Also, just noticing that you asked about weight loss walking Sarria to Santiago. That doesn't take longer than a week, so you cannot safely lose much. However, it could be a great part of a longer program.
Yes, I agree.
When I walked St. Jean to Santiago, I lost the equivalent of my pack weight, but I did not notice any real change until we were at Santo Domingo, then at Burgos, then at Astorga.... My sense was that significant differences were roughly 2 weeks apart, and I ended up with bruises on my iliac crest at the end... and scrapes on my collar bones.
When I walked Fromista to Santiago, I noticed no real drop until Santiago...
and when I walked from Coimbra to Santiago I had a modest drop followed by a radical one but only because I became ill and could not eat due to acute tonsillitis.
On my first camino I had lost about 8 pounds before leaving... and I lost another 13 on the way... and then another few at home. I went from 147 to 115, and I'm 5'4.5"
I did eventually regain some, and usually sit at 125-130 now... which I like and maintain because of how the camino preparation and after-effects altered my day-to-day (Covid aside).
With Covid, I'm sitting in the higher 130's and I don't love it.... but I think it could have been so much worse if I had still been stuck in my pre-camino habits. What I am missing is my daily 7 km to campus and back, and the 2-3 I put in walking around campus...
To me, the weight I carry matters most in the sense of spine health, sciatic presurre etc. At 54 years old, I don't worry so much about aesthetics anymore.
But to the original poster I would say that the general lifestyle changes that come of preparing to walk long distances, and the habits might stick after a camino can really enhance your objective health measures and your sense of well-being.
That said: a caution is in order about camino weight losses.... my spouse lost his pack weight on his 2018 SJPDPD to Santiago.... and then a little more as he changed his habits at home. Generally fit, he has a genetic heart condition and the cardiologist said the rapid weight loss could have had a significant role in the development of the arterial occlusions that have led to the unstable angina he now lives with. Something about the plaques being able to take advantage of rapid changes in body chemistry.