Cool I didn't know this. I assume it is metabolism-related? Could you provide a short explanation or an article if possible? I'd be very interested to read the science behind this
There's some basic chemistry involved. Freebase MDMA is weakly basic - its nitrogen and oxygen atoms have exposed electrons that can attract free-floating protons found in the acidic stomach. MDMA with a proton attached is positively charged and polar. It is more difficult for positively charged MDMA molecules to pass from the intestine to the blood, because the surface membrane of the intestine is non-polar and non-polar membranes prefer non-polar molecules.
When the stomach is more acidic, more of the MDMA becomes positively charged, so less of it is absorbed by the intestine. By bla-ljus's reasoning, this means reduced intensity and duration, because more of the MDMA is passed out in urine rather than heading for the brain. Personally, I think this depends on how much urine is secreted - a more acidic stomach could also
extend a roll's duration, because the basic MDMA molecules are more reluctant to leave the acidic stomach and enter the intestine, so they do so more slowly. No hard claims though, and this would still result in a less intense roll.
By contrast, if MDMA is taken with an antacid like calcium carbonate (Tums) to reduce the stomach's acidity, it will leave the stomach faster, be less charged, and pass more quickly and easily from the intestine to the blood. This can intensify the effects of a roll, and may either shorten or extend the duration depending on the competing reasoning I mentioned above.
I really don't know how much science has been done on this, but the reasoning is chemically sound and people (myself included) seem to agree that the effect is noticeable.
Here is a corroborating source, sort of.