鉄瓶と鉄分

How much iron does an iron kettle actually give you?

An unlined iron kettle does put iron into the water, and it is the readily absorbed kind. How much you actually get is a question that has not been answered. It is the most common reason people buy one, and it is the part of the object with the least evidence behind it — not because the evidence is bad, but because it has not been gathered yet.

What is settled: the form, and what draws it out

What is established is the form. Konno and Oikawa, publishing in the Journal of Cookery Science of Japan in 2003, report that 78 to 98% of the iron dissolved from iron pans and iron kettles is ferrous iron, the readily absorbed form, and that with vinegar added it reaches 98% and is stable. The same study puts the order of what draws iron out as vinegar, then tomato ketchup, then salt, with vinegar far ahead; longer heating dissolves more, oil dissolves less, and food cooked in an iron pan carried more iron than the same food cooked in stainless.

The one figure with a number on it

The one measurement with a number attached to it is a maker’s. Wahei Freiz, a cookware maker in Tsubame-Sanjo, had ebi chili analysed by the Ken-O Research Institute: 0.2 mg of iron per 100 g in the ingredients, 5.7 mg per 100 g once cooked in an iron wok — 5.5 mg drawn out of the pan. The company says it chose that dish deliberately, because iron dissolves more readily in the presence of both salt and acid. Note what that figure is and is not: it is a dish, simmered, salted and acidic. It is not a cup of hot water.

What is not settled: the kettle

For the kettle itself there is no equivalent figure. In July 2026 the Le Ber Vie Institute of Preventive Medicine, a Japanese analytical laboratory, opened a nationwide collection of tetsubin water — participants send in the water before and the water after, and the laboratory measures both — on the stated grounds that there is almost no data on how much iron real kettles release. It lists the variables it expects to matter: maker and production method, years of use, how the kettle has been cared for, the water boiled in it, how often it is used, and the region. Until that returns, a single number for “a kettle of water” is not something anyone can honestly give you, and a number offered without one of these conditions attached should be read with that in mind.

What the requirement actually is

It is worth knowing what the requirement is, because the figures in circulation do not match it. Japan’s Dietary Reference Intakes, 2025 edition, put the recommended daily iron intake for ages 18 to 29 at 7.0 mg for men, 6.0 mg for women who do not menstruate and 10.0 mg for those who do; for ages 30 to 49 the figures are 7.5, 6.0 and 10.5 mg. A specialist Nambu ironware retailer states the daily requirement as 10 mg for adult men and 12 mg for adult women on the same page where it explains iron leaching — higher than the current standard on both counts. This page is not advice about your own intake; the point is only that the requirement is a published, checkable figure and the kettle’s contribution is not.

What follows from it

• ✓The form is the strong part of the claim. 78 to 98% ferrous is a published, peer-reviewed figure, and it is the half of “iron from cast iron” that holds up.
• ✗The amount is not. No one has published what a kettle of boiled water gives you under ordinary use — which is why a laboratory is currently collecting it. Treat any confident milligram figure for a kettle as unsourced until that lands.
• ✓Salt, acid and time are what move the needle. Vinegar far ahead of ketchup, ketchup ahead of salt; longer cooking more, oil less. That is a cooking pan’s working conditions, not a kettle’s.
• ✗An enamel lining gives none of it. Glossy black or blue inside means the water never touches iron.

Unless yours is lined

One thing here is settled and easy to check: an enamel-lined piece contributes nothing at all. The Nambu Ironware Cooperative Association’s figures, quoted by the specialist retailer above, are explicit that the glassy lining fired onto the iron prevents rust and prevents iron dissolving with it. Since a great many pieces sold abroad as “tetsubin” are lined, that is the first thing to establish about any piece bought for this reason — and it is visible from the inside, which is the subject of the guide below.

This page reports what these sources say and is not advice about your own diet or your own health. Iron intake in particular is something the Japanese standard itself treats carefully, and a question about your own levels belongs with a doctor rather than with a page about kettles.

Where to look

There is no shop link on this page, deliberately. The honest summary of the evidence above is that the amount is unmeasured — and putting a kettle to buy underneath that sentence would be selling the very claim the page has just said nobody has established. If you are buying a kettle, buy it for the water and the object; the guides below cover what decides whether it is the kind that meets iron at all, and what its makers say about the inside of it.