On Heart Rate Variability (HRV), Weight Loss and Low-Carb Intake
A case study and some considerations
I’ve collected some interesting resting physiology data in the last month, highlighting once again differences between resting heart rate and HRV - as well as possible misconceptions about HRV - which I’d like to report here.
My data collection protocol is to measure first thing in the morning, after going to the bathroom if needed, and while seated, using the HRV4Training app. I believe this is the most effective way to track our stress response.
As you might know already, I’ve been experimenting more with periodized nutrition for endurance performance. In particular, we have two main events in an otherwise rather standard month:
Caloric deficit (leading to weight loss), starting mid-December.
Switch to a low-carb diet, starting at the beginning of January.
With the caloric deficit and weight loss, my resting heart rate showed a large drop below my normal range, and similarly, my HRV increased. This is an expected behavior, which I discussed in more detail in this blog. Based on this data, I speculate Normalized HRV might be a more useful parameter to track our stress response at the individual and not only at the group level - with respect to just HRV - as normalized HRV accounts for changes in resting heart rate (for example, if our HRV is within normal range but resting heart rate is highly suppressed, our HRV is in fact lower than our normal).
Normalized HRV is something we have added now to HRV4Training Pro for athletes and coaches, so that you can experiment with it as well (try it here, if you already use the app).
Recently, I switched not only to a caloric deficit but also to a low-carb diet, for reasons that are mostly training-related (covered here). The response to this dietary change was a marked drop in HRV, with no change at all in resting heart rate, which remained suppressed below my normal range:
I find the decoupling between resting heart rate and HRV particularly interesting, as it shows that there is information in HRV that is clearly not present in resting heart rate alone.
The change in HRV is consistent with what was reported in a study by Dostal et al. (including also Dan Plews among the authors, who is one of the major experts both when it comes to low-carb diets and to HRV - as well as my coauthor for an upcoming book 👀).
Below is the data reported in the study, where you can see the HRV drop at week 2:
I’m curious to see if/when my HRV will bounce back and re-normalize (in the study, the drop was after 2 weeks, then HRV went back to normal and did not change any further for the 12-week study).
I also wonder if this is a stress signal or just an artifact. These days we are quick to associate any change in HRV to a “meaning” (thanks wearables), but the reality is much more complex. A reduction in blood volume (from glycogen and water loss) can alter HRV, without directly reflecting a change in parasympathetic tone. Always keep in mind that HRV is an indirect measure or a proxy of parasympathetic tone, we are not able to measure parasympathetic activity directly, and many other factors can impact HRV without any link to our stress response (as I discuss in the context of continuous HRV measurements, here).
I speculate we have a bit of both here, i.e. there is an actual stress response detected, but it might be overamplified by other physiological changes during a phase of transition to a low-carb diet. For example, glycogen depletion can lead to reduced blood volume, impacting stroke volume and possibly leading to a change in HRV that is not to be considered a negative stress response. However, we might have also a transitory stress response, e.g. associated with increases in cortisol or adrenaline and reduced parasympathetic activity, which may further contribute to a reduction in HRV, and might not be picked up by changes in resting heart rate (which typically require a stronger signal, as resting heart rate is less sensitive to stress). As blood volume stabilizes and metabolic systems adjust to fat oxidation, HRV trends should return to baseline.
Let’s see how it goes in the next weeks.
February 13th, 2025: An Update
Consistently with published research, my HRV data has also re-normalized after an initial phase in which there was a large suppression, possibly due to a combination of a reduction in blood volume (another physiological artifact that should not be overinterpreted as “positive or negative”, regardless of HRV), and an actual stress response.
In terms of resting heart rate, it seems to be a good meter for caloric deficit, and nothing else (in this case!). We can see how in resting heart rate the shift to a different macronutrient distribution does not impact the data, and how resting heart rate re-normalizes as soon as the caloric deficit stops.
Related readings:
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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Marco lovely for you to share the data becuase I was looking for a reference and this gives me a lot! I am in a low carb diet myself at the moment on Day 12 with the intention of losing some visceral fat (I am very healthy and fit just have stubborn waist area fat). I observed very similar results as yourself. I had my HRV shoot up(87 from baseline of 65) in the initial days which was accompanied by great mental sharpness. From Day 8 the HRV dropped to 45 and has remained there since. I have lost the sharpness and my body does feel under stress which reflects in the HRV. I am wondering if this is a temporary phase or do I introduce carbs? How did you navigate this and what was your take on this?
Thanks as usual for your insight here. You seem to be one of the few people consistently writing on HRV & health, especially unusually high HRV. I end up directed back to your blog time and time again in my searches. I use HRV4Tracker in the mornings as well as Stress Monitor which tracks rMSSD via Apple Watch throughout the day, as I am trying to find correlations for managing my health conditions.
I’m curious if, purely theoretically, you’d have any ideas on what might drive very high HRV at the end of an intermittent fast. I’ve been fasting 16-18 hours overnight for probably a decade. As I have been monitoring my hour to hour HRV I have noticed that my HRV will often spike to over 200 (E.G. rMSSD 242, HR 56 which is typical RHR for me) coinciding with hunger at the end of my fast. After eating it will drop, either down to normal or worse than normal depending on other factors. I am doing my own experiments to reduce this swing, but I’m extremely curious whether you have any ideas about the underlying mechanics generally speaking (no medical advice :) )