Why ester cthe traditional form of vitamin c is ascorbic acid which in some cases causes discomfort through indigestion ester cis an advanced patented form of vitamin c developed through many years of research ester ccombines ascorbic acid with a vital mineral in a unique process that yields a vitamin c that is effectively absorbed and utilised ester cis therefore acid neutralised and gentle on the digestive system vitamin c contributes to maintain the normal function of the immune system