Nutrient absorption in runners is impaired after long runs or intense workouts. With a strenuous effort, blood is predominantly flowing to the muscles, heart, lung, and brain. The gut is the bottom of the blood totem pole at this point. Unfortunately, this results in poor digestion and absorption of nutrients due to villus blunting. Villi in the small intestine are responsible for nutrient absorption. Blood flows to the top of each villus "mountain," but when the blood isn't flowing to the gut, the villus tip temporarily dies and cannot absorb nutrients. As blood returns to the villi, absorption is resumed, but in the meantime, malabsorption and diarrhea are common.
Runners also are at a higher risk of anemia as a result of iron lost through sweat and red blood cell destruction from mechanical trauma, ie the pounding of each foot strike. Training at altitude further puts an athlete at risk.
Mechanical trauma, also called foot strike hemolysis (hemo- meaning blood and -lysis meaning death of a cell) occurs from vascular and red blood cell trauma in the foot. Also, red blood cells are more prone to oxidative damage, naturally occurring during exercise, which will speed the destruction of the cells.
Pseudoanemia, common in athletes, is caused by an expansion in overall blood volume. This dilutes the red blood cells, causing a false representation of anemia in blood tests, but does not usually cause problems with athletic performance. Actually, the increased blood flow aids in oxygen delivery.
Symptoms of anemia or iron deficiency include fatigue, pale skin, shortness of breath, dizziness, cold hands and feet, and headache. These symptoms normally do not appear in pseudoanemia. It is important to still get a blood test (CBC or complete blood count) to confirm a deficiency because iron can be toxic and harmful to the liver if too much is stored. A blood test will look at the following markers and help determine the cause of anemia:
- Serum Ferritin - levels of stored iron, 18-270 mcg/L (m), 18-160 mcg/L (f)
- Total iron binding capacity - protein that carries iron in the blood, measures how much transferrin in the blood is not carrying iron, 20-50%
- Red blood cell (RBC) - total number of red blood cells in that sample of blood, 4.2 - 5.6 mill/mcl (m), 3.9 - 5.2 mill/mcl (f)
- Hemoglobin (Hb) - protein in red blood cells that carries oxygen, 14-18 g/dl (m), 12-16 g/dl (f)
- Mass Cell Volume (MCV) - average volume of a red blood cell, clue to the cause of the anemia because cells are small in iron deficiency anemia as compared to a B12 or folate deficiency (pernicous anemia), 80-100 fl
- Hematocrit (Ht) - proportion of red blood cells in overall blood, 42-54% (m), 38-46% (f)
- Mean corpuscular hemoglobin (MCH) - average hemoglobin in the average red cell, 27-33 pg
- Mean corpuscular hemoglobin concentration (MCHC) - average concentration of hemoglobin in a given volume of red cells, 32 - 36 %