Conclusion
In people who do not meet the criteria for anemia but have low ferritin, a marker of iron stores, several studies report reduced fatigue after taking an iron supplement.
No similar benefit has been demonstrated in people whose blood tests show adequate iron levels. The benefit appears to depend on having low iron stores to begin with.
How much does it help?
In a trial of menstruating women with low ferritin (roughly below 50 micrograms per liter) but no anemia, 12 weeks of iron supplementation reduced fatigue scores by close to half compared with placebo.
Reviews pooling several studies also report modest improvements in fatigue and physical capacity among non-anemic adults with iron deficiency. Benefits typically take a few weeks to about three months to appear.
Trials in people with adequate iron levels have not shown a similar improvement.
What the research shows
The evidence base includes a systematic review of non-anemic, iron-deficient adults and a double-blind randomized trial in menstruating women. Most participants had blood tests confirming low ferritin.
Differences across studies in how iron deficiency was defined, including the ferritin threshold used, likely contribute to some of the variation in results.
Cautions
- The benefit of iron supplementation appears limited to people who actually have low iron stores
- Taking iron supplements on your own without testing can lead to iron overload; a blood test and a physician's guidance before starting are recommended
- Constipation and other digestive discomfort have been reported
A simple analogy
Iron supplementation works a bit like refueling a tank that has run low. Adding fuel to a tank that is already full does not change how the car drives, and people with adequate iron are unlikely to see a similar benefit.