No supplement has strong evidence for raising resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate variability (HRV). Fish oil comes closest: across 15 randomised trials, it raised high-frequency power HF power The variance of the heartbeat intervals that falls in the 0.15 to 0.40 Hz band, in ms²: the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power , the breathing-linked vagal part of HRV, by a small amount, while leaving 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 and SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN unchanged (Xin and colleagues, 2013). Magnesium, ashwagandha and creatine each rest on one small study or none. This page grades what exists and recommends no product.
How to read the grades
Most supplement trials are small (a few dozen people), short (weeks), and often run or funded by the maker of the product tested. Many measure HRV as a side outcome, which makes a positive result easier to find by chance. We grade evidence as moderate when a meta-analysis of randomised trials agrees on a direction, weak when it rests on one or two small randomised trials, very weak when there is no randomised trial, and none when nothing usable exists. “Effect size” (SMD) compares groups on a common scale: about 0.2 is small, 0.5 medium, 0.8 large.
The measures matter too. Most apps report RMSSD or SDNN, defined in the 1996 Task Force standards; high-frequency (HF) power is the breathing band of the spectrum and mainly reflects the vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve (Task Force, 1996). A supplement that moves HF but not RMSSD may not move the number on your wrist.
The evidence at a glance
| Supplement | What was found | Evidence | Would it show in your app? |
|---|---|---|---|
| Fish oil (omega-3) | HF power rose (SMD 0.30) across 15 trials; SDNN and RMSSD did not change; LF/HF LF/HF ratio LF power divided by HF power, with no unit. It was proposed as a gauge of balance between the sympathetic and parasympathetic systems, but research shows it cannot measure that reliably. More about LF/HF ratio fell only when low-dose trials were removed | Moderate for HF; null for RMSSD and SDNN | Probably not in RMSSD |
| Magnesium | In 100 inactive adults aged 40 to 65, 400 mg a day for 90 days alongside weekly strength-endurance training raised 24-hour pNN50 pNN50 The percentage of neighbouring NN intervals that differ by more than 50 milliseconds. It tracks fast, vagally driven changes, like RMSSD, but as a share rather than a size. More about pNN50 against a no-supplement control; LF/HF (p = 0.06) and a stress index Baevsky stress index A number built from the histogram of heartbeat intervals, with no unit. It rises as the rhythm gets steadier and less variable, which Baevsky read as strain on the body's regulation. More about Baevsky stress index (p = 0.39) did not differ significantly; cell magnesium fell in both groups | Weak (one open-label trial, no placebo) | Unknown |
| Ashwagandha | In 120 overweight adults over 12 weeks, RMSSD rose 9% on the extract (not significant) and fell 19% on placebo; the group difference was significant, the primary stress score was not | Weak (one trial; HRV a secondary outcome) | Unclear; the placebo drop drove the result |
| Creatine | No randomised trial. In 48 men, 32 of them bodybuilders, the bodybuilders taking creatine lacked the parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system shift seen in those who did not | Very weak (cross-sectional) | Unknown |
What each study found
Fish oil
The 2013 meta-analysis pooled 15 randomised controlled trials of fish-oil supplementation and HRV. High-frequency power rose by a small, significant effect (SMD 0.30, p = 0.005). Low-frequency power LF power The variance of the heartbeat intervals that falls in the 0.04 to 0.15 Hz band, in ms²: rhythms repeating every 7 to 25 seconds. Once read as sympathetic activity, which is now disputed. More about LF power did not change. The LF/HF ratio showed a trend downward that became significant only after dropping the two trials under 1,000 mg a day. SDNN (SMD 0.10) and RMSSD (SMD 0.05) did not change. The authors concluded that short-term fish oil may raise vagal tone Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone as seen in the frequency domain. Subgroup analyses by predefined study characteristics found nothing further.
For a wearable user, the honest reading is: a real but small shift in one spectral measure, and no evidence for the time-domain number most apps show.
Magnesium
The one randomised trial is German, published in 2016 in a general medical journal. One hundred adults aged 40 to 65 who exercised little were split into two groups for 90 days; one took 400 mg of magnesium a day, the other took nothing, and both did a weekly supervised strength-endurance session. Ninety-four finished (49 on magnesium, 45 controls). On 24-hour recordings, pNN50 rose from 6.0% to 8.0% in the magnesium group and fell from 5.0% to 4.3% in the controls, a significant difference at the end (p = 0.024). The LF/HF ratio fell from 5.6 to 4.9 on magnesium and stayed at 6.0 in the controls, which did not reach significance (p = 0.06), and the stress index did not differ (p = 0.39). Cell magnesium fell in both groups (Wienecke and Nolden, 2016). The abstract says LF/HF and the stress index “decreased”, which the paper’s own tests do not support. Because both groups trained, the training is matched, but there was no placebo and no blinding, and the paper reports no registration, so expectation and attention cannot be separated from the mineral.
Ashwagandha
An Australian trial randomised 120 overweight or mildly obese adults aged 40 to 75 with high stress and fatigue to 400 mg a day of a standardised root extract or placebo for 12 weeks. On the primary outcome, perceived stress, both groups improved by about the same amount (39% and 36%), with no difference between them. On a 5-minute RMSSD measurement, the extract group rose from 26.4 to 28.8 (not significant) while the placebo group fell from 31.4 to 25.5, and the interaction favoured the extract (p = 0.003) (Smith and colleagues, 2023). HRV was one of many secondary outcomes, the “effect” came largely from the placebo group falling, and the extract was the manufacturer’s product. That is a lead for a future trial, not a result to act on.
Creatine
We found no randomised trial of creatine with HRV as an outcome in humans. One study compared 24-hour Holter HRV in 16 bodybuilders taking creatine, 16 not taking it, and 16 non-training controls. The bodybuilders without creatine showed a lower heart rate and a shift toward the parasympathetic side compared with controls; those taking creatine did not, apart from one frequency measure (Mert and colleagues, 2017). The authors could not separate creatine from differences in training. A snapshot of 48 men cannot say what creatine does to anyone’s HRV.
Why capsules are unlikely to move the needle much
In morning readings from 28,175 app users, age, sex, body mass index and activity level together explained only about 15% of the differences in HRV between people, with age the strongest (Altini and Plews, 2021). Much of the rest is genes and the make-up of each person’s nervous system. The levers with real evidence are habits: regular aerobic exercise, enough sleep and less alcohol, as the page on how to increase HRV sets out lever by lever. A supplement would have to beat day-to-day noise of about 12% in RMSSD to be visible in your own data at all (Buchheit, 2014).
If you want to test one on yourself
- Build a baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline first, measured the same way every day. The lesson on building your HRV baseline shows how.
- Change one thing. Starting a supplement, a new training block and an earlier bedtime in the same month tells you nothing about any of them.
- Compare monthly averages, not days. The experiment widget in what moves HRV shows how many tagged days a small effect needs before it stands out from noise.
- Expect nothing. If the average has not moved after two months, the honest conclusion is that it did not move.
Diet changes have their own pages: keto and low-carb diets, losing weight and caffeine.
When to talk to a doctor
Supplements can interact with medicines and conditions. Talk to a doctor or pharmacist before starting one if you take any medicine, are pregnant or breastfeeding, or have a heart, liver or kidney condition. A low HRV is not a reason to take anything, and this page is not medical advice. If your trend stays well below your usual range for weeks with no clear cause, that is a conversation for a doctor, not a shop.