I used to play a trick on myself where I'd hold a particular hand position and meditate on a state of mind, one intended to clear my mind and get centered, the other to get me amped up to go do some work. (I worked in construction at the time) It worked amazingly well, and I spent several weeks / months feeling no other emotions. I'd feel the state slipping, and just hold the hand position and the state would come back. Only reason I stopped was that the emotional monotony got old after awhile.

I have no trouble believing that fish oil as a mood stabilizer can work fantastically, and yet have no physiological effect whatsoever.