I have another explanation too - there is some stuff that connect spirituality to the physical world we don't fully understand yet. The "true righteous believer" as you put it, actually can to an extent, channel spiritual energy into their physical bodies, to some extent having positive effects both on hunger levels as well as positive hormonal levels. In this context, I believe its actually possible to maintain good hormone levels even while fasting. In this instance, the faster would NOT have high serotonin levels.
Maybe after all, "Man does not live on bread alone" (an infamous quote in the NT by Jesus). But yeah, nobody likes a hypocrite or someone who pushes their lifestyle on others especially when they're not living it themselves.
"When you fast, do not look somber as the hypocrites do, for they disfigure their faces to show others they are fasting. Truly I tell you, they have received their reward in full. But when you fast, put oil on your head and wash your face, so that it will not be obvious to others that you are fasting, but only to your Father, who is unseen; and your Father, who sees what is done in secret, will reward you." - Matthews 6:16