Can Home Cooking Reduce Heavy Metals? Figures Shown for Hijiki, Rice, Innards, Tap Water, and Canned Food

Short answer

Hijiki: soaking, boiling from dry, and soaking then boiling in fresh water

These are the results of MAFF’s tests (fiscal 2014, 20 g of dried hijiki, 600 cc of water). The reductions are given as rounded figures such as “about 50%.”

More than 70% of the iron, dietary fiber, and calcium remained with every method. MAFF recommends not using the soaking water for cooking. For details, see Should You Avoid Hijiki?

Rice: polishing, rinsing, and cooking

Cooking in extra water and discarding it lets nutrients such as water-soluble minerals and vitamins escape along with it. We found no test under Japanese water conditions (water low in arsenic). There is also a report that cooking with water containing arsenic (10 and 50 μg/L) increased arsenic in the rice (Menon 2024).

Cadmium: barely reduced in rice, and far higher in innards

• Rice: polishing reduces it only 2–5%, and rinsing and cooking change it almost not at all. With brown rice as 100, polished rice is an average of 95% (2022–24 crops) and 96.8% (2001 test).
• Processing soybeans: 61–66% of the cadmium in soybeans moves into tofu, and the rest into okara and the like. About 80% (43–83%) moves into miso and 40–50% (43–60%) into soy sauce.
• We found no public data on how much cooking such as soaking then boiling in fresh water reduces cadmium.

In seafood, cadmium is far higher in the innards than in the muscle (MAFF, fiscal 1997–2002).

• Worked example (body weight 55.1 kg, absorption rate not considered, Urayomi’s calculation): the Food Safety Commission’s TWI of 7 is 386 μg per week. 11 g of Japanese flying squid liver gives 386 μg; 20 g of shiokara (mean 2.63) gives 52.6 μg (14% of the TWI); and for one at the maximum of 9.9, 198 μg (51%, or 144% against EFSA’s 2.5).
• The Consumer Affairs Agency explains that the TWI is a guide to long-term averages, and that exceeding this share at a single meal does not immediately become a problem. Public agencies do not say “don’t eat innards”; they advise watching out for the imbalance of eating a lot of the same food every day, and keeping a balanced diet.

Mercury: no difference by part, and no tests of reducing it by cooking found

• A Food Safety Commission document (a literature review) summarizes that there is no difference in mercury content between parts of a fish.
• In what we checked, we found no public tests on how much cooking reduces mercury in fish.
• What the government gives is the kind and amount of fish (recommended amounts for pregnant women; How Much Fish During Pregnancy?). Mercury in the body falls by half in about 2 months (MHLW).

Lead: peeling, tea, tap water, canned food, and tableware

• Taro (satoimo): with the skin on, the maximum was 0.09–0.36 mg/kg, but every peeled sample was below the limit of quantification (MAFF).
• Green tea: for lead in tea leaves, 77 of 120 samples were below the limit of quantification, and the maximum of 2.4 was one sample of bancha. With 10 g of tea leaves in 450 mL of 90°C water for 60 seconds, about 8.8% of the lead goes into the infusion. MAFF concludes that tea is not a major source of lead intake.
• Tap water: the Food Safety Commission advises that, until lead water supply pipes are replaced, the first water drawn in the morning and water after a long absence should be used for purposes other than drinking (a guide is 10–15 L).
• Canned food: an industry association advises that tinplate cans with no interior coating let tin dissolve more easily if left open in the can, so food should be moved to a glass, ceramic, or plastic container and refrigerated (treated here as the industry association’s explanation; tin intake is about 0.3% of the tolerable intake).
• Tableware: the Food Safety Commission advises not using decorative ceramics, which are not made for tableware, as tableware.

What you lose in exchange for reducing it

• Hijiki: more than 70% of the iron, dietary fiber, and calcium remains, but because the soaking water is discarded, what dissolves into water is lost.
• Rice: cooking in extra water and discarding it also reduces water-soluble minerals and vitamins. Brown rice is richer than white rice in vitamins B1 and E, iron, and dietary fiber (MAFF).
• The more the rice is polished, the more arsenic goes down, but the nutrients that are abundant in brown rice also go down.
• None of these methods marks a line of “do this and it is safe.” The reduction rates are values under the conditions shown.

Sources

• MAFF, arsenic in hijiki (home-cooking tests)
• MAFF, “Guide to Reducing Arsenic in Rice”
• MAFF, survey of lead in agricultural products
• MAFF, fiscal 2019 survey of lead, aluminum, and other contents in green tea
• MAFF, cadmium concentrations in domestic agricultural, livestock, and fishery products (seafood)
• MAFF, changes in cadmium through processing
• Consumer Affairs Agency, “Q&A on Cadmium in Food”
• FSCJ, Q&A on the “Lead” assessment report
• Japan Canners, Bottlers and Preservers Association, Q&A (storage after opening)
• Naito S et al. Food Chem 2015（PMID 25172713）
• Carey M et al. PLoS One 2015（PMID 26200355）
• Menon M et al. Sci Total Environ 2021（PMID 33153748）

Last checked 2026.10.03 (against the original government documents and papers, collected and cross-checked in Gemini Notebook).

Other explainers

• Should you avoid hijiki?
• How much fish during pregnancy?
• Arsenic and cadmium in brown and white rice
• Baby food, infant food, and powdered formula
• Differences between Japanese and overseas standards
• Fifty years of heavy metal intake in Japan
• Lead: tap water, tableware, cans, and spices
• Do detox and chelation work?