The recommendation comes from "Food, Nutrition, Physical Activity, And The Prevention Of Cancer: A Global Perspective":
- Eat at least five portions/servings (at least 400 g or 14 oz) of a variety of non-starchy vegetables and of fruits every day
- Eat relatively unprocessed cereals (grains) and/or pulses (legumes) with every meal3
- Limit refined starchy foods
- People who consume starchy roots or tubers as staples also to ensure intake of sufficient non-starchy vegetables, fruits, and pulses (legumes)
The paper also contains a quick summary of the justification, plus two chapters dedicated to the specific findings:
An integrated approach to the evidence shows that most diets that are protective against cancer are mainly made up from foods of plant origin.
Higher consumption of several plant foods probably protects against cancers of various sites. What is meant by ‘plant- based’ is diets that give more emphasis to those plant foods that are high in nutrients, high in dietary fibre (and so in non- starch polysaccharides), and low in energy density. Non- starchy vegetables, and fruits, probably protect against some cancers. Being typically low in energy density, they probably also protect against weight gain. Further details of evidence and judgements can be found in Chapters 4 and 8.
Non-starchy vegetables include green, leafy vegetables, broccoli, okra, aubergine (eggplant), and bok choy, but not, for instance, potato, yam, sweet potato, or cassava. Non- starchy roots and tubers include carrots, Jerusalem arti- chokes, celeriac (celery root), swede (rutabaga), and turnips.
So basically the recommendation is to eat plants, not to eat vitamins. Vitamins are not a good substitute for eating a lot of veggies in this sense.