When diets are ‘measured’ (either by food record or questionnaire), the recorded foods are translated (using computer based ‘food tables’) into nutrients. So, for example, a typical apple contains water, sugar, fibre and various vitamins and minerals. the
One of the main problems arising from this is the over interpretation of the ‘benefits’ of single nutrients. There is no doubt that diets containing more fruit and vegetables do result in better overall health; but just which of the constituents, in which amount s and in which combinations with which other nutrients confers health benefits is simply unknown. Vitamin C, for example, is undoubtedly associated with health, but dietary vitamin C is always consumed along with fibre and many other dietary micronutrients too.
There is no evidence at all that supplemental vitamin C has any benefit. Moreover, the known vitamins (Vitamins A, B complex, C,D and E) were characterised in the early 20th century and more recently, many other biochemically active, and powerful vitamin-like compounds have been identified (for example lycopene, found in tomatoes); these are not included in most ‘food tables’ and thus overlooked.