The most common cause of this is low levels of B vitamins and/or zinc. The zinc and B vitamins needed for stomach acid also happen to be acid dependent for absorption. So the use of anything that reduces or neutralizes stomach acid will further impede stomach acid formation by blocking the absorption of these nutrients. These include antacids, acid blockers (proton pump inhibitors), calcium oxide/hydroxide, calcium carbonate (oyster shell, coral, dolomite), magnesium oxide and alkaline waters. Aging, lack of methylation or low levels of histamine can also reduce stomach acid.