Food Sources of Iron“I might be low on iron”….have you ever had that thought or possibly been diagnosed with iron deficiency? It is one of the most common nutritional issues experienced by endurance athletes, particularly women. This blog aims to give you an understanding of the role that iron plays in energy production, how to identify if you might be deficient and how to change your diet in order to ensure an adequate intake of iron from food. Iron is one of those nutrients where more is definitely not better, so please don’t assume that taking an iron supplement is something you need to do in order to give yourself a performance edge.

Iron is an essential component of haemoglobin, the protein that carries oxygen and carbon dioxide in your blood. It also plays a role in the electron transport chain within the mitochondria – the “batteries” of your muscle cells – to produce ATP, your body’s primary energy source. Iron is therefore critical to exercise performance. The transport and use of iron is tightly regulated by your body. It is stored as ferritin in your liver until needed. If your ferritin level is low – as identified via a blood test – this is a sign that you do not have adequate iron stored. You can have low ferritin levels but normal haemoglobin levels. A medical diagnosis of anaemia means that you have a low haemoglobin level in your blood, which adversely affects the transport of oxygen. The most common, but not the only reason for developing anaemia, is inadequate iron because you cannot make haemoglobin without it. This is Iron Deficiency Anaemia. If you have low ferritin stores but normal haemoglobin levels, this is known as Iron Deficient Non-Anaemia (IDNA) or simply iron deficiency.

Research shows that having low ferritin stores (usually defined as <20 µg/L) can result in fatigue and ultimately could lead to reduced endurance. Having Iron Deficiency Anaemia produces heavier fatigue and a more dramatic decline in performance, linked at least in part, to a reduction in VO2 Max. Anaemia is usually diagnosed as haemoglobin < 12g/dL and ferritin < 12 µg/L and is usually resolved through iron supplementation. The dose varies according to the form of iron supplemented, as absorption rates vary. Ferrous sulphate is often used; 100mg has 18mg available for use in the body.

Whether correcting low ferritin stores via iron supplementation produces a subsequent improvement in endurance performance is less clear cut. A 2013 review published in Current Sports Medicine Reports 1 showed that three of the 11 studies reviewed found no effect of iron supplementation on performance despite serum ferritin levels being improved. However, the other 8 studies reviewed did show improved endurance times, faster time trials and increased energy efficiency when compared to placebo supplementation. All the studies reviewed involved female athletes. A more recent 2014 review published in the British Journal of Sports Medicine 2, which included 17 studies, concluded that iron supplementation did improve both the iron status and the aerobic capacity of iron-deficient non-anaemic endurance athletes. The researchers also noted, however, that the effect of iron supplementation on serum ferritin levels appeared to decrease if continued for more than 80 days.

Athletes can be more prone to low iron than non-athletes due to losing red blood cells from striking the foot on the ground through running. They also lose iron through sweat and sometimes gastro-intestinal bleeding. Female athletes with heavy blood loss through menstruation are particularly at risk of iron deficiency. It’s also worth knowing, however, that endurance training itself can affect your blood measurement of iron, as training produces an increased blood volume. This dilutes haemoglobin, making it appear low in some athletes when iron stores (ferritin) are normal. This is known as ‘sports anaemia’ and has no harmful effect on performance. There’s no benefit from taking iron supplements in this case.

So, should you get your haemoglobin and ferritin stores tested?

If you are experiencing more fatigue than usual, plus poor recovery and an unexplained drop in performance, you should ask your doctor to run a blood test for both these markers. I suggest asking to have your folate and vitamin B12 levels tested as well, as these two nutrients are also involved in the production of red blood cells. Equally, it’s important not to take iron supplements without medical guidance and to re-test your blood after a period of supplementation.

To avoid developing low iron levels in the first place, it’s important to be aware of getting enough iron in your diet. This isn’t just dependent on eating plenty of iron rich foods, but also on ensuring that you absorb as much iron from these foods as possible. There are two forms of iron in food: haem and non-haem. Haem iron is found in animal foods and about 25% of the iron is absorbed. The richest sources are red meat such as beef, lamb and venison. Offal, eg liver, kidney, is also high in iron. Dark chicken or turkey meat and seafood like prawns and oysters also provide a reasonable level of haem iron. All meat and fish contains some iron, as do eggs.

Non-haem iron is found in plant foods, and a smaller proportion of this is absorbed. This means that vegetarians need to take particular care over their iron intake. Absorption of iron from plant foods is improved if you add foods high in vitamin C to the same meal. Think peppers, oranges, tomatoes, berries, orange juice and dark green leafy vegetables. Some plant foods with the highest amounts of iron are dried apricots, red lentils, beans, tofu, pumpkin seeds, prunes, chick peas, wholemeal bread, cashew nuts and broccoli. Some breakfast cereals are fortified with iron. Contrary to popular belief, spinach is not one of the best plant sources of iron. It is also worth knowing some substances found in coffee, tea and alcohol inhibit the absorption of iron, as do foods high in calcium. It’s best not to eat your iron-rich foods at the same time as enjoying a cup of tea with milk!

So, in summary:

  • Get both your haemoglobin and ferritin levels tested if you experience unusual fatigue, poor recovery or an unexplained drop in performance
  • Only take iron supplements under medical guidance
  • Consume iron-containing foods regularly and be aware of the issues with absorption
  • If you are vegetarian, make sure that you include 3-4 sources of iron in your diet every day

Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sportBased in London, she also works as a Registered Nutritional Therapist, conducting onetoone consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit


1 DellaValle DM. Iron supplementation for female athletes: effects on iron status and performance outcomes. Curr Sports Med Reports. 2013. 12(4): 234-239.

2 Burden RJ et al. Is iron treatment beneficial in iron-deficient but non-anaemic (IDNA) endurance athletes? A meta-analysis. Br J Sports Med 2014 Oct 31. bjsports-2014-093624 [e-pub ahead of print]