In his book, Dr Cobb refers to l-ascorbic acid and d-ascorbic acid, inferring that the latter is worthless and must be excreted by the body. He says most artificial vitamin c is a mixture of the two types.
I've never seen any vitamin c product which makes a distinction. It just says ascorbic acid on the packaging. Should we now stick to taking l-ascorbic acid? Assuming we can find a product of that type.