That being said, some runners may have nutrient deficiencies that may have an impact on their performance and health. For example, some female runners are low in calcium, putting them more at risk for stress fractures. While they can increase their consumption of calcium rich foods such as low-fat dairy products, their doctor may also recommend a calcium supplement to help reach the goal of 1,000 to 1,300 mg of calcium per day. An iron deficiency can also wreak havoc on runners' training because it can lead to feelings of sluggishness and low energy. If your doctor determines through bloodwork that you're low in iron, he or she may recommend an iron supplement.
The bottom line? It's not necessary for most runners to take supplements, but make sure you go for regular medical check-ups and talk to your doctor and/or a registered dietician about any concerns about nutrient deficiencies.
Also see: Diet and Nutrition for Runners
Nutrition and Hydration for Distance Runners