GLP-1 drugs raised resting heart rate 3.2 bpm and cut HRV by 6.2 ms

The common side effects of GLP-1 medication are well known: nausea, fatigue, indigestion, heartburn, gut trouble. But a different kind of report has been accumulating among people who train — the weight comes off while performance markers go down with it.

That is counterintuitive. VO2 max is calculated in ml/kg, so when bodyweight falls the number should rise on the arithmetic alone. Instead it has been reported moving the other way.

What did 12 weeks of tracking show?

A researcher at a wearables company tracked 66 active adults, average age 42, across 12 weeks on a GLP-1, publishing the results in the American Journal of Physiology. Three things happened at once.

• Body mass fell — which is what the drug does.
• Resting heart rate rose — by an average of 3.2 beats per minute
• Heart rate variability fell — by an average of 6.2 milliseconds

Those numbers sound small, and for a trained person they are not. In the researcher's framing, a shift of that size is what you would see after taking two weeks off vigorous training. The HRV reductions were statistically significant as early as four weeks.

The mechanism the study points to is recovery: on a GLP-1, heart rate does not fully downshift during recovery the way it should.

Why does this happen?

Much of it is not the drug itself but the eating it prevents. A GLP-1 turns two dials at once: it turns down food noise, the background hum of hunger, and turns up fullness. For fat loss, that is the entire mechanism. For someone training, it creates a quiet problem.

One performance dietitian described the pattern he saw in clients: sleep tanking, increased moodiness, brain fog, and rising perceived effort for their normal workload. He read those as yellow-to-red flags for low energy availability — and they resolved once proper nutrition was back on board.

The risk this points at is RED-S, relative energy deficiency in sport: when exercise output consistently outpaces caloric intake, the body starts borrowing against itself, and the debt builds quietly until injury, bone loss and hormone crashes appear. You do not have to reach that point to feel it — a modest deficit shows up in output, recovery and focus long before injury does.

What to do if you train on one

• Eat on a schedule — hunger cues are no longer trustworthy, so three meals and one snack at consistent times. On training days, pre-workout fuel and the window right after are non-negotiable.
• Aim for 3:1 carbohydrate to protein — for endurance work, roughly 90g of carbohydrate to 30g of protein. A grain bowl with a cup or so of rice, legumes, vegetables and a protein gets you there.
• Check vitamin D and B12 first — both are commonly deficient in GLP-1 users and both affect energy, mood and recovery. Look there before changing your programme.
• Watch your wearable — the data tells you when a nutrition deficit has become a performance problem before your body does.
• Do not fight the medication — pushing through when your body says stop is the wrong instinct here.

The framing that matters is the goal. As the researcher put it, the goal should not simply be weight loss — it should be maximising fat loss while preserving skeletal muscle, maintaining adequate nutrition, and continuing to train.

How this relates to your strength score

There is a trap here specific to a bodyweight-corrected score. Because the score is corrected for bodyweight, losing weight raises it even if your lifts do not move. It is easy to read a rising score during GLP-1 use as evidence that training is going well, when by the data above muscle and recovery may be heading the other way.

So do not read the score alone during this period. Watch the absolute Big 3 numbers alongside it. Weight falling while squat, bench and deadlift hold or rise means you are losing fat and keeping muscle. Both falling together means calories or protein are short. The same trap from another angle is in calorie density dieting for lifters.

One honest gap to leave open: the study above ran 12 weeks. Whether these markers stabilise past six months, or whether HRV finds a new baseline while the medication is still active, nobody yet knows.