Re: Criminal offence to impose a diet lacking in essential elements ?
It is difficult, though not impossible, to have a healthy vegan diet without taking some supplements, slightly less so for vegetarian. You can look at any reputable source, such as the NHS advice on vegetarian and vegan diets for children for a rundown on the supplements that will probably be needed.
You could certainly argue that a diet that requires supplements in order to provide the necessary nutrition is, by definition, "inadequate". However with the appropriate supplements is probably as healthy as a proper balanced diet. So I guess it depends which of those two cases you're referring to as to whether the diet is inadequate or not.