Normal anion gap metabolic acidosis is caused by-

Metabolic Acidosis This calculation, normally 10 to 15 mmol/L, represents the unmeasured negative ions, predominantly albumin, in blood. Normal anion gap acidosis (low serum HCO3 but normal anion gap) is caused by excess bicarbonate loss from either the gut (diarrhea),cholera or kidney (renal tubular acidosis Ref Davidson 23rd edition pg 364