4. "Everybody" knows iron supplements give stomach problems.
For a very long time there was nothing like heme iron tablet preparations available for iron supplementation. All oral preparations were built on synthetic/chemical non-heme ingredients like Iron Sulphate, Iron Fumarate and the like.
In order for non-heme iron to be absorbed into the organism it needs to separate from its carrier i.e. dissolve for iron ions to be taken up. Free iron ions do not exist in nature and are considered toxic and very reactive. This is the reason why side-effects are so common. Since the uptake of non-heme iron is only 2 - 4 % most of the free iron ions are left in the stomach to irritate.
The remaining alternative is very expensive and inconvenient intravenous iron. There are potential risks with repeated iron injections over a longer period of time: overdosing and increased oxidative stress, which can be and often is fatal in weak patients.
The mechanism of absorption of natural heme iron is totally different. Heme iron is easily taken up in the intestinal cells as a whole unit, only to disintegrate later and be transported around the body. What is not absorbed of heme iron is totally inert and will not irritate. In all studies heme iron therefore shows a basically perfect tolerance.