Iron and Hemoglobin: How They Impact Your Performance

Summary: This article explains how hemoglobin and iron affect oxygen transport, influencing your endurance and performance. The key question is what occurs when your iron stores are low, even if you don't have obvious anemia, and what levels you should monitor as an athlete. The conclusion is that iron deficiency can slow down both oxygen transport and cellular energy management, but excess iron isn't beneficial without a deficiency. First: review your diet and measure relevant values. Supplements may be necessary with low stores but should be based on test results and preferably discussed with healthcare providers.

What is hemoglobin?

Hemoglobin is a protein in red blood cells that, with the help of iron, binds oxygen so it can be transported through the blood to muscles and other tissues in the body that need oxygen. Hemoglobin consists of a heme group — the iron-rich part (ferrous iron, Fe2+) that gives blood its red color — and globin, a globular (rounded) protein.

What hemoglobin does in the body

Hemoglobin binds oxygen in the lungs where the environment is more basic (rich in oxygen) and releases it in tissues where the environment is more acidic (less oxygen). It's a clever natural mechanism: an equilibrium reaction that ensures tissues with lower oxygen availability receive more oxygen. During activity, oxygen release is directed both by blood circulation and by the local environment's oxygen and pH levels. When you exert with your legs, the blood flow to the capillaries in the leg muscles increases, these muscles use more oxygen, thus having less oxygen available — so more oxygen is delivered right there.

Generally, males have more iron in their bodies than women (males have about 4 g iron in their bodies, while women have about 2.5 g). Approximately 80 percent of the iron is active (for example in hemoglobin, myoglobin and enzymes) while the remaining 20 percent, in the form of ferritin, functions as storage. Hemoglobin is usually stated as Hb value in g/dl or g/L. The reference range is approximately 138–180 g/L for males and about 121–151 g/L for women. This is often mentioned as an Hb value of 120, 130, etc.

Iron Turnover and Our Daily Needs

Males lose about 1 mg of iron per day, while women lose around 2 mg. Iron absorption from food is typically between 5–15 percent. This means that to compensate for our losses, males need to consume around 10 mg of iron daily, and women around 20 mg. According to Riksmaten 2010 — the latest report on food habits in Sweden — an average Swedish woman consumes 9.5 mg, and males consume 11.5 mg per day. The recommended intake is 15 mg for women of childbearing age and 9 mg for males.

As you can see, males generally do well with the current intake, although some eat too little. Women, however, have a harder time reaching the recommended intake, especially in the 18–30 age group where iron deficiency (anemia) is most common. This is partly because women eat less, lose more iron through menstruation, and don't consume more iron-rich food than males.

Measuring iron levels in the blood often involves checking ferritin, the iron stored in reserves that correlates with the body's total iron. For active women, a ferritin value below 15 µg/L is considered deficient, 15–30 µg/L is low, and over 30 µg/L is acceptable. A ferritin level of 50 µg/L is a reasonable target. Low ferritin stores correlate with low Hb, but the relationship isn't always entirely causal: you can have low Hb and simultaneously high ferritin, and vice versa. If this happens, you should discuss it with your doctor.

How Does Athletic Activity Affect Our Needs?

Whether you run, cycle, swim, or ski, the body undergoes an inflammatory response — which is normal as it's part of the adaptation to training. However, during this inflammatory response, levels of the hormone hepcidin, produced in the liver, increase. Hepcidin inhibits iron absorption and transport — which is not ideal for us. Athletes, therefore, have a slightly elevated risk of iron deficiency or anemia compared to the inactive population, especially if they are in a negative calorie balance.

There's also the phenomenon of "foot-strike hemolysis," essentially the mechanical destruction of red blood cells due to running. This typically doesn't affect the body's iron stores and isn't usually something to worry about initially.

How does the iron level in your blood affect your performance?

Low iron levels impede oxygen transport and can slow down performance. It's important to note that it's not just the low level itself—research shows that a decrease in iron level, even without reaching levels classified as anemia, can reduce the number of mitochondria and the activity of enzymes important for cell respiration and oxygen management (research on animal models). This means that iron deficiency affects both oxygen transport and cellular oxygen management.

There are no exact levels that say "this much iron deficiency leads to x percent performance reduction." But values below 30 µg/L should be avoided. Studies have shown that women with low reserves who received iron supplements during a four-week training period experienced significantly better training effects than those who did not receive supplements (study). The greatest effect was seen in those with the lowest initial values—those who most needed supplements to reach normal levels.

Check Table 2 in this overview study for a summary of how anemia affects performance and what extra supplements, even intravenous, can do. Summarized data shows that women with very low reserves (<16 µg/L or Hb < 120 g/L) can achieve increased performance with supplements of 50–200 mg of iron (tablets) per day.

Is extra iron beneficial if you don't have a deficiency?

There is no evidence that more iron is better if you do not have a deficiency—on the contrary, too high iron levels can be harmful. When looking at the Hb value, it is the total mass of hemoglobin that determines the oxygen transport capacity, i.e., how many red blood cells you have. You can also get your Hb expressed as a percentage of blood volume. Typically, the hematocrit value is around 45 percent in males and 40 percent in women (erythrocyte volume fraction) (hematocrit value). A low hematocrit value together with high plasma volume is not necessarily negative—it is the total mass of red blood cells that is crucial for your oxygen transport ability. Good to know when interpreting test results.

Help with Iron Absorption

Yes, who didn’t have juice with blood pudding in school? Vitamin C aids in iron absorption. Coffee, tea, and calcium-rich products like cheese and milk can inhibit absorption. Good to know — plan meals and supplements accordingly.

Iron Supplements

Studies recommend supplementing around 20–60 mg of iron per day for athletic women (20 mg – 60 mg). The gut can significantly increase its absorption — in some cases up to fourfold — if the amount of consumed iron increases, so there’s usually no waste. Doses of 40–60 mg are also a safe daily dosage in combination with a varied diet.

However, as an athlete, you should first review your diet. Consider increasing the intake of red meat, which contains heme iron that we absorb at approximately 15–35 percent compared to non-heme iron from vegetables where absorption is at 2–20 percent source. A well-planned vegetarian diet can also meet the needs, but the recommendation is that the iron content in such a diet should be at least 1.8 times higher than a mixed diet containing meat, according to the American Dietetics Association. Important to consider when planning your diet.

If you can't boost your iron levels despite supplements — besides consulting with healthcare — it might be worth checking the levels of vitamin B12 and folate, as deficiencies in these can affect your iron absorption source.

Summary

• Women are primarily at risk of anemia, especially in sports where weight matters.
• If you don’t have a measured low value indicating anemia or iron deficiency, you should not start with iron supplements.
• If you are a fertile woman, possibly losing weight, training hard, and have decreased your meat intake, risk factors for low iron stores accumulate. Review your diet and supplement as needed — but only after you have a measured value.
• If you feel inexplicably tired, check your values and adjust in consultation with a doctor if ferritin is below 30 µg/L.

“Based on the cited results and according to several review articles, a ferritin cut-off of 30 mcg/l in adults seems to be most plausible [8, 83–90].”

Found a readable C-thesis which can be interesting if you are curious but don't feel like reading primary studies or need an easier entry.