In Japan, “boiling water in an iron kettle adds iron to the water” is a widely known benefit of using a traditional iron kettle.
And in a kettle where the water comes into direct contact with the iron surface, some iron does in fact dissolve into the water as it is heated.
However, not every iron vessel works the same way.
A useful example is the difference between an iron kettle and an iron teapot. Many Nanbu ironware teapots have an enamel coating on the inside. In that case, the water or tea does not come into direct contact with the iron, so they are not suitable if your goal is to obtain iron from the vessel.
Some traditional Nanbu iron kettles, on the other hand, are not enamel-coated inside. Instead, they may undergo a treatment known as kanakedome, in which the kettle is heated at high temperature to help prevent rust.
With this type of kettle, the hot water comes into direct contact with the iron, allowing some iron to dissolve into the water. If getting some iron from the vessel is your goal, this is the better choice than an enamel-coated iron teapot.
The amount of iron released, however, is not always the same.
Research that included tests using iron kettles found that the amount released into the water changed with repeated use. The amount fell considerably after the first few rounds of boiling, and later measurements also showed variation.
That means it is too simple to say that one use of an iron kettle will always provide a fixed number of milligrams of iron.
The research also found that much of the iron released into water from the kettle was in the form of ferrous iron, or Fe²⁺, a form that is generally considered easier for the body to absorb than ferric iron.
This helps explain why iron kettles have long been associated with increasing iron intake.
But iron dissolving into the water is not the same as saying that an iron kettle alone can prevent or treat iron deficiency or anemia. Research on iron cookware as a whole has produced mixed results, so an iron kettle should not be treated as a substitute for an adequate diet or medical care when it is needed.
For foreigners buying Nanbu ironware, appearance alone can be misleading.
Do not look only for the words “cast iron.” Check whether the item is a kettle meant for boiling water or a teapot with an enamel-coated interior. If your goal is to obtain some iron from the vessel, what is on the inside matters more than how it looks from the outside.
References
- Nanbu Tekki Cooperative Association — “Manufacturing Process of Nanbu Ironware”
- Iwachu — “Kyusu (Iron Teapots)”
- Journal of Cookery Science of Japan — “Elution of Iron from an Iron Pot during Cooking,” Akiko Konno and Keiko Oikawa
- Iron-containing cookware for the reduction of iron deficiency anemia among children and females of reproductive age in low- and middle-income countries: A systematic review
