Exertional Dyspnea in otherwise fit 45M

I am 45 years old, male, 5'10", white. Never smoked, never drank alcohol, no recreational drug use. Approximately 170lbs.

Current medications: Adderall, Symbicort

Primary complaint: exertional dyspnea/exercise intolerance

Background:

I lost about 100lbs starting in 2019 through diet and exercise. I lift weights at least three times a week and walked on the treadmill every day.

My daily routine was to walk between 1.5 and 2 hours on the treadmill nightly at 13% incline and 4.3 mph. I took a Sunday off in October, and on Monday I was exhausted doing 11% and 4 mph for an hour. I thought I might just need sleep, but after 7.5-8 hours/night for the following week I was still at diminished capacity. I had a lot of coughing, postnasal drip, and exhaustion during my workouts in the first half of the week. I thought I might have caught COVID, though the previous times I got it I wasn't slowed down much. I became more concerned when I did some single-arm rows (sets of 10) and was really winded after the set (obviously should not be a cardiovascular challenge).

I followed up with my PCP, who suggested taking a COVID test. The OTC test came back as negative. By the end of January, there was no improvement. My cardio performance did not improve, and I found myself winded sometimes after relatively light activity (examples: dragged a light (<75lb) tub around to the back of the house, had to stop and catch my breath; walking up and down stairs sometimes leaves me somewhat winded). Dyspnea is not consistent.

PCP ordered bloodwork and a lung x-ray. No anemia and the x-ray was clear. PCP referred me to a cardiologist; stress test was the first four stages of the Bruce protocol. Result was normal with excellent HRV (though I was gasping for air when the test stopped). EKG/ECG were unremarkable. PCP referred me to a pulmonologist, who ordered a lung test. All values were normal except low TLC at 73%, but the pulmonologist did not think it was clinically significant. They gave me a FeNO test just prior to meeting with the pulmonologist, which was elevated at 49, which the pulmonologist said can be a clue for airway inflammation. She prescribed a Symbicort low dose inhaler to use as needed before exertion to see if it improves dyspnea. The Symbicort inhaler has not improved symptoms after two weeks.

I still use the treadmill, but hitting 4mph and 13% is extremely difficult and virtually impossible to do two days in a row, even if I only stay at that level for ten minutes of the workout. I am frequently winded between sets when lifting weights. The other day I evaluated my diet and realized that I am only getting between 10 and 15mg of Vitamin C, so I have been supplementing Vitamin C for the past three days (500mg 3x/day, spaced an hour away from the Adderall) just in case the dyspnea is caused by hypovitaminosis.

I'm getting very discouraged and I'm worried that I am just going to be stuck like this.