Does my body need to re-acclimate after +/- two days at lower altitude?

Question

I am heading to Peru and will do a 17 day trip with a friend. From day 10 to day 14 we are in Cusco - so at an altitude of 3399 meters. Sometimes we will even go up higher, to a maximum of 5000 meters (Rainbow mountain), but will descend back down to Cusco the same day. Day 14 from the morning and day 15 we are at Machu Picchu (2430). Then we take a transfer back to Cusco and fly to Lima. We will stay in Lima for one day, and then my friend flies back home and I was planning on taking the night bus to Huaraz at 3052 meters.

My question is: Do I have to acclimate again? I was looking into the Santa Cruz trek, which would start right after I get off from the night bus and I would sleep that night at 3900 meters, and the next night at 4250 meter before descending again.

If that is not a great idea with regards to altitude sickness, I could stay one night in Huaraz and then do the shorter version (3 days instead of 4) of the trekking.

Of course the first option is the most appealing to me, because then I would see all the beautiful views, whilst the 3 day one skips a beautiful lake. Unfortunately I cannot go one day later, because of my flights.

Answer 1

Does my body need to re-acclimate after +/- two days at lower altitude?

That question is a matter hard to answer, because some individuals will not need to acclimatize again, while some individuals will need to re-acclimatize again.

Generally speaking if one is away from trekking for some time (2 months or more) one would probably need to re-acclimatize again. This is not a clear cut rule for everyone. Ever individual is different and acclimatization will differ for person to person.

From my experience, acclimatization is very temporary and takes 1 to 3 days to acclimatize. In any case, stay hydrated and trek slowly in higher elevations. I generally take some additional supplements before trekking to help with my red blood count and oxygen levels some 10 days prior such as iron and B12 and magnesium for muscle cramps.

Answer 2

While individual experience with altitude may vary, a short break of 2 days is unlikely to affect your acclimatization negatively. Quite the contrary, one important recommendation for acclimatization is "climb high, sleep low". By climbing to 5000m and then going back down, you are doing exactly that. Your body will do a better job acclimatizing when you give it some rest at lower altitude.

Regarding the matter of sleeping high...you might run into altitude sickness and nights might be uncomfortable. However, given the prior acclimatization and nights at 3000, 3900 and 4200m it is very unlikely that altitude sickness will be dangerous. As a rule of thumb, HACE and HAPE unlikely to occur below 4500m.
However, you should watch out for symptoms and prepare an alternative plan if you start feeling unwell. Also, it would be more textbook style to spend the second night at ~3600m instead.