Is my cardio interfering with lean-mass retention, or am I simply not recovering at this intake?

I have added a lot of cardio because it felt like the responsible thing to do and now I am not sure it is helping. Current week: three full-body lifting sessions, four 50-minute runs, and roughly 12,000 steps on non-running days. Intake is 1,150-1,300 kcal, protein 130-140 g, body weight 74 kg down from 93 kg over ten months on semaglutide.

Symptoms that made me start asking: resting heart rate down from 61 to 48, which I initially read as fitness but now suspect is not; always cold; sleep is broken between 3 and 4am most nights; lifting numbers have been flat or falling for eleven weeks; two menstrual cycles missed; mood is flat in a way that is unlike me.

Two questions. First, is there a real interference effect where the cardio directly blunts the lean-mass retention from lifting, or is that a myth from the 1980s? Second, and I suspect this is the actual issue, is there a way to tell whether my problem is interference between modalities versus simply not having enough energy available for any of it?

On the resting heart rate specifically, since OP flagged it as the thing that first made them suspicious and it is a genuinely ambiguous marker.

A resting heart rate of 48 in someone running four times a week is entirely consistent with improved cardiovascular fitness. It is also entirely consistent with the reduced sympathetic tone and lowered thyroid output of chronic underfeeding. The two look identical on a wrist tracker and the distinction matters, because one is an adaptation and the other is a warning.

Ways to tell them apart:

• Heart rate variability trend. Fitness-driven bradycardia usually comes with stable or rising HRV. Underfeeding-driven bradycardia often comes with falling HRV, and specifically with a widening gap between how you feel and what your tracker says about readiness.
• Orthostatic response. Lie down five minutes, record heart rate, stand, record again after one minute. Fit people show a modest rise. People in energy deficit frequently show an exaggerated rise plus lightheadedness, which reflects reduced plasma volume and blunted autonomic regulation.
• Exercise heart rate ceiling. If your maximum achievable heart rate during hard efforts has fallen alongside your resting rate, that points to central fatigue rather than fitness. Genuinely fitter people have lower resting rates and unchanged maxima.
• Thermoregulation. Fitness does not make you cold. Cold intolerance alongside bradycardia is the underfeeding pattern.

OP has cold intolerance, falling performance and missed cycles, so all four indicators point the same way. But the general method is worth having, because plenty of people on these drugs see their resting heart rate fall and cannot tell which story they are in.

Worth checking with bloodwork rather than inference if the pattern persists: thyroid panel including free T3, morning cortisol, ferritin and a full blood count. Low free T3 with normal TSH is a classic low-availability signature and it is invisible on a TSH-only panel.

A dissenting note on step counts, which nobody has addressed and which is a bigger share of OP's expenditure than the structured exercise.

12,000 steps on non-running days is roughly 350-450 kcal of additional expenditure for a 74 kg person, and it does not appear in the accepted answer's exercise energy calculation at all. Include it and the availability figure drops further, into single digits on some days.

I would push back gently on cutting steps, though, and here is why: non-exercise activity is the component that falls spontaneously and invisibly during energy restriction, and it is one of the main mechanisms of the metabolic adaptation people complain about. Deliberately maintaining daily movement is protective against that adaptation in a way that structured cardio is not. Walking is also nearly free in terms of recovery cost, does not damage the tissue you are trying to load, and carries the strongest independent association with long-term weight maintenance of any single behaviour.

So the order I would cut in: running first and hard, structured cardio duration second, steps last and only if the intake correction fails. And if steps do get cut, cut them explicitly rather than letting them decay, so you know what you changed.

One more thing: OP describes adding cardio because it "felt like the responsible thing to do". That instinct is a residue of the pre-pharmacology era where the deficit had to be manufactured through expenditure. On a drug that produces the deficit for you, the responsible thing is usually the opposite: the scarce resource is energy for recovery, not additional expenditure, and training should be selected for stimulus per unit of fatigue rather than for calories burned.