Vitamins are great when you include them as part of an all around healthy diet. However, it is important to try to get as many nutrients from fresh, whole foods as you can. When taking vitamin supplements, you single out one nutrient and send it into your body alone. You do not have all the other compounds working together with it as you would from consuming whole foods. Your body does not absorb this as well as it would through food. Also, considering how much work is going on in the nutrition field, their is still a lot we do not know. An example is with Omega 3 fatty acids in fish. Scientists single out this specific part of fish as having health benefits so they make supplements for it. However we do not know if that is what makes it healthy, or some combination of Omega 3's and other nutrients in the fish.
High doses of vitamins(more than your daily recommended amount) can actually be harmful to your body. Vitamins B and C are water-soluble so over consumption of these will really not have much of an effect. Vitamins A, D, E, and K are fat-soluble and can be stored for long periods of time, and too much can result in harmful levels in the body.
This is not to say taking vitamins and supplements are bad. They can be added into your diet as a bonus, just not something to replace a food group. The best way to get in the recommended amount of vitamins and minerals is still though the consumption of fresh foods with minimal processing.
