Yes. On average, people with diabetes have lower heart rate variability (HRV), and over the years diabetes can damage the nerves that control the heart. That damage is diagnosed with standard tests in a clinic, not with the HRV number from a watch or ring.
What the research shows
A 2018 review pooled 25 studies that recorded 24-hour ECGs in 1,356 people with type 2 diabetes and 1,576 people without it (Benichou and colleagues, 2018). The diabetes groups had lower HRV on almost every measure, including 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 and RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD , the measures wearables most often report. Links between HRV and blood sugar levels or time since diagnosis were mixed, so the review cannot say how much HRV changes as blood sugar rises.
As with any group comparison, the ranges overlap. Plenty of people with diabetes have ordinary HRV, and plenty of people with low HRV do not have diabetes.
Cardiac autonomic neuropathy
Over time, diabetes can damage nerves, including the autonomic nerves that speed up and slow down the heart. Blood sugar control is a main risk factor, and in type 2 diabetes so are high blood pressure, high cholesterol and obesity. When this affects heart and blood pressure control, doctors call it cardiovascular or cardiac autonomic neuropathy (CAN). An international expert panel defines it as impaired control of the heart and circulation in someone with diabetes, once other causes have been ruled out (Spallone and colleagues, 2011).
What the expert groups report about CAN:
- About 1 in 5 people with diabetes have confirmed CAN, and the share rises to as many as 65% with older age and longer diabetes (Spallone and colleagues, 2011).
- The American Diabetes Association (ADA) calls CAN an independent risk factor for death from heart disease, irregular heart rhythms and silent ischemia, a heart problem without the usual chest pain (Pop-Busui and colleagues, 2017).
- Early on it may cause no symptoms at all. Later signs include a 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 over 100, poor exercise tolerance, and a drop in blood pressure on standing without the usual rise in heart rate (Pop-Busui and colleagues, 2017).
How CAN is diagnosed
HRV is part of the diagnosis, but measured in a specific, controlled way. According to the ADA, early CAN may show up only as a smaller change in heart rate during deep breathing. Doctors can test this in the clinic with an ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG taken during 1 to 2 minutes of deep breathing, or as the person stands up from sitting (Pop-Busui and colleagues, 2017). These are called cardiovascular reflex tests, and they remain the reference standard for assessing CAN (Benichou and colleagues, 2018).
The expert panel stages CAN by how many of these tests are abnormal. One abnormal heart rate test suggests possible or early CAN, two or more confirm it, and a fall in blood pressure on standing on top of that marks advanced CAN (Spallone and colleagues, 2011). Doctors also need to rule out other illnesses and medicines that can mimic CAN.
The 1996 international standards for HRV, which defined measures such as SDNN and RMSSD, named reduced HRV as an early warning sign of diabetic neuropathy (Task Force, 1996). That refers to HRV measured under clinical conditions and read by a doctor, not to a nightly wearable score.
Why a wearable cannot screen for CAN
- Different measurement. Clinic tests measure how your heart rate responds to a set challenge, like deep breathing or standing. A watch or ring measures resting HRV during sleep or at a time you choose.
- Many other causes. Poor sleep, alcohol, illness, hard training, age and many medicines also lower HRV. See what low HRV can mean and do medications affect HRV?
- No wearable criteria. The diagnostic criteria rely on clinic tests, and none of them uses a wearable score.
- A normal reading is not a clean bill of health. Some rhythm problems can even push HRV up. See can HRV detect heart problems?
What helps
The ADA recommends good blood sugar control as early as possible to prevent or delay CAN in type 1 diabetes, and an approach that targets blood sugar along with other heart risk factors in type 2 diabetes (Pop-Busui and colleagues, 2017).
Exercise may also help HRV. A 2021 review of 21 studies with 523 people with type 2 diabetes found that exercise training raised SDNN, RMSSD and other HRV measures (Picard and colleagues, 2021). Endurance training had the strongest evidence, and supervised training seemed to help most. If you track HRV, a gradual rise with regular exercise is a reasonable thing to look for, but it is not proof that nerve damage has improved.
When to talk to a doctor
If you have diabetes, ask your diabetes team whether you should be checked for CAN, especially if you have other complications such as eye, kidney or foot nerve problems. The ADA lists the most common symptoms as light-headedness, weakness, palpitations or faintness when standing up (Pop-Busui and colleagues, 2017). Talk to your doctor if you notice these, a resting pulse over 100 without a clear reason, or that you no longer feel the usual warning signs of low blood sugar.
Get emergency help for chest pain, fainting or trouble breathing. Because CAN is linked to heart problems without the usual chest pain, take sudden, unexplained breathlessness seriously too, whatever your HRV shows.