Does heme iron cause melena (black stool)?

From the Guidelines

Heme iron can cause black stool, but the provided evidence does not directly support this claim, and the relationship between heme iron and black stool is more complex than a simple cause-and-effect relationship. The evidence provided focuses on haemochromatosis, a condition characterized by iron overload, and discusses dietary recommendations and lifestyle modifications to reduce iron accumulation and prevent organ damage [ 1 ]. While the evidence mentions heme iron and its absorption, it does not explicitly address the relationship between heme iron and black stool. However, it is known that heme iron can be absorbed and metabolized in the body, and any excess iron can be excreted in the stool, potentially causing a change in color. Some key points to consider when evaluating the relationship between heme iron and black stool include:

• Heme iron is well absorbed and its bioavailability is little affected by meal composition [ 1 ]
• Individuals with haemochromatosis show a greater dietary iron absorption, for both heme and non-heme iron [ 1 ]
• Meat consumption, especially red meat, should be limited to reduce iron intake [ 1 ]
• Certain diet components, such as black tea, may have an inhibitory effect on iron absorption in patients with haemochromatosis [ 1 ] It is essential to note that black stool can also be a sign of a more serious medical condition, such as bleeding in the upper digestive tract, and should be evaluated by a healthcare professional if it occurs without a clear explanation, such as iron supplement use. In clinical practice, it is crucial to consider the potential causes of black stool and to evaluate patients accordingly, taking into account their medical history, symptoms, and laboratory results.

From the Research

Heme Iron and Black Stool

• The provided studies do not directly address the relationship between heme iron and black stool, but rather focus on the effects of oral iron supplementation, particularly ferrous sulfate, on gastrointestinal symptoms and stool color.
• However, it can be inferred that iron supplementation, in general, may cause black stools due to the oxidation of iron in the gut, which can lead to the formation of melanin-like compounds that give stool its dark color 2, 3.

Gastrointestinal Side-Effects of Iron Supplementation

• Ferrous sulfate supplementation has been shown to cause significant gastrointestinal side-effects, including nausea, vomiting, heartburn, abdominal pain, diarrhea, and constipation, as well as black stools 4, 2, 5.
• The frequency and severity of these side-effects can vary depending on the dose and type of iron supplement used, with ferrous bisglycinate appearing to have a more favorable gastrointestinal side-effect profile compared to ferrous sulfate and ferrous fumarate 3.
• A systematic review and meta-analysis found that ferrous sulfate supplementation significantly increased the risk of gastrointestinal side-effects compared to placebo and intravenous iron therapy, with an odds ratio of 2.32 and 3.05, respectively 5.

Black Stools and Iron Supplementation

• The presence of black stools has been reported as a side-effect of iron supplementation, particularly with ferrous sulfate and ferrous fumarate 2, 3.
• The frequency of black stools has been shown to increase with the dose of iron supplementation, with higher doses of ferrous sulfate and ferrous fumarate associated with a higher frequency of black stools 3.
• However, it is essential to note that the relationship between heme iron and black stool is not directly addressed in the provided studies, and more research is needed to fully understand this relationship.