Thanks for the insightful question, L.
Unfortunately, I don’t like using AHAs with retinol. I stand by this one. I’ve been called out by other bloggers and scientists. I’ve read their rebuttals. But I still don’t agree.
First of all, the optimal pH for retinol to work in is significantly higher than the optimal pH range for AHAs.
For the ultra-scientific or the ultra-curious, the enzymes responsible for the oxidation process – dehydrogenases (DHs) – underlie every pathway of retinol activation within the skin. Specifically, two types of dehydrogenases must convert retinol to retinylaldehyde and then to all-trans retinoic acid within the skin for retinol to be activated. This is optimized at a neutral pH (Nature).
Secondly, retinoids are pretty unstable ingredients to begin with. To elaborate, I consulted with the site of Dr. Leslie Baumann, M.D., a practicing dermatologist and the co-founder and chief of the Cosmetic Dermatology department at the University of Miami School of Medicine. According to Dr. Baumann:
“Your question about order is a great one. Retinoids should not be mixed with BHA (i.e., salicylic acid) or AHA (i.e,. glycolic acid) because the BHA and AHA can inactivate the retinoid. Always use retinoids at night because the sun can also make the retinoid less effective.”
Or, use retinoids at night for 3-4 months, and then cycle off, and move to AHA products for 3-4 months. Your call.
Hope this helps!
All the best,