Tells your doctor: If you have low iron levels; sometimes done if you're experiencing fatigue, weakness or headaches.
Running's effects: Low levels, when combined with fatigue and low hemoglobin (see below), may mean you need iron supplements. However, ferritin also increases with inflammation. A recent cold or flu can artificially inflate your numbers, masking the underlying problem.
Tells your doctor: If you have anemia or a low red-blood-cell count; sometimes done as part of a regular check-up or if you have unexplained tiredness.
Running's effects: Slightly lower levels can actually be normal in a runner; your body produces extra fluid to keep your sweat and blood flowing freely, diluting your red blood cells. But iron-deficiency anemia is common, especially in women, and can cause fatigue that hampers your running. If your hemoglobin is low, your doctor may also want to run a ferritin test.
Tells your doctor: Whether you have kidney disease, a metabolic disorder or urinary-tract infection; sometimes part of a wellness exam
Running's effects: Blood and protein in your urine—normally red flags for kidney disease or even cancer—can occur for up to a day or two following a long or fast run.
Tells your doctor: Your blood levels of this nutrient; sometimes tested if you're experiencing frequent fractures, low energy or sleep issues.
Running's effects: Outdoor runners in warm climates often get plenty of the "sunshine vitamin." But if you live north of Nashville, have dark skin or are super vigilant about your SPF, you could be deficient. Low levels are thought to contribute to muscle pain, insomnia and a greater risk for stress fractures.race.