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Pure Lab Vitamins: Carbonyl Iron. Carbonyl® Iron powder is metallic iron (Fe) with high (> 98%) iron content. It is not a salt. It absorbs at 69%, compared to only 10 to 15% with most other forms of iron. The high iron content and high absorption rate results in lower dose per capsules, achieving the same or better daily intake levels, compared to ferrous (Fe2+) salts (e.g., ferrous sulfate, ferrous fumarate, or ferrous gluconate), while at the same time and most importantly, side effects like constipation and gastro-intestinal discomfort and pain are avoided. In addition to the high iron content, a key physical property of Carbonyl® Iron is its fine spherical particle size and porousity with huge surface area, both resulting in higher human bioavailability than other iron forms. Carbonyl® Iron is far less toxic than soluble iron salts, it causes fewer side effects. With its low dose level, excellent bioavailability and low toxicity, Carbonyl® Iron is a better choice for iron supplementation. With greater than 98% iron, Carbonyl® Iron has a much higher iron content than ferrous salts, such as ferrous gluconate, ferrous sulfate and ferrous fumarate (Table 2). As a result, substantially less Carbonyl® Iron is required to reach the same unit dose than a ferrous salt, resulting in lesser side effects. The amount of iron absorbed by the body per unit dosage from a particular iron source is a function of several factors: particle size, surface area, ionic charge, iron content. The first three of these factors contribute to the relative biological value (RBV) of the iron source - a measure of how quickly the iron enters the blood stream. Table 1 shows that the high iron absorption from Carbonyl® Iron results from its small particle size, which contributes to a higher RBV than other forms of elemental iron, and its high iron content relative to ferrous sulcate. ("Ferronyl Iron" is Carbonyl® Iron)