Effects of concurrent treatment of niacin on cell viability under low pH. Cells were cocultured with niacin (0, 20, 40, 80, 160, or 320 mM) and low pH (5.8, 5.6, 5.2 or 5.0) for 12 h. 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).