Is your Low Resting Heart Rate due to Athleticism – or Malnutrition?
As a REDs and eating disorder specialist, I often see athletes, particularly endurance athletes, bragging about their low resting heart rate as if it’s a badge of honor. Longevity experts and fitness influencers are often touting their low resting heart rate as if it’s the ultimate marker of health and athleticism.
In athlete communities, boastful statements like this are common:
“My resting heart rate went down to 35 when I started running marathons.”
“My coach said that I’m fitter this year because my resting heart rate went down from 58 to 42.”
“Ever since I went keto and started Cross Fit, my Apple Watch alarms at night because my heart rate is so low.”
But here’s the truth - sometimes, a low resting heart rate in active individuals is NOT a marker of health. In fact, it can often be one of the first and most severe warning signs of something more dangerous, like Relative Energy Deficiency in Sport (REDs) or an eating disorder.
Unfortunately, the fitness and medical communities often don’t consider this nuance. I cannot tell you the number of times I’ve had a patient with severe anorexia nervosa, for instance, whose heart rate dropped after rapidly losing a large percentage of their body weight, be told their heart rate is low only because they are “in shape” by a medical professional.
A low resting heart rate is known by the medical term, “bradycardia”. If bradycardia is solely due to an athletic training effect, then there are typically no medical concerns. Conversely, if bradycardia is due to malnutrition, it can be potentially life-threatening and predispose an individual to severe medical complications like cardiac arrhythmias, fainting episodes, and even sudden cardiac death.
Given the potential dangers of bradycardia from malnutrition, it’s essential to not automatically assume someone’s low resting heart rate is only a sign of fitness.
Before we get into how to determine if your low resting heart rate is healthy, here are some facts about resting heart rates:
For most adolescents and adults, a normal resting heart rate is 60-100 beats per minute (bpm). Average for males is around 74 bpm, and females around 79 bpm.
Well-trained athletes commonly can drop to the 50s, and even 40s, due to a training effect. This is because their heart has learned to be more efficient and pump more blood with each beat.
Males tend to have slightly lower heart rates (by about 5-10 bpm) than females due to a larger heart size.
MOST well-nourished female athletes have heart rates about 50, and MOST well-nourished male athletes have heart rates above 45.
Bradycardia due to malnutrition can occur regardless of one’s BMI. In fact, some of the sickest, most bradycardic patients we see are in larger-bodied individuals who lose weight large amounts of weight very quickly, often while receiving societal praise
Why does heart rate drop in states of malnutrition?
Inadequate calories can do a number on virtually every organ system in the body. One adaptation is to try to slow down some of the body’s functions, like the heart, as a survival mechanism. It’s almost like the body is trying to hibernate to preserve itself, so it slows the heart down in an attempt to slow you down until you can be better fueled.
How can I tell if my low resting heart rate is due to athleticism or something like REDs?
In these cases, context matters. I’ll present two fictional cases to explain what I mean by context.
Case 1: Sabrina
In the first example, we have Sabrina, a 24-year-old triathlete. Sabrina started training for triathlons about 2 years ago, and noticed that her resting heart rate dropped from about 60 beats per minute to 47 beats per minute (based on smart watch data) over this 2 year period. She eats a wide variety of foods, fuels before, during, and after her training, and has not had any significant weight/BMI fluctuations. She also maintains regular monthly periods, has good energy levels, and has not had any significant injuries despite her training load.
Case 2: Nico
Nico is a 24-year-old triathlete who started training for triathlons about 2 years ago, and noticed that his resting heart rate dropped from 60 beats per minute to 40 beats per minute over this period. He initially started triathlons to lose weight, and dropped from 200 lbs to 135 lbs, a weight loss of about one-third of his body weight. He consistently gets told he looks great by his friends and teammates after his weight loss, and his primary care doctor is thrilled about his weight loss and low resting heart rate. Nico follows a strict, 1800 calorie, low carb diet. However, Nico is struggling with fatigue, decreased libido, GI symptoms, poor concentration, depression, and recently sustained a stress fracture.
As you can see from the above examples, context is the distinguishing factor that helps us determine whether or not Sabrina and Nico’s resting heart rates are healthy (Sabrina), or due to an energy deficient state/malnutrition (Nico). Both are 24 year old triathletes, and both have bradycardia. And yet, there are some potential dangers in Nico’s case that cannot be ignored. In real life, however, often there is a mixed picture where both a training effect and inadequate nutrition are playing a role.
Questions to ask:
Have there been any recent weight changes, or in growing kids/teens, dropping off weight/height/BMI percentiles on growth charts?
Are there any other warning signs for energy deficiency? Common signs include fatigue, poor concentration/mood, injuries, GI problems, hair loss, lost periods for females, decreased libido/ability to sustain erection for males, dizziness with position changes.
What is my baseline resting heart rate and how does it compare to my current resting heart rate? Often, this can be found on notes from prior doctor’s visits.
Things to keep in mind:
If your medical provider, coach, or teammate praises your low resting heart rate, keep in mind that often they do not have proper education on other potential causes of bradycardia, like bradycardia due to improper nutrition
If heart rate drops substantially in the context of an eating disorder or REDs, sometimes there may be an indication for hospitalization for closer cardiac monitoring. For instance, the Society for Adolescent Health & Medicine considers a heart rate less than 50 during the day and less than 45 at night as being “unstable” and needing closer medical monitoring, possibly in an inpatient setting.
What to do if you’re concerned your low resting heart rate may not be so healthy:
Seek a medical evaluation from a REDs savvy physician. Often further testing, like by getting an EKG and labs, are warranted.
One way to test if there’s a malnutrition component to your low heart rate is to work on improving your fueling, ideally by working with a Registered Dietitian savvy with REDs. In these cases, your resting heart rate should come up as you renourish, likely to above 45-50 beats per minute. This is NOT a sign of lost fitness, but rather a sign that your body is feeling better nourished.
Check out some educational resources, with some of my favorites below:
Lane 9 Project (for female athletes)
F.E.A.S.T. (for parents of those struggling with an eating disorder)
