Low Ferritin and Iron Deficiency Anemia in Distance Runners — A Scientific Guide for Athletes and Coaches

You may or may not know John Davis — his blog runningwritings.com is a fantastic resource for the open-minded runners and coaches alike. He reports to be a biomechanics PhD student and like Steve Magness and myself, he has a passion for learning and sharing useful information related to running and performance.

He has two excellent short booklets on running training: Modern Training and Physiology for Middle and Distance Runners and Basic Training Principles for Middle and Long-Distance Running. They are useful primers — or reminders — on some of the fundamental training principles for runners. Buying both will cost you less than $10 total and it’s money well spent.

His last post was in 2018, but his archives are still up and relevant.

Today’s link is to his guide to Low Ferritin and Iron Deficiency Anemia in runners. John is not a doctor, nor purports to be one. He is not offering medical advice. If you struggle with iron-related blood issues, you should go see a qualified doctor to resolve it.

However, his guide is based on sound science. I think it’s valuable for coaches and runners to understand the basics of low ferritin and iron deficiency anemia, its impact on running performance, and possible steps to consider when addressing this condition. And John’s guide accomplishes this in a clear and succinct way.

Enjoy. | jm

Low Ferritin and Iron Deficiency Anemia in Distance Runners — A Scientific Guide for Athletes and Coaches

by John Davies

When I see a runner getting fatigued early on in workouts or struggling mightily in races for no good reason, there's one potential cause I always consider first: low iron.

Iron deficiency is a significantly underdiagnosed problem in distance runners. Low levels of hemoglobin in the blood, or low levels of the iron storage protein ferritin, can have a profoundly negative impact on your ability to have successful workouts and races.

Hemoglobin is the main building block for red blood cells, which carry oxygen from your lungs to your muscles. If you don't have enough hemoglobin, you can't make enough red blood cells, and as a result, your distance running performance will suffer. Furthermore, research and practical coaching experience suggests that low ferritin levels can cause poor performance, even when hemoglobin levels are normal.

We'll take a close look at the science behind low iron and distance running performance, then analyze the best ways to treat and prevent iron deficiency in runners.

THE BIOLOGY OF IRON AND RED BLOOD CELLS

One red blood cell contains millions of hemoglobin proteins

Hemoglobin is an essential part of your body's oxygen delivery system. It's a protein with four iron atoms at its core, and these iron atoms are what grant red blood cells their ability to transport oxygen (as well as give them their red color).

Because red blood cells must be replaced fairly frequently, your body keeps extra iron on-hand in a storage protein called ferritin. Your body's iron reserves are mostly locked up in ferritin, which can be called upon when needed to synthesize hemoglobin for new red blood cells, or other proteins and enzymes in your body that also require iron.  Low ferritin by itself is termed iron deficiency.

As you might guess, when ferritin levels in the body are inadequate, hemoglobin synthesis slows down and your body can't produce as many red blood cells. Abnormally low hemoglobin levels is a condition termed anemia, and when the cause is low iron, this is iron deficiency anemia.

THE PREVALENCE OF IRON DEFICIENCY AND ANEMIA IN DISTANCE RUNNERS

According to research from the Centers for Disease Control and Prevention, between 9 and 11% of teenage and adult women are iron deficient, while only 1% of teenage and adult men are iron deficient.1 In this context, "iron deficient" means serum ferritin levels below the standard lab reference ranges for the general population (typically 12 ng/mL). As we'll soon see, these ranges need to be increased for endurance athletes.

Iron deficiency anemia (meaning low hemoglobin in addition to low ferritin) occurs in less than half of those who have low ferritin. 

Among endurance athletes, iron deficiency and iron deficiency anemia are far more prevalent. A 1989 study in the Journal of Pediatrics found that 34% of female and 8% of male high school cross country runners had serum ferritin levels below 12 ng/mL.2 Another study in the Journal of Adolescent Health Care found similar results.   

In a sample of high school cross country runners, about 3% of boys and 40% of girls were iron deficient at the beginning of the season.3 This study went a step further and followed the same sample of runners throughout the fall. By the end of the season, the prevalence of iron deficiency had increased to 17% of boys and 45% of girls. Even among those who were not classified as iron deficient, the majority had a decrease in their serum ferritin levels.

Adult male recreational runners are at less of a risk for iron deficiency, but the same is not true for women. A 2010 study by researchers in Switzerland reported that 28% of female marathoners had iron deficiency, versus only 1.6% of male marathoners.4

Data from swimmers provides some interesting contrasts. Thomas Rowland and John Kelleher at Baystate Medical Center's Department of Pediatrics report a similarly high incidence of iron deficiency in female swimmers—47%—but a complete absence of low iron in male swimmers. Further, swim training does not seem to deplete iron levels like run training does.5 Rowland and Kelleher found no decrease in ferritin as the season progressed, unlike what happens with cross country runners.

WHY ARE FEMALE RUNNERS MORE LIKELY TO BE IRON DEFICIENT?

The data from the above scientific studies suggests three primary reasons for why female runners are at increased risk for iron deficiency.

First, their dietary intake of iron tends to be very poor. Rowland and Kelleher, for example, found that the female athletes in their study averaged less than half their recommended daily intake of iron.3

Second, the menstrual cycle induces a substantial loss of blood. This increases the body's demand for iron, since it must manufacture new red blood cells to replace the ones it loses.

Third, running training induces an additional loss of iron. This happens primarily through gastrointestinal blood loss. A very clever 1996 study by researchers in Germany used radiolabeled iron to estimate the body's iron turnover during periods of rest, intensive training, and intensive racing among high-level distance runners.6 They found that periods of intensive training or racing were associated with a three to six-fold increase in gastrointestinal blood loss. No trace of iron was found in the sweat or urine of the volunteers, indicating that blood loss from minor bleeding in the gastrointestinal tract is the prime cause of high iron turnover in runners (in addition to menstruation in females).

The high baseline level of iron deficiency in women and girls, coupled with the iron losses associated with menstruation and running training, results in a prevalence of iron deficiency in women that approaches 50%. And this is using the fairly conservative standards for ferritin levels used in the general population. As we're about to see, there is good evidence that distance runners can suffer from impaired performance even at iron levels at the low end of the "normal" reference range.

HOW TO GET YOUR IRON LEVELS CHECKED

The only way to determine if low iron is the cause of a runner's troubles is to get a blood test. You will need to request two separate tests: A "CBC," which stands for complete blood count, and a serum ferritin test.

Complete blood counts include an entire panel of tests, but the one you are interested in is your hemoglobin level.  Hemoglobin is reported in grams per deciliter (g/dL), or sometimes grams per liter (g/L), which is just g/dL multiplied by ten.

A serum ferritin test, measured in ng/mL

A ferritin test requires a separate blood draw; the value you are interested in here is your serum ferritin level, which is typically reported in nanograms per milliliter, or ng/mL. You may also see it reported as micrograms per deciliter, or µg/dL—these units are equivalent.

When your doctor calls to report your test results, make sure you ask for the specific number and units for both hemoglobin and serum ferritin.

Most medical labs now provide patients with the data from their blood tests (indeed, you are entitled to it because it is part of your medical record), so this is less of a concern than it used to be, but some runners have had issues with doctors who simply report that their values are "normal" or "a little low" without any context. You need to know the number to figure out the level of iron deficiency you are dealing with.

Another option is direct-to-consumer lab testing. If your insurance will not cover a lab test, or you have trouble convincing your doctor to order one, or if you simply don't want to bother with an office visit, you can pay for a lab test through so-called "direct access" labs.  These tend to be far less expensive than going through a doctor if you are paying out-of-pocket.

Unfortunately, because of legal restrictions, direct access lab testing is currently unavailable in New York, New Jersey, Rhode Island, and Maryland. Residents of those states will have to see their doctor or go out-of-state.

There's a plethora of independent labs that offer consumers blood draws for relatively little expense. Amid concerns about the accuracy of independent labs, it can be tricky to figure out what labs you can trust.

HOW CAN LOW FERRITIN IMPACT RUNNING PERFORMANCE WHEN HEMOGLOBIN IS NORMAL?

The most plausible explanation for the paradox of impaired performance in athletes with iron deficiency but not anemia has nothing to do with red blood cells. Remember in the introduction, when I mentioned that ferritin is an iron storage protein for hemoglobin manufacture, as well as other proteins that require iron?

Well, it turns out that there are several iron-containing enzymes that play a critical role in aerobic metabolism. As DellaValle and Haas write,

There are several iron-dependent enzymes involved in the transformation of chemical to mechanical energy during oxidative metabolism, which is the main energy pathway used by rowers [and runners] during endurance training and most competitive events performed submaximally, around the lactate threshold. During high-intensity activities such as a 2-km ergometer time trial [or a running race], impaired O2 transport capacity, even in the absence of frank anemia, may result in increased reliance on anaerobic metabolism to produce energy.8

So, in layman's terms, iron deficiency can impair your production of enzymes important for creating aerobic energy, so you're forced to dig into your anaerobic energy systems earlier, thus becoming fatigued more rapidly.

After reviewing the research, this much becomes clear: Runners should aim for ferritin levels above 40 ng/mL to avoid fatigue and impaired performance from iron deficiency.

WHAT TO DO WHEN YOU HAVE LOW IRON

In most cases, the easiest and most straightforward way to fix iron deficiency is the most obvious one: increase your iron intake.  This can involve increasing your dietary intake, taking an iron supplement, or, preferably, both…

….continue reading John Davis’ article on his blog runningwritings.com here.