One friend has suggested I must have discovered the secret of nuclear fission and am hiding it in my marrow, and another suggested I'm an autotroph. The doctor and dietician had no explanation but seemed less inclined to propose explanations.
After 2.5 weeks on a low-carb diet (the dietician checked over my eating record), with my usual exercise routine, I haven't lost any weight. Normally, she said, she first puts people on a healthy diet (which I had when I first saw her), and if that isn't effective, she tries the low carb diet, which has never before failed to deliver results. She gave me instructions for gradually re-introducing carbs into my diet, so I'm no longer low-carbing, although I still have a number of low-carb recipes to post. Then she sent me back to the doctor.
The doctor couldn't find anything medically wrong with me, but also doesn't know what could be causing this. She referred me to the obesity clinic (although I'm not technically obese), so in the next month or so I'll find out what some more experts have to say about it.
In the meantime, I'll enjoy the fact that I'm otherwise in better physical condition due to picking up exercise again, and continue to enjoy my (mostly healthy) food.