Eleanor Whitney offer this explanation in “Understanding Nutrition”:
Getting vitamins and minerals from food is best because they’re diluted and dispersed among substances that help your body absorb them. When vitamins and minerals are concentrated in pills, they interfere with each others absorption and with the nutrients in the food you’ve eaten. And there’s a synergy of the parts that’s greater than the sum.
Why can’t we just take a vitamin pill and eat junk food?
Because vitamin pills don’t have the fiber or ephemeral (temporary) components like phytochemicals, which are only found in fresh food.
Phytonutrients often give plant foods their characteristic colors and flavors (there may be 80,000 of these nutritional chemicals, most of them are not identified yet).
Large epidemiological studies have shown that consumption of fruit and vegetables (5 or more portions daily) is associated with decreased risk of several diet-related diseases such as cancers and heart disease, and it is thought that this protective effect is at least partly due to the phytonutrients. Evidence suggests that it is the synergistic effect of several of these phytonutrients — the combination of thousands of these phytonutrients — that provides protective effects against disease. Therefore to gain these benefits, you need to eat the unprocessed foods they’re found within, i.e. vegetables, fruits, grains, nuts, or seeds.
Large sub-groups of phytochemicals include the carotenoids, phytosterols, saponines, glucosinolates, and polyphenols. These substances appear to produce very interesting anti-cancer, anti-microbial, anti-oxidant, anti-thrombotic and anti-inflammatory effects. Non-starchy vegetables naturally contain more micronutrients per calorie than any other food.
Other plant foods such as pulses, nuts, berries and seeds also provide a range of these phytonutrients; coffee and tea contain polyphenols. Nutrient-dense plant foods, especially non-starchy vegetables (leafy greens, cabbage, carrots, etc), help to promote health and satisfy appetite while reducing calorie intake, and are thought to increase the chances of living a long, healthy life.
(Nestle) 2008. Food and Nutrition Communication.