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Brian is a 30 year old YouTuber who drinks alcohol once a week.
Recently, he’s noticed trouble sleeping, and thanks to a recent smart watch purchase,
he’s become more aware of his heart rate. His resting heart rate usually stays around 75 beats
per minute. This is within the normal range, but he’s a young guy. He knows that a low resting
heart rate is indicative of good cardiovascular fitness, so he decided to do something about it.
Assuming a relationship between the alcohol, sleep, and heart rate,
Brian decided to give up alcohol cold turkey and tracked his results on his smart watch.
Within a few days, he noticed a trend in his resting heart rate. 75 beats per minute became 70,
then 65, then eventually down to 60 beats per minute where it hovered.
Wanting to keep the health improvements going, Brian enrolled in a local gym and
started exercising multiple times per week. As his body adapted to exercise,
his resting heart rate dropped even lower, eventually getting into the low 50s and high 40s.
Brian’s case got my attention because he made two changes around the same
time that are both known to result in a lowered resting heart rate. So how much
of his new heart rate was from quitting alcohol, and how much was from exercise?
[intro]
Resting heart rate depends on a lot of factors, and everything from fitness, disease,
alcohol and caffeine consumption can change it. Brian is otherwise healthy and doesn’t take any
medications that affect his heart rate. So his resting heart rate is straightforward.
The heart’s main pacemaker is the sinoatrial node, sino- for sinus, -atrial for atrium.
It’s a bundle of heart muscle cells that send electrical impulses through the rest of the heart.
This thing keeps the heart beating at a steady 60 to 100 beats per minute but of course,
it can be sped up or slowed down.
The SA node is influenced by the two branches of the autonomic nervous system,
that branch of the nervous system you don’t consciously control. These
influences can be part of the sympathetic nervous system and make it beat faster,
or part of the parasympathetic nervous system which makes it beat slower.
In a healthy person, heart rate is ultimately the result of these two influences,
the speed up and the slow down.
The main nerve we care about in this video is the awesomely named vagus nerve. It comes off
the brainstem, down the neck and hooks up to the sinus node and the atrioventricular node.
If this thing is doing its job, it’ll keep the heart rate comfortably low.
So let’s get to our case study. Our data is coming from his Fitbit,
and according to a community moderator on the Fitbit message boards, the device measures
resting heart rate both during sleep /and/ during inactivity (when no steps are detected).
Heart rate changes based on which phase of sleep you’re in, so they sprinkle some
algorithm on top of it to come up with a number. It doesn’t matter exactly how
they get it as long as it’s consistent, since this is the number we’re comparing over time.
So, after you drink alcohol, your heart rate speeds up. This is something that
happens in nearly everyone, and quite a bit of research has looked into it.
A review from 2011 looked at research that investigated the short term response of alcohol
consumption in healthy, nonalcoholic participants. While all the studies measured outcomes in
slightly different ways, 6 studies found an association with alcohol consumption and
increased heart rate while 3 found no change. None found that alcohol slowed down HR.
We’ve known about this association for a long time, so to get a more nuanced understanding,
researchers started studying the balancing act between the sympathetic and parasympathetic
influence on heart rate. And each study measured that balancing act differently.
One of the most common ways to study that balancing act is to look at vagal tone,
how well the vagus nerve can conduct an electrical signal and slow down heart rate.
And you can see it in action if you hook someone up to an ECG,
a device that measures the electrical activity of your heart. That classic little waveform
you’ve seen on medical TV shows is a sinus wave, although sometimes it's really bad. This indicates a normal rhythm where the sinoatrial
node is operating as usual. And it’s usually beeping along like a perfectly constant metronome.
But in reality, no heart beats like a perfect metronome. Sometimes it’s extremely consistent,
sometimes it’s off by a few milliseconds.
We measure that with a metric called heart rate variability. I’ll explain:
This circle is clicking at a constant rate. If our sinus wave had zero variability,
it would land perfectly in the middle every time. But now, let’s introduce
low variability. It’s still pretty steady, it’s only off the mark by a few milliseconds. Now,
add even more variability and it’s all over the place.
It’s still firing at a constant rate, but the heart beat is far from a perfect metronome. Now,
I know it seems counterintuitive, but for most health metrics, a /high/ variability is associated
with good outcomes. It shows that your sympathetic and parasympathetic systems are well matched.
Back in that meta analysis, 6 of the 8 studies of healthy, nonalcoholic participants concluded
that alcohol consumption was associated with an overall decrease in heart rate variability.
The heart acted more like a consistent metronome, indicating that parasympathetic activity was lower
and sympathetic activity was the main system keeping pace.
2 of those studies also concluded that sympathetic activity was /increasing/.
The main takeaway is the same though — alcohol speeds up heart rate because
sympathetic activity dominates over parasympathetic activity.
So far, we think that’s because alcohol inhibits the main parasympathetic input.
So alcohol doesn’t necessarily speed up heart rate, it inhibits its slowing down.
This change in heart rate variability also explains
why chronic alcohol consumption affects your quality of sleep.
A study from 2006 looked at 28 male participants:
14 alcohol dependent and 14 controls and recorded their vagal tone with an ECG
and sleep quality with an EEG for a single night sleep study. They found that alcohol dependent
subjects had decreased vagal tone right before falling asleep /and/ while they were sleeping.
They also found that alcohol-dependent participants self reported worse sleep
quality and more sleepiness and fatigue in the morning. Kind of like waking up with a hangover.
Got it. Alcohol consumption screws up sleep quality.
So after Brian quit drinking, his heart rate plateaued around 60 beats per minute.
That’s when he started getting serious about exercise and started doing Crossfit and running.
For those unfamiliar, Crossfit is mostly strength training but does have a cardio component to it.
Now, according to a 2018 meta analysis, /just/ strength training doesn’t impact
resting heart rate. It has plenty of other benefits obviously, but aerobic exercise
like running or bike riding are the most impactful for reducing resting heart rate.
The biggest adaptation is an increase in cardiac output,
equal to heart rate times stroke volume. Basically how much blood your heart is pumping per minute.
As you do more intense aerobic exercise,
your body has to be able to deliver more blood to adapt to an increased demand for oxygen delivery.
You can’t increase your maximum heart rate, so the only way to increase cardiac output
is to squeeze more blood out of your heart with each pump, or increase stroke volume.
That’s because your body will increase the volume of blood in your body by
adding more plasma. Red blood cells and exercise is a whole video by itself.
To accommodate more blood per pump, the chambers of the heart muscle cells have
to stretch a bit. And this stretch on the heart muscle allows them to contract more forcefully.
This ultimately squeezes out more blood per heart beat.
If you want to get nerdy, this whole stretch mechanism is called the Frank Starling Law.
It’ll probably be on the test.
Bringing it all together, Brain’s resting heart rate kept going down
because his improved fitness caused his heart to pump more blood per pump.
That allows the heart to pump less frequently to deliver the same amount of blood at rest.
Finally, we can’t untie alcohol and exercise from all the other factors in one’s life like stress.
Chronic stress elevates resting heart rate, but that’s harder to get data for.
Researchers /have/ studied whether endurance exercise has any effect on parasympathetic
activity, but the evidence so far says it doesn’t. Interestingly, the only known
exception might be yoga, which is probably a testament to its ability to reduce stress.
But I want to hear from you all. If you drink, do you notice a change in your heart rate?
How about the quality of your sleep? Let me know in the comments.
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