Iron supplements with low risk of constipation
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Iron Supplement Types and Constipation Risk
Iron supplements are commonly used to treat or prevent iron deficiency, but gastrointestinal side effects—especially constipation—are a frequent concern, particularly among pregnant and premenopausal women. Research shows that the risk of constipation varies depending on the type and dose of iron supplement used, as well as individual factors such as pregnancy status and history of constipation 135.
Low-Dose Iron Supplements and Gastrointestinal Tolerability
Several studies have found that low-dose iron supplements are less likely to cause constipation and other gastrointestinal complaints compared to higher-dose formulations. For example, a study comparing different iron doses in pregnant women found that doses up to 60 mg of elemental iron per day did not increase gastrointestinal side effects, but an 80 mg dose was associated with significantly higher rates of constipation and laxative use . Another study in premenopausal women with iron deficiency (but not anemia) showed that a low-dose (27 mg) liquid fermented iron-bisglycinate supplement improved iron status without increasing constipation or gastrointestinal distress compared to placebo .
Iron Formulations: Ferrous Bisglycinate vs. Ferrous Fumarate and Ferrous Sulphate
The type of iron compound also affects the risk of constipation. Ferrous bisglycinate, a chelated form of iron, has been shown to have a more favorable gastrointestinal side effect profile than traditional forms like ferrous fumarate and ferrous sulphate. In a randomized study, women taking 25 mg of ferrous bisglycinate daily reported significantly fewer gastrointestinal complaints, including constipation, than those taking 40 mg of ferrous fumarate or 50 mg of ferrous sulphate . This suggests that ferrous bisglycinate may be a preferred option for individuals sensitive to gastrointestinal side effects.
Iron Supplementation in Pregnancy and Constipation Risk
Pregnant women are particularly susceptible to constipation, and iron supplementation can increase this risk. One study found that iron supplements were associated with a more than threefold increase in the odds of constipation during pregnancy . However, a supplement with lower iron content (35 mg) and a formulation that separates iron from calcium (PregVit®) resulted in a 30% reduction in constipation rates compared to a higher iron (60 mg) supplement (Materna®), while maintaining similar compliance and iron absorption . This highlights the importance of both dose and formulation in minimizing side effects.
Mechanisms Behind Iron-Induced Constipation
Oral iron can alter the gut microbiota, promoting the growth of certain bacteria and methanogenic archaea that may slow intestinal transit and contribute to constipation and bloating . This effect is more pronounced with higher doses and less absorbable forms of iron, which leave more unabsorbed iron in the gut.
Conclusion
To minimize the risk of constipation, low-dose iron supplements—especially those containing ferrous bisglycinate—are preferable, particularly for pregnant and premenopausal women. Formulations that separate iron from calcium and use lower elemental iron content can further reduce gastrointestinal side effects without compromising iron absorption. Choosing the right type and dose of iron supplement is key to improving tolerability and adherence.
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