Saw some discussion about reta raising heart rate so figured I'd explain the actual mechanism since it gets misunderstood a lot.
The short version: GLP-1 receptors are directly expressed in the heart, specifically in the SA node (your heart's pacemaker). Reta raises HR through direct receptor activation, not primarily through fitness or how much cardio you do.
The mechanism: GLP-1 and the GLP-1/GIP/glucagon agonists like reta bind receptors in the SA node and increase the firing rate. It's a direct chronotropic effect the drug is essentially nudging your pacemaker to fire faster. This is well documented across the whole class (sema, tirz, reta all do it), typically raising resting HR 5-10 bpm.
Here's where the fitness thing comes in, because a lot of guys assume it's a conditioning issue: cardiovascular fitness modulates HOW MUCH your HR rises, but it isn't the cause. A fitter heart with higher stroke volume and better vagal tone shows a smaller increase because the baseline is lower and the autonomic buffering is better. So if you're well conditioned you'll see a smaller bump, but you'll still see one. Someone with elite cardio still gets the HR increase from reta, just less than an unfit person.
There's probably a small secondary contribution from the metabolic changes too altered sympathetic tone during active fat loss, the glucagon component bumping metabolic rate slightly via thermogenesis. But the primary driver is the direct cardiac receptor effect.
So the "is my heart just unfit" theory explains the magnitude of your response, not whether it happens at all. The HR increase is pharmacological, not a sign your conditioning is bad. Doing more cardio will lower your resting baseline and shrink the bump, but it won't eliminate it because you can't out-train a direct receptor effect.
Worth tracking your resting HR trend if you're on reta. A few bpm up is expected and generally considered tolerable. If you're seeing a large sustained jump (15+ bpm) or resting HR climbing into the 90s+, that's worth paying attention to, especially if stacked with other compounds that also raise HR.