What are some conditions that would cause severe exercise intolerance?

Question

I'm making a character and trying to be biologically accurate. Said character has transfusion-dependent anemia (from Gunther disease/congenital erythropoietic porphyria) so I suppose exercising would be hard by default, but what are some other conditions that would make physical training unhealthy? Ideally, I'm trying to make it so even light exercising is bad, but severe would cause vomiting/fainting/etc.

I posted this question on biology, but someone suggested I try asking here instead. I think that the question is less relevant there, so might as well ask here since I didn't get an answer anyway. I apologize for asking the same question twice.

Answer 1

If you're looking for a disease with childhood onset that requires exercise restriction, Long QT Syndrome (LQTS) may work, specifically LQT1. Here is a lay overview. For more detail, see this review. I'm not a cardiologist, and have never seen a patient with LQTS, so I'm honestly not sure how variable the presentation is. Generally, for LQT1, exercise or stress induced tachycardia can cause a life threatening arrhythmia called torsades de pointes and sudden death.

Treatment of LQT1 includes avoidance of stress, exercise restriction, and beta blockers. As discussed in the linked review above, certain extreme phenotypes may not be fully controlled with beta blockers. Pain, nausea, or perceived exercise intolerance (e.g., fatigue), the symptoms you mentioned in your comment, are not part of the described symptom complex. Rather elevated heart rate leads to the malignant arrhythmia. The symptom complex is that of the arrhythmia (palpitations, fainting, seizures, and death).

Answer 2

Here are medical conditions that come to mind, each with different symptoms:

1. Severe exercise-induced asthma. Symptoms will be severe wheezing, O2 drop, potentially even death. Long-term anti-inflammatory medications can prevent exacerbations, though exacerbations can still be triggered in specific situations. Imagine a deconditioned character, in fight-and-flight, running from danger in cold weather, and especially with bronchi inflamed from a respiratory infection.
2. POTS - postural orthostatic hypotension. A type of dysautonomia, a likely autoimmune condition that can trigger O2 drops, brain fog, nausea, fainting from minor exercise, or (in very severe cases) even from standing up and staying upright. It used to be thought that graduated exercise would help everyone. Not true.
3. Chronic fatigue syndrome. Often infection-triggered, likely autoimmune, with antibodies to adrenergic receptors (adrenergic - think adrenalin). Causes post-exercise malaise, meaning dysproportionately severe symptoms for days/weeks after overdoing things, including much increased odds of fainting from POTS, overwhelming fatigue, GI symptoms (nausea, inability to digest..). Depending on the patient, the "overdoing" may be physical or cognitive, ranging from doing a college entrance exam, to cooking for a family visit, to walking around the block.