Three RCTs (106 participants) were included.
In all the included studies, the method of randomisation was not mentioned, allocation concealment was unclear and the percentage of patients lost at follow-up was below 3%. Two studies were double-blinded; the method of blinding was unclear in the third study.
At 5, 6 and 7 miles per hour running speeds, the pooled WMDs in heart rates after exercise between the iron and the placebo groups were -7.3 (95% CI: -19.6, 4.9, p=0.241), -6.6 (95% CI: -19.9, 6.6, p=0.327) and -8.0 (95% CI: -19.7, 3.7, p=0.182), respectively, suggesting no significant effect of iron supplementation. There was evidence of statistical heterogeneity for these outcomes (test for heterogeneity, p<0.001). After excluding a study with iron-deficient but non-anaemic participants, the WMDs were -13.1 (95% CI: -23.2, -3.1, p=0.01), -14.2 (95% CI: -22.3, -6.1, p=0.001) and -12.7 (95% CI: -23.5, 1.9, p=0.021), respectively, suggesting a significant positive effect of iron supplementation in terms of reducing heart rate after exercise. There was no evidence of publication bias on the funnel plot or when using the Egger's or Begg's methods.
One study found significantly lower levels of blood lactate in patients receiving 30- or 40-mg iron supplements in comparison with placebo both before (7.71 and 7.55 mg/dL versus 8.43 mg/dL) and after (14.36 and 14.35 mg/dL versus 16.48 mg/dL) exercise (p<0.05).
One study observed an increase in oxygen consumption in both the placebo and iron supplementation groups at 5, 6 and 7 miles per hour running speeds. The difference in oxygen consumption was not statistically significant between the treatment groups. Another study showed no significant differences in maximal or submaximal oxygen consumption among 5 patients treated with iron or placebo.
Treadmill endurance time.
Iron supplementation was associated with an improved endurance time (mean +0.57 minutes, range: +0.03 to +1.92), whereas a reduction was observed in patients receiving placebo (mean -0.67 minutes, range: -0.07 to -1.3).