In a 2025 study, the most common deficiency-related complication for participants starting GLP-1 therapy was nutritional anemia with iron-deficiency anemia developing in 1.6% of subjects after six months and 3.2% after one year.
Nutrient deficiency occurs when GLP-1s cause a dramatic reduction in how much people eat. Additionally, GLP-1s slow gastric emptying, which can affect the absorption of nutrients in the gut.
SloIron is a supplement that can reportedly bypass the need for gastric acid, which other non-heme iron require for absorption, the company said. SloIron does not rely on the DMT1 transporter for absorption but is absorbed through a completely different mechanism called receptor-mediated endocytosis.
“Iron supplemented through SloIron (ferritin iron) will not be affected by reduced gastric acid exposure,” said Vincent Hackel, SloIron CEO. “In addition, because the reactive ferritin iron is encased in a ferritin protein ‘cage’ and never exposed to the gut, it does not cause irritation to the gut like every other iron supplement.”
The absorption mechanism allows the ferritin iron to enter the gut cell without ever having to expose iron to the gut. It stays encased in a ferritin “cage” the entire time. The cage also serves as a natural enteric coating for the ferritin iron and is not broken down inside the stomach. Instead, when it reaches the gut, the gut cell absorbs the cage using a dedicated absorption mechanism, avoiding direct exposure of the iron to the gut wall, which can cause irritation. Ferritin-iron is bioavailable and has been shown in human clinical studies to be just as well absorbed in humans as ferrous sulfate, SloIron said.
A plant-based source
SloIron first became commercial as an ingredient in 2019. Its co-founder, Elizabeth Theil, a researcher who has a PhD in biochemistry from Columbia University, pioneered the science behind iron and ferritin, shedding light into the body’s ability to absorb ferritin iron from legumes.
Theil has authored several studies characterizing ferritin iron and its potential as a plant-based iron supplement that has an absorption mechanism different from any other iron form.
Legumes contain high amounts of ferritin which is not the case for other plants and vegetables. Soybeans naturally contain more ferritin than other legumes and ferritin iron from soybeans were the main source that Theil studied before the commercialization of SloIron. The first version of SloIron contained soy, and the company developed a non-soy alternative using peas for people concerned about soy as an allergen.
“The science behind SloIron is not based on the bioavailability or the dose,” Hackel said. “Ferritin iron has shown to be just as bioavailable as ferrous sulfate. However, what sets it apart is its unique absorption mechanism.”
The company also documents testimonials from people who struggle with increasing their iron and ferritin levels even when sufficiently supplementing with traditional iron supplements. SloIron said these individuals can experience a significant improvement in their iron and ferritin levels after taking the supplement, something they did not experience with traditional iron supplements.
As for GLP-1 users, ferritin iron may be more successful in preventing iron efficiency that may arise from the delayed gastric emptying effect of GLP-1s, which directly affects absorption for traditional non-heme iron forms but will not affect ferritin iron.
SloIron does not contribute to the common side effects of traditional iron supplements such as nausea and constipation, Hackel added. The supplement may be more successful in preventing iron deficiency that can arise from the delayed gastric emptying effect of GLP-1s, which directly affects absorption of all traditional non-heme iron forms but will not impact ferritin iron.
