Low-Carb Diets May Worsen Post-Exercise Iron Absorption

Carbohydrate restriction may matter more than a calorie deficit

A hard workout can temporarily make dietary iron harder to absorb. The more surprising finding is that severe carbohydrate restriction may amplify that effect even when athletes eat enough calories, while a much larger energy deficit with carbohydrates maintained did not change the same iron and immune biomarkers in one small trial.

That result comes from McKay et al. (2022), a six-day controlled dietary intervention in 28 elite race walkers. The low-carbohydrate, high-fat group ate fewer than 50 g of carbohydrate per day while consuming about 40 kcal per kilogram of fat-free mass. A low-energy-availability group maintained carbohydrate intake but received 15 kcal per kilogram of fat-free mass—a 62.5% calorie reduction relative to the low-carb group. The low-carb diet, not the low-energy diet, raised post-exercise interleukin-6, hepcidin, cortisol, and white blood cell count versus baseline.

The finding deserves attention, not a victory lap. It is a short trial with a small subgroup in each diet arm, and no study in this evidence base shows that a low-carb diet causes iron deficiency over months or years. Still, it identifies glycogen availability as a plausible risk factor that low-carb athletes should not wave away because total calories look adequate.

Why timing matters: exercise raises hepcidin

Hepcidin is a liver hormone that regulates how much iron reaches the bloodstream. After strenuous exercise, working muscle releases IL-6, which signals the liver to increase hepcidin production. Hepcidin then causes ferroportin—the protein that exports iron from intestinal cells into circulation—to be internalized and degraded. Iron can remain on the wrong side of the intestinal wall, which is an impressively bureaucratic way for the body to manage a mineral.

An individual-participant-data meta-analysis by Fensham et al. (2023) found that hepcidin typically peaks three to six hours after exercise. That window overlaps with the recovery meals and supplements athletes often use to improve iron intake. The analysis also identified baseline ferritin as a significant moderator of the hepcidin response. Ferritin reflects iron stores, so athletes with low stores may face impaired absorption at precisely the time they need replacement iron most.

Hepcidin is a biomarker and regulatory hormone, not a diagnosis of iron deficiency. A post-exercise rise does not tell us how much an athlete's ferritin will change over a season. It does, however, provide a biologically credible reason to separate hard training from iron supplementation when possible.

The energy-deficit study points to glycogen, but does not isolate it

A randomized crossover trial by Hennigar et al. (2021) showed that metabolic stress can worsen this bottleneck. Ten healthy men completed a 72-hour simulated military operation under energy balance and under a 45% calorie deficit. IL-6 elevations were nearly identical between conditions, yet hepcidin was 108% higher in the deficit condition: 7.8 ± 12.2 ng/mL, with P<0.0001.

The study also measured a functional absorption outcome. At 120 minutes, plasma iron-isotope appearance was 74% lower in the deficit condition and 49% lower in the energy-balanced condition than at rest; both reductions were statistically significant at P<0.01. Muscle glycogen fell significantly only during the deficit condition, by 188 ± 179 mmol/kg (P<0.01). That pattern suggests glycogen depletion may help explain why hepcidin rose more sharply despite similar IL-6.

But this design changed both energy availability and glycogen status in the deficit arm. It cannot prove that carbohydrate restriction alone caused the difference. McKay and colleagues supplied the cleaner comparison: in elite race walkers, the low-energy group with carbohydrate maintained showed no statistically significant difference from control in the measured iron or immune biomarkers, while the adequately fueled low-carb group did.

In that low-carb arm, post-exercise IL-6 rose significantly (P=0.019), as did hepcidin (P=0.011), total white blood cell count (P=0.026), and cortisol (P<0.001). Post-exercise blood glucose declined only in the low-carb group (P=0.049), consistent with lower carbohydrate availability during the exercise bout. These results support, but do not establish, a glycogen-depletion pathway.

The current evidence supports a plausible low-carb-specific risk signal, not a proven long-term iron-deficiency outcome.

Low energy availability still stresses athletes

The null iron-biomarker result in McKay et al. does not mean low energy availability is harmless. A 2026 systematic review by Guisado-Cuadrado et al. found broad biochemical stress responses to experimentally induced short-term low energy availability in athletes. Iron-specific markers were inconsistent across the included studies, however. The review therefore challenges any claim that calorie restriction is physiologically neutral, while remaining compatible with the narrower finding that carbohydrate restriction may be more important for this particular post-exercise iron pathway.

Training context also matters. In Fensham et al. (2022), sequential submaximal training in elite male rowers did not amplify IL-6 or hepcidin. Exercise intensity, modality, accumulated load, training adaptation, iron status, and diet can all influence the response. Race-walking data should not be treated as a universal prediction for every sport.

Where GOS fits—and where it does not

Galacto-oligosaccharides (GOS) are fermentable prebiotic carbohydrates. They may improve iron availability in the gut before iron encounters ferroportin. In a crossover stable-isotope study of iron-depleted women with ferritin concentrations of 15–30 μg/L, Husmann et al. (2022) found that GOS increased ferrous fumarate solubility at pH 4 and pH 6, but not at pH 2. The isotope curves supported absorption in the proximal intestine rather than the colon. In this iron-depleted population, 15 g of GOS increased fractional iron absorption by up to 61%; earlier crossover data discussed in the paper found improvement with 7 g as well.

That is an upstream effect on luminal iron chemistry. It cannot reopen ferroportin after exercise-induced hepcidin has triggered its degradation. GOS may therefore complement better supplement timing or adequate carbohydrate availability, but it cannot substitute for either.

Duration is another problem for anyone hoping to toss a scoop of GOS into one post-workout shake. A 2021 stable-isotope study found that a single GOS dose did not improve iron absorption from micronutrient powders in Kenyan infants. By contrast, a 2024 randomized trial found that chronic prebiotic supplementation increased iron absorption and reduced adverse gut-microbiome effects in Kenyan infants. Infant studies support a chronic prebiotic effect, but infants are not trained adults, and no trial has tested GOS in athletes combining hard exercise with a very-low-carbohydrate diet.

GOS also should not stand in for all fibers. A 2023 systematic review of dietary fiber and iron absorption found mixed results that depended on fiber type, dose, iron status, and host context.

What athletes can do now

The strongest practical move is timing. Because hepcidin peaks about three to six hours after exercise, athletes who use iron supplements or deliberately iron-rich meals can discuss placing them outside that window with a sports dietitian or clinician. Iron supplementation has real downsides when unnecessary, so blood testing should guide it.

• Monitor ferritin as well as hemoglobin if training volume is high or symptoms raise concern. Nabeyama et al. (2023) documented iron deficiency without anemia among university athletes; normal hemoglobin can miss depleted stores.
• For athletes following ketogenic or very-low-carb diets, consider carbohydrate availability around demanding sessions, especially when iron status is already low.
• Treat chronic GOS as an unproven adjunct for athletes, not a hepcidin antidote. Adult athlete-specific doses and treatment duration remain unknown.

The verdict

Two controlled studies support moderate confidence that low carbohydrate availability can amplify the post-exercise hepcidin response more than calorie restriction alone. The best isolating study lasted six days and involved 28 elite race walkers, so the result needs replication across sports, sexes, and longer time frames. No cohort data show whether iron deficiency is more common in low-carb athletes than in carbohydrate-adequate athletes.

For now, a very-low-carb athlete with low ferritin has a reasonable case for taking the risk seriously: measure iron status, avoid treating hemoglobin as the whole story, and do not assume a prebiotic can outmuscle hepcidin.