Recap of our latest paper, part 1

Blog post by Marco Altini

​In our recent publication, we analyzed the relationship between heart rate and HRV with respect to individual characteristics such as:

• age
• sex
• body mass index (BMI)
• physical activity level

In a large sample of 28 000 individuals. What did we learn?

Some of the findings are larger-scale replications of what we knew already Consistent results with published literature that used different data collection procedures is a good first step It gives confidence in the quality of the data when we start digging a bit deeper

Sex

​Women have higher resting heart rate than men, but very similar HRV In fact, at a younger age, women have a slightly higher HRV. This is of interest as a higher heart rate would normally be associated with lower HRV. The discrepancy might be due to hormonal differences.

BMI

Both underweight and overweight/obese categories show what we have called in the paper a suboptimal physiological profile, meaning that resting heart rate increases and HRV reduces when deviating from the normal range. The strongest deviation is for the obese category.

Age

There was no correlation between resting heart rate and age, and a moderate correlation between HRV and age. This is one of the most interesting relationships, as heart rate and HRV clearly decouple and are representative of different processes (more on this later).

Physical activity level

The association between physical activity level and resting physiology is stronger for heart rate (r = 0.30, moderate effect size) than for HRV (r = 0.21, small effect size). When we break this down by age group, things get even more interesting.

The correlation between physical activity level and HRV reduces by age, getting to r = 0.13 for older individuals. Only for very young individuals (20-30 age group) there is a decent association between fitness and HRV.

Finally, we built models to determine how much variance age, sex, BMI and physical activity level could explain. Are they sufficient to get a good understanding of inter-individual differences? Not really, as they explain 19% of the variance in heart rate and only 15% in HRV.

What are the implications of these findings?

A low HRV in aging individuals might be associated with a deterioration of regulatory mechanisms. The weak link between physical activity and HRV as we age might similarly be associated with reduced baroreceptor sensitivity. On the contrary, increased stroke volume due to high levels of physical activity maintain resting heart rate low even for older age groups. In terms of explained variance, it is clear that genetic factors are key in explaining differences in heart rhythm between people.

An important implication here is that in our opinion, targeting improvements in HRV as intervention goals might not be realistic, given the strong heritability coupled with reductions with age and low explained variance associated with lifestyle factors such as physical activity level.

But there's an important caveat. In this work, we had a large sample. However, this sample is not representative of the whole population, but only of relatively healthy or health-conscious individuals. There might be more to gain for e.g. who never exercises, is overweight, etc.

This is why HRV as an absolute value is of little interest (in our opinion). On the other hand, HRV was able to capture day-to-day stressors within individuals with high sensitivity, as I will cover in a future blog.

You can find the full text of the paper, here. Thank you for reading!