Vitamins strengthen our bones, muscles, the immune system and they can aid in metabolism. Most of these vitamins can be found in fruits and vegetables, however grains, meat, fish and milk also contain essential nutrients. It's almost impossible to overdose on the vitamins found in natural foods. But taking too many vitamin supplements can be too much of a good thing. Still, it's hard to know what constitutes an overdose. "We can assume that anything up to three times the Dietary Reference Intake is safe. Beyond that it's a grey area," says Hans Konrad Biesalski, head of the department of Biological Chemistry and Nutritional Science at the University of Hohenheim. Usually the body simply excretes the unneeded nutrients. But excess vitamins can trigger diarrhea, dizziness and vomiting. And new research suggests that taking more dietary supplements than needed may actually increase your risk of developing cancer and heart disease. So it's wise to take supplements only if your doctor establishes that you have a vitamin deficiency. That's often the case with older people. They generally eat less and therefore ingest fewer vitamins. Their metabolism is also slower, as is their skin's synthesis of vitamin D, so older people are often prescribed Vitamin D supplements. However you can't expect dietary supplements to counteract the effects of an unhealthy lifestyle. Natural foods contain far more nutrients and minerals that promote good health than tablets can ever hope to.