Iron Strength: What Endurance Athletes Should Know About Iron Deficiency Anemia and Ferritin Screening

By Holly J. Benjamin, MD | Aug. 27, 2019, 12:30 p.m. (ET)

swimming in cleveland

“Doc, can you please check my ferritin?”

Endurance athletes often want to know what their ferritin level is in addition to knowing if they are truly anemic (low red blood cells levels). Working muscles need oxygen to maximize energy production from the breakdown of glucose. Oxygen travels to working muscles bound to an iron atom within the hemoglobin molecule in red blood cells, much like boxcars on a train. 

There is abundant evidence that when the body’s ability to carry oxygen lowers due to iron-deficiency anemia, work capacity decreases, thus VO2max is affected. However, there are also concerns that low ferritin, even in the absence of true iron deficiency anemia (IDA), may have significant negative effects on both performance and general health.  

So why check ferritin?

Ferritin is a valuable blood marker for total body iron stores. However, its level in the blood can vary depending on what acute body stresses are occurring. Lack of sleep, infection, liver or chronic diseases, inflammation or intense exercise can have immediate effects on ferritin. Iron metabolism is a tightly regulated process in the human body. Ferritin helps control iron effects on the body and is an integral part of iron metabolism.

Running low

Ferritin levels have a broad range of normal, typically 10-300 ng/mL.  Low ferritin is highly specific for iron deficiency anemia. One is considered iron deficient with a level below 10-20 ng/mL.  The use of a higher cutoff value of 40-50 ng/mL improves the likelihood of identifying the presence of iron deficiency and may be more appropriate for endurance athletes.  In 1992, a systematic review concluded that a ferritin of < 15 ng/dL had a 52-fold greater likelihood ratio of correlating with the presence of iron deficiency and 15-35ng/dL was predictive of low iron stores.  Two major scientific studies in 2011 and 2012 demonstrated that in female athletes with chronic fatigue who had ferritin levels less than 50 and hemoglobin > 12 had significant increases in ferritin level, hemoglobin and more importantly, resolution of fatigue with iron supplementation.  This finding may be due in part because iron is not only critical for oxygen transport but also is involved in other cellular processes in our body.

Iron strength

Normal iron metabolism requires a meticulous balance between dietary uptake and loss. A certain amount of iron loss must occur daily and therefore a daily source of iron is needed. Seventy percent of the body’s iron is bound to hemoglobin in blood or stored in myoglobin in muscles, while 25% of the body’s iron is bound to ferritin. Iron is the active catalyst to bind oxygen to hemoglobin and myoglobin in the body. 

Simply put, hemoglobin is essential for transferring oxygen from the lungs to the blood to get to exercising muscles. So, whether you are swimming, biking, running or simply running errands, every breath you take is dependent on iron and ferritin to get the work done.

Am I iron deficient?

Iron deficiency is the number one nutritional deficiency in the United States, occurring in approximately 11% of women,1-2% of all adults and in approximately 12.5% of athletes. It is the No. 1 cause of anemia in athletes. Reports of iron deficiency rates (with or without anemia) in athletes range from 20-50% in women and 4-50% of men, according to the National Health and Nutrition Examination Survey (NHANES) II. However, despite the frequency of occurrence, making a clinical diagnosis of iron deficiency is challenging. 

There are many reports of athletes presenting with vague complaints of weakness, fatigue, decreased physical endurance, feeling hot or cold, diminished immune response, changes in energy levels, cognitive performance and behavior.  Iron deficiency is not the only cause of these common symptoms.  Causes range from a variety of health issues to simply overtraining or even lack of sleep.  Any athlete that experiences a decrease in training or performance coupled with even vague symptoms should see a qualified health care provider for a thorough evaluation.

Stage of iron deficiency

There are three stages of iron deficiency called prelatent, latent and true deficiency. The first two stages result in iron deficiency without anemia. Only the third stage reflects true anemia.  In the prelatent phase, iron stores are low, yet blood test findings are subtle and usually fairly normal except for low ferritin levels. In the latent phase, iron stores are exhausted, therefore both serum iron and ferritin are low, but hemoglobin remains normal. In true deficiency, measures of ferritin, serum iron and hemoglobin are all low.

Transferrin is the main protein in the blood that binds iron and ferritin then transports it throughout the body. Total iron binding capacity (TIBC) is an important blood marker and is a measure of the ability of transferrin to bind iron. As iron stores and depleted and iron is less available, TIBC rises. A simple example of supply and demand is illustrated here. As iron supply goes down, transferrin’s demand goes up and its capacity to bind all available iron is maximized (Table 1: Stages of Iron Deficiency).

Therefore, screening for anemia by measuring ferritin alone or obtaining a complete blood count (CBC) without ferritin is inadequate.  Iron deficiency might seem complicated but getting a thorough checkup coupled with complete iron testing allows a health care provider to identify the various stages of iron deficiency that affect many athletes. 

Let’s get technical

The most comprehensive clinical laboratory picture of a body’s iron stores involves obtaining a CBC, ferritin and complete iron studies (Table 2:  Critical laboratory values in iron deficiency).  Athletes should discuss their needs for comprehensive iron screening with a qualified health care provider as well as any test results.  

How do athletes lose iron? 

Iron losses occur from blood loss in the gastrointestinal (GI) tract, microscopic losses in urine, hemolysis red blood cells (RBC) breakdown, menstrual cycle, sweat loss, and intense exercise. Frequent use of medications with GI side effects such as aspirin and non-steroidal anti-inflammatories like ibuprofen or naproxen can cause or worsen iron deficiency.  Diet is also critical as both lack of intake as well as poor absorption can result in low iron stores. 

How much iron do I need?

Your body is a high-performance race car when it comes to iron and energy. You need fuel to train and race (gas in the tank). The harder you train and the more stress your body experiences will increase the need for iron. Similarly, your iron losses, iron absorption, and quality/quantity of iron sources will affect your needs. If you compete in the Indianapolis 500 or drive high speeds on the Autobahn in Germany, you will need extra fuel to burn. Suffice it to say, the amount of iron any athlete needs on a daily basis will vary from person to person.  It is affected by genetics, intake, absorption, utilization, and ongoing stresses.  Iron supplementation to maintain adequate stores is effective. 

Generally speaking, non-pharmacologic dietary supplementation is 10 mg/d for males and 15 mg/d for females. Eating foods high in iron is recommended.  Heme sources of iron such as red meat is more easily absorbed than non-heme sources (plant-based0.  Other dietary strategies such as concurrent Vitamin C intake can further increase iron absorption. Conversely, foods that reduce absorption such as tannins (black tea and coffee) and brans which bind iron should be limite

Supplementing iron

If your doctor has suggested an over-the-counter iron supplement, look for ferrous sulfate and ferrous gluconate as the iron source. These forms are widely available, inexpensive and time tested. Ferrous sulfate 325 mg (65 mg elemental iron) and ferrous gluconate 325 mg (36 mg of elemental iron) can be taken up to three times a day, preferably at the same time of day. 

Avoid enteric-coated forms, couple iron with vitamin C intake and take on an empty stomach in order to maximize absorption. Be aware that iron supplements may cause side effects like constipation, diarrhea, nausea, and dark stools so do your research first. Stained teeth can sometimes be seen with liquid forms of iron so use a straw. 

Make sure to follow recommended intake guidelines, as excess iron supplementation can have adverse health consequences. In the rare cases of extreme iron deficiency with failed oral treatment then intravenous iron supplementation is an available option, usually administered under the guidance of a hematologist or an experienced primary care physician.

Iron periodization-more is not always better

Every endurance athlete looks for a competitive edge.  So, one might consider the possibility that high dose iron supplementation might improve performance?  This, however, is not the case. Iron supplementation does NOT improve performance in non-depleted individuals. Only proper training will improve overall maximal performance.  So, iron supplementation, whether pharmacologic or dietary, should be aimed at maintaining adequate iron stores and availability, not trying to exceed the body’s demands.

Be mindful that during periods of increased training or stresses such as illness or injury, iron needs may rise, but during periods of low training or recovery, iron needs may be lower.  Therefore, strategic periodization of iron intake can be beneficial. Monitoring iron and ferritin through periodic bloodwork will help detect suboptimal iron levels.  Plan your testing strategically and be aware that serum ferritin can be falsely elevated for up to 72 hours after a long, strenuous session (IRONMAN 70.3, IRONMAN, marathon), or if you are experiencing significant inflammation or infections. The bottom line is ferritin and iron are your friends, not your foes, when it comes to endurance sports and performance.  Work with your health care provider to monitor your iron and ferritin levels and maximize your iron availability.


Holly J. Benjamin, MD, FACSM, is a specialist in sports medicine and non-surgical musculoskeletal injuries.  She is currently chair of the committee on sports medicine and fitness for the Illinois Chapter of the American Academy of Pediatrics, and a fellow of the American College of Sports Medicine. She serves as director of the Primary Care Sports Medicine Program at the University of Chicago Student Care Center.   She is a medical officer for the Chicago Marathon and Chicago Triathlon and is part of the medical team for the WNBA Chicago Sky. In addition, she is head team physician for the University of Chicago Lab School, and a team physician for the varsity athletic program where she specializes in the treatment of medical conditions in the athlete.

Andrew Getzin, MD, FACSM, is the head team physician for USA triathlon.  He is a USAT level 1 coach, many year USAT age-group All-American and has qualified and competed in Kona.  He is a fellow of the American College of Sports Medicine.  Dr. Getzin is the medical director of Cayuga Medical Center Sports Medicine and Athletic Performance in Ithaca, NY, www.cayugamed.org/sportsmedicine and the director of their shortness of breath in the athlete clinic, www.cayugamed.org/sob.


The views expressed in this article are the opinion of the author and not necessarily the practices of USA Triathlon. Before starting any new diet or exercise program, you should check with your physician and/or coach.


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