lowering blood pressure
Decreasing sugar cravings, especially in the afternoon
Eliminating the “wired yet tired” feeling at night
For those that love exercise, be mindful of what the rest of your life looks like. It’s probably fine to train for an endurance event like a marathon or triathlon if you have:
Low stress
Get 8 hours of sleep EVERY night
Eat right
Have adequate rest days
If you find yourself exhausted and it feels like you’re running through water when training, you probably need to BACK OFF!
The caveat of this entire thing should be that endurance events likely need to be few and far between because of the risks of heart disease and elevated cortisol levels from overtraining. I will say the GOOD part of high intensity exercise is it has been shown to help blunt the response of the HPA-axis allowing for less cortisol to be produced in stressful circumstances, but take your overall health into consideration when you decide how much exercise to get every week.