Caloric Deficit and Heart Rate Variability
A possible use case for Normalized HRV, which you can find in HRV4Training Pro (https://hrv4training.web.app/)
A few weeks ago I wrote a blog about Normalized HRV, introducing a new feature we have released in HRV4Training Pro, where you can look at your HRV normalized by your resting heart rate.
While this type of normalization seems necessary for population-level analysis, at the individual level it is somewhat less clear if we gain more insights by looking at normalized HRV as opposed to just looking at HRV or heart rate in isolation.
In my previous blog, I mentioned how I was going to pay more attention to normalized HRV in my own data and see what considerations I could add.
Well, thanks to a little dieting (a classic Christmas project for me), I do have some interesting data to report.
In particular, I think it could be more useful to look at normalized HRV in situations in which we have abnormal reductions in resting heart rate. We’ll see some data soon, but first, let’s add some context.
What to expect with reduced caloric intake?
Common thinking when it comes to a stressor, is that HRV should reduce. This is exemplified by “artificial intelligence” getting it wrong, as shown below:
The reality of caloric deficit is however quite different. Both in the scientific literature and in my own experience, often the opposite happens, i.e. HRV increases, and in particular resting heart rate is highly suppressed, with reduced caloric intake.
There are simple ways to study this, for example, ramadan offers an opportunity to study fasting without forcing people to do anything different from what they’d do anyways, and most ramadan studies do report lower heart rate and increased HRV. Other studies more explicitly targeting caloric deficit, show the same.
Possibly, during a caloric deficit, the body may reduce its metabolism as an energy conservation mechanism, which can also lower heart rate. The body might think you are starving and therefore shift towards energy saving. It is unclear to me if this means higher parasympathetic activity (higher HRV does not mean higher parasympathetic activity), or if it is via other mechanisms, but the result is a marked suppression in resting heart rate, typically. Maybe the change in HRV is just a byproduct of the change in heart rate, something I will discuss again later.
Normalized HRV: a better tool?
Below is my resting heart rate in the past week, since I’ve been restricting caloric intake, which is markedly reduced, with a suppression well below my normal range (and well below what I had experienced as acute drops in the past months):
and here is my HRV for the same time period:
According to the data above, I am doing great. Wearables might advise you to go and smash it, due to overly simplistic interpretations of your resting physiology typically used in made-up scores. However, the data above got me thinking. If my resting heart rate is abnormally low and my HRV remains within my normal range, it means that my HRV is in reality reduced, since a lower heart rate means there is more room for variability.
This seems a great case for Normalized HRV, i.e. a way to look at changes in HRV that accounts for how resting heart rate is also changing.
When looking at the data we indeed see that Normalized HRV (which you can pick as HRV:HR in HRV4Training Pro) shows data that better reflects my current situation (i.e. quite tired and low on energy, due to the caloric deficit).
In other cases in which I restricted calories, I dealt better with it than I am currently doing, feeling better and training harder. In those instances, my HRV was abnormally high, as opposed to the current barely normal values. Hence there is information in what HRV is showing, which is not just a change in response to the change in resting heart rate. This means that Normalized HRV would have still been within normal range back then, and not suppressed as shown for this past week, once again reflecting my subjective feeling and capacity to take on additional stress.
Normalized HRV seems a better proxy of our response even at the individual level, and not only at the group level. Food for thought and something to keep experimenting with.
If you use HRV4Training, you can find HRV:HR (Normalized HRV) in HRV4Training Pro, under Overview. If you do not use HRV4Training, maybe it’s time to start.
Thank you for reading and I hope this was informative.
Marco holds a PhD cum laude in applied machine learning, a M.Sc. cum laude in computer science engineering, and a M.Sc. cum laude in human movement sciences and high-performance coaching.
He has published more than 50 papers and patents at the intersection between physiology, health, technology, and human performance.
He is co-founder of HRV4Training, advisor at Oura, guest lecturer at VU Amsterdam, and editor for IEEE Pervasive Computing Magazine. He loves running.
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spot on with this analysis! i've been seeing the very same thing with me and other friends for 3+ years (consistently). the functional medicine view of these events is thru the lenses of the freeze state of the nervous system, which you did touch on, the body is trying to conserve energy. crystal clear example when higher hrv does **not** mean one ought to push harder.