Female Physiology

Iron and the Absorption Week: Why the Quiet Weeks Are Training Too

The fatigue that lingers into an easy week is often not a fitness problem at all.

Every big training block is followed by a quiet one. The volume drops, the sessions ease off, and by day two or three most athletes expect to feel lighter and more capable. For a meaningful number of women, that lift does not come. The legs stay heavy through an easy week that should have felt easy, and the instinct is usually to blame fitness, motivation, or not recovering hard enough. Often, the real answer is iron.

Iron deficiency is one of the most common and most under-diagnosed causes of unexplained fatigue and stalled adaptation in endurance athletes, and female athletes are affected disproportionately at every stage. Nabeyama and colleagues (2023) found that 47 percent of female university athletes had ferritin at or below 30 ng/mL, the threshold for iron deficiency, and half of those athletes reported a subjective performance decline tied to it. Other research puts the figure as high as 60 percent of female athletes carrying some degree of iron deficiency. This is not a rare problem sitting at the margins. In a group of women training through a heavy vertical block, it is reasonable to expect a substantial share are running on suboptimal iron stores with no obvious explanation beyond persistent, disproportionate tiredness.

What makes this a female-specific issue rather than a generic athlete issue comes down to two things: monthly blood loss as an ongoing drain on iron stores, and a hormone called hepcidin that actively works against female athletes trying to replace it. Hepcidin is the hormone that regulates how much iron the gut actually absorbs from food, and it spikes for three to six hours after a hard training session. Eating an iron-rich meal or taking a supplement inside that window is, physiologically, close to wasted effort. Alfaro-Magallanes and colleagues (2020) confirmed this hepcidin response specifically in endurance-trained women, which makes the timing of iron intake an active training decision, not a background nutrition detail. A pre-session meal or a next-morning dose absorbs far better than the recovery snack eaten straight off the trail.

There is also a diagnostic trap worth knowing about. Standard blood markers are not always sufficient on their own. Haemoglobin can be distorted by training-related plasma volume changes, and ferritin itself is what is called an acute-phase reactant, meaning inflammation from training can push it up and mask a true deficiency underneath. This is why Pedlar and colleagues (2018) recommend longitudinal tracking, read alongside cycle pattern and training load, rather than a single point-in-time result. A ferritin target above 40 μg/L is generally considered adequate for high training load, 20 to 35 is worth watching and adjusting through diet, and below 20 warrants working with a physician rather than self-treating with supplements, since iron overload carries its own risks.

The practical takeaway is simple, and it fits neatly into a quiet week: this is exactly the moment to check in. Note where you are in your cycle, whether a ferritin test is due, and whether the fatigue you are feeling matches the volume on the page. If you are menstruating, testing roughly twice a year, or quarterly if you know your periods are heavy or your levels run low, gives you a real picture rather than a guess. Pair haem-iron food sources with vitamin C, keep coffee, tea and calcium away from iron-rich meals, and protect the pre-session or next-morning window for anything iron-focused. An absorption week that looks easy on paper is still building fitness, provided the body underneath it actually has what it needs to adapt.

Sources: Nabeyama et al. (2023), PMID 38018828. Pedlar et al. (2018), PMID 29280410. Alfaro-Magallanes et al. (2020), PMID 33348847.