Clinical trials of vitamin supplements are of no value or even harmful, have a flawed methodology, says research.
Many projects have tried to study nutrients that are naturally available in the human diet the same way they would a powerful prescription drug.
Balz Frei, professor and director of the Linus Pauling Institute at Oregon State University, said that this leads to conclusions that have little scientific meaning, even less accuracy and often defy a wealth of other evidence.
He said that almost every animal in the world, unlike humans, is able to synthesize its own vitamin C and doesn't need to obtain it in the diet, which makes it difficult to do any lab animal tests with this vitamin that are relevant to humans.
The new review has been published in the journal Nutrients.