It can, at least in the first weeks. In a randomised crossover trial, eight trained male runners spent 31 days on a tightly controlled ketogenic diet and 31 days on their usual diet, taking a morning heart rate variability (HRV) reading every day. On the ketogenic diet, RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD fell by 9.8 ms on average and swung more from day to day (Maunder and colleagues, 2021). That is one small trial in athletes, and the only other one is a six-month diet study in people with obesity, so the honest answer for most readers is “unknown, possibly a dip at first”.
What the two trials found
Runners switching to keto. The New Zealand team wanted to know why some athletes lose performance on a ketogenic diet and others do not. Each runner did a treadmill test to exhaustion before and after each 31-day block. On keto, morning RMSSD dropped by 9.8 ms and its day-to-day variability rose by about 2 percentage points. The drop was concentrated in the four runners whose exercise capacity fell, at about 22 ms, which just missed statistical significance, while the four who kept their capacity showed no change (a 5 ms rise, not significant). A weekly recovery-stress questionnaire did not change. The authors suggested morning HRV could flag, person by person, who is struggling with the switch.
A very-low-calorie ketogenic diet. An Italian trial randomised 26 people with obesity to a very-low-calorie ketogenic diet or a standard low-calorie diet for a six-month programme. Heart rate fell and high-frequency HRV rose in both groups, and the changes were statistically clear only in the ketogenic group, which the authors read as less sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system drive (Polito and colleagues, 2022). Here the diet also caused weight loss, which tends to raise HRV on its own; see does losing weight raise HRV?
So the two trials point in different directions, for different reasons: a stress response to a sudden dietary shift in lean, hard-training athletes, and a benefit of losing weight in people who had weight to lose.
Why the first weeks may be rough
A ketogenic diet changes the fuel the body runs on, and the switch takes time. In the first week, the body sheds water and salt, which lowers blood volume and can raise heart rate. Training hard on a new fuel mix is its own stress. A 2022 review of weight-loss studies found that most showed better vagal HRV after losing weight, but could not separate the type of diet from the weight lost (Mattos and colleagues, 2022). Nobody has run the trial that would answer the direct question: the same people, weight stable, low-carb against normal, for months.
How to read your own data
- Tag the switch. Note the day you change diet and compare the next few weeks with your baseline, not with single days.
- Expect a dip and more bounce. A lower, jumpier first fortnight matches the runners’ trial. A 7-day average that keeps sliding while you feel flat in training is a reason to ease the training load, add salt and fluid, or rethink the timing of the change.
- Separate the diet from the weight. If you are also losing weight, a slow rise over months may be the weight, not the carbs.
- Keep the reading routine fixed. Measuring before breakfast on some days and after it on others will add more noise than any diet.
For the wider picture of what moves the number, see what affects HRV and, for pills and powders, supplements and HRV.
When to talk to a doctor
Talk to a doctor before a ketogenic or very-low-calorie diet if you have diabetes, kidney or liver disease, a heart condition, or take medicines for blood pressure or blood sugar, since doses may need changing. Get checked if you feel faint, have palpitations or chest pain, or if your HRV stays well below your usual range for weeks and you feel unwell. HRV cannot diagnose anything, and this page is not medical advice.