Most of the kelp people buy in the market is dried. Due to reasons like seawater pollution, some commercially available kelp now has a high arsenic content and must be carefully processed before consumption.
Arsenic and its compounds are often toxic, and long-term excessive intake can lead to chronic poisoning. The World Health Organization stipulates that the allowable daily intake of arsenic for humans is 0.05 mg per kg of body weight. However, the arsenic content in dried kelp, even after initial processing, already greatly exceeds this standard. Experiments have shown that after soaking dried kelp in water for 5 to 10 hours, its arsenic content remains as high as 5 mg per kg. Only after soaking in water for 24 hours does the arsenic content reach the food safety standard. When soaking, it is best to use weakly acidic water, such as rice washing water or tap water with a few drops of vinegar added. This way, the arsenic compounds in the kelp will react with the acid, becoming soluble substances that dissolve into the water.
Some people worry that soaking kelp for too long will cause a loss of nutrients. Indeed, after prolonged soaking, kelp will lose some nutrients. Among them, vitamin B1 and niacin will be partially lost, and some trace elements and mannitol will be lost in greater amounts, with iodine also experiencing some loss. However, this does not have a significant impact on the nutritional value of kelp. This is especially true for iodine, as its content in kelp is very high, so the loss of a small portion is negligible.
Everyone knows that patients with hyperthyroidism should not eat kelp because of its rich iodine content, which can worsen the condition. Additionally, pregnant and lactating women should not eat too much kelp. This is because the iodine in kelp can enter the fetus and infant's body through the bloodstream, causing thyroid dysfunction.
Kelp is rich in iron. After eating kelp, do not drink tea immediately (as tea contains tannic acid) or eat sour and astringent fruits (which contain plant acids) right away. This is because tannic acid and plant acids can interfere with the absorption of iron.