Effects of pre-treatment of niacin on cell viability under low pH. Cells were cultured with niacin (0, 20, 40, 80, 160, or 320 mM) for 12 h before 12 h treatment with low pH (5.8, 5.6, 5.2 or 5.0). Relative cell viability was presented as a percentage relative to the control group. a,b,c Mean values within different letters were significantly different (P < 0.05).