Can supplements replace food?
Not really. Nutrients from food are still indisputably the most effective and best absorbed. Supplements may be able to fill in some blanks but it can never replace food.
For example, if you want your child to get the full benefit of health-promoting phytochemicals (plant nutrient-chemicals), you have to encourage your child to eat fruit and vegetables. The more colourful his repertoire is the better it is. Vitamin supplements may not necessarily contain these phytochemicals. Even if some included them, the nutrients are not necessarily in the same form as those in foods. For example, beta-carotene supplements contain mainly "trans" beta-carotene . Food like carrots and papaya contain mostly the "cis" forms of beta-carotene, which has shown to be the stronger antioxidant compared to the "trans" form.
Vitamin E is another good example. In food, vitamin E may be present in eight forms. Alpha-tocopherol is the best known of these forms present in supplements. But in reality, gamma-tocopherol is the most abundant in our diet. This may explain why higher levels of gamma-tocopherol in the blood are associated with a lower risk of heart disease but vitamin E supplements in form of alpha-tocopherol failed to show the same lowered risk.
Nutrients from food besides being present in full force and in the more 'effective' form, it is also present in a more absorbable form. Calcium in milk has always been known to be the most absorbable form. Milk calcium is distributed as follows: 20 % in an organic form bound to casein (solid part of milk protein) and 80% in a mineral form. Currently, most doctors agree that calcium supplements, in the form of calcium carbonate, is best. However, the absorption still falls far behind compared to milk calcium.
Another reason why getting nutrients from food is best is that there usually are other nutrients or components that aid absorption. For example lactose, the sugar naturally present in milk, helps in the absorption of calcium. Another bonus, milk also provide lots of other goodies for nourishing a healthy body such as high quality protein.
Eating a balanced diet to get all the nutrients you need is best. We need more than 40 known nutrients to function and possibly many others, less understood substances to be completely nourished. Only eating a wide variety within each food group will ensure that we get everything we need. Focusing on individual nutrients may prove too narrow an understanding of nutrition and provide a false sense of security.