In recent years, the debate on salt iodization has been raging, with many different opinions, attracting a lot of media attention, so they also joined in, making people foggy for a while, not knowing at once whether they should use iodized salt or not. In order to trace the root of the problem and set the record straight, I had to search up and down through the vast literature and finally came up with some clues that I would like to share here.
1. What is the effect of iodine on the human body?
In fact, we are not unfamiliar with iodine, such as iodine for skin disinfection, iodine volts, it can also be used in the field of pharmaceuticals, dyes, etc. It is also one of the necessary trace elements for the body. Iodine in nature comes mainly from the ocean, where it can follow the air and rain into the soil inland, and from the earth’s crust, where it can travel to the surface in groundwater.
Most of the iodine that the human body naturally takes in comes from food and iodine fortification (e.g. iodized salt), and to a lesser extent from drinking water. The human body is “hungry” for iodine, and it is largely absorbed in about one hour and almost completely absorbed in three hours. After absorption, most of the iodine finds its home in the thyroid gland: like a vacuum cleaner, the thyroid gland “captures” iodine from the blood, and the body excretes iodine mainly in the urine, with a small percentage excreted in the feces, and a very small percentage in sweat, breath, and breast milk.
The physiological function of iodine is relatively single, mainly involved in the synthesis of thyroid hormone, which has the function of enhancing metabolism (such as maintaining normal body temperature, regulating human blood lipids, etc.) and promoting growth and development, especially brain development.
2. What is the damage caused by iodine deficiency and iodine excess?
The main consequence of iodine deficiency is thyroid dysfunction, which manifests itself in various ways, the well-known “big neck disease” (scientific name ‘endemic goiter’) being the most typical one, in addition to some intellectual and physical problems, especially in children and adolescents. It is worth noting that iodine deficiency in children and adolescents can affect growth and intellectual development.
Maternal iodine deficiency is actually secondary to its own effects, but mainly affects the brain development of the fetus and infant, and in severe cases even causes fetal abortion, malformation or death. Iodine deficiency can cause irreversible mental retardation, with “cretinism” (also known as cretinism) being the most serious risk caused by iodine deficiency.
Excess iodine can also affect thyroid function, leading to both hyperthyroidism and hypothyroidism, with excess iodine in pregnant women also leading to neonatal goiter and hypothyroidism. Most studies now suggest that excess iodine increases the prevalence of “autoimmune thyroid disease,” but the overall effect of “iodine supplementation” is small. For example, a Danish study showed that the overall incidence of hyperthyroidism increased by only 0.04% after 6 years of iodine supplementation.
The relationship between excess iodine and thyroid tumors has been a hot topic of concern in recent years, and there have been many scary and inaccurate reports. In fact, no correlation has been found between iodine intake and thyroid cancer in areas where it is adequate.
A Swiss study even found that the incidence of thyroid tumors gradually decreased after salt iodization, and the malignancy of thyroid cancer gradually changed from highly malignant to less malignant after iodine supplementation, and the World Health Organization (WHO) also believes that the incidence of thyroid cancer in areas with adequate iodine intake is much lower than that in areas with iodine deficiency. Therefore, there is no scientific basis for the claim that “salt iodization causes high incidence of thyroid cancer”.
3. Are we in a state of iodine deficiency or iodine excess?
Most areas in China are iodine deficient to varying degrees, with a population of over 700 million. In the 1970s, there were about 35 million people with “great neck disease” and about 250,000 people with typical “endemic cretinism”. By the 1990s, patients with abnormal thyroid function were found in 1,807 counties and 27,128 townships, and millions of patients with subcretinism, and the IQ of preschool children in the affected areas was 10-11 percentage points lower than normal.
With the promotion of salt iodization policy, the general iodine deficiency in China has been significantly improved, and according to recent research, most of the population in China is basically in a state of iodine nutrition in moderation, except for a few areas where people are still iodine deficient.
However, in 2007 and 2010 surveys, it was found that pregnant women in Nanjing and Hangzhou were still mildly iodine deficient, and in 2006, 34 villages in 11 villages in Xinjiang had new cases of cretinism, with a total of 76 cases under 15 years of age diagnosed, which was directly caused by a significant decrease in iodized salt coverage, suggesting that this current state of iodine adequacy is not easy to come by. If we blindly listen to the rumors and give up iodized salt without permission, it is entirely possible that “after working hard for half a year, we will go back to liberation overnight”.
4. What is iodized salt all about?
In view of the fact that the world’s population is suffering from iodine deficiency, the World Health Organization (WHO) has implemented a global strategy of salt iodization for the prevention and control of iodine deficiency disorders, believing that the health benefits of iodine supplementation far outweigh the potential health risks associated with iodine fortification. Some countries even add iodine to animal feed to increase the iodine content of food of animal origin.
The “global elimination of iodine deficiency disorders” was proposed by the United Nations in 1990, and the leading internationally recognized measure to prevent and treat iodine deficiency disorders is salt iodization. The implementation of salt iodization has reduced the prevalence of iodine deficiency among children and youth in areas with a high prevalence of “Great Neck Disease” by 40-95 percent worldwide. In Kangra district of India, for example, the prevalence of “Great Neck Disease” was 55% in 1956, but it dropped to 20-30% after 5 years of salt iodization and 8.5-9.1% after 15 years.
However, in 2000, the Indian government suspended salt iodization in response to “public opinion”, but in a short time, iodine deficiency disorders resurfaced everywhere, and in 2005, the Indian government had to resume salt iodization and ban the sale of non-iodized salt. Using “public opinion” rather than science to validate public health policies is a very idle move!
The WHO recommended 20-40 mg/kg of iodine in salt, and the first 20-60 mg/kg of iodine in our salt was adjusted downward to 35±15 mg/kg in 2000, and again to 20-30 mg/kg in 2007. This was an appropriate adjustment to the amount of iodine added based on the monitoring data during implementation, with the aim of avoiding excessive iodine supplementation as much as possible. Now it is even clearer that localities can flexibly control the amount of iodine added within the standard range according to the iodine nutrition status of local residents.
As for the rumor that iodized salt can be heated or exposed to the sun to reduce the iodine content by volatilization, it is completely outdated. The iodine used in the production of iodized salt is “potassium iodate” (chemical formula KIO3), not potassium iodide (chemical formula KI), which was used in the past (before the 1990s). Potassium iodide is indeed not stable enough and can be easily volatilized, destroyed or lost, but potassium iodate is more stable and not easily destroyed.
According to the test, 10 grams of iodized salt was placed in a 250 ml iodine bottle, dissolved in 200 ml of distilled water, heated and boiled for 30 minutes, and then the iodine content was measured, resulting in only 1.7% loss of iodine; heated to 1 hour, the iodine was also only 2% loss. This shows that the iodine in iodized salt is rarely destroyed by long time stewing. When dissolved iodized salt was heated to 100 degrees and maintained for 10 minutes, only 1.2% of iodine was lost; when heated to 300 degrees (the normal frying temperature is 180 degrees) and maintained for 10 minutes, only 5.2% of iodine was lost.
This shows that high-temperature cooking destroys very little of the iodine in iodized salt. It is also reported that the effect of sunlight exposure on potassium iodate salt is not significant, even if exposed to sunlight for 128 hours, the iodine loss is less than 5%.
5. What is the current recommended daily intake of iodine for humans?
WHO has established the recommended daily intake of iodine for different groups of people: 90 micrograms for 0-5 years old, 120 micrograms for 6-12 years old, 150 micrograms for 13 years old and above, and 200 micrograms for pregnant women and lactating mothers. By maintaining the intake at 150-300 micrograms through salt iodization, it is possible to ensure that the iodine intake of all people is in a safe range. The recommended daily intake of iodine set by the Chinese Nutrition Society is 50 micrograms for 0-3 years old, 90 micrograms for 4-10 years old, 120 micrograms for 11-13 years old, 150 micrograms for 14 years old and above, and 200 micrograms for pregnant women and lactating mothers.
If we calculate the average value of 25 mg/kg of iodized salt concentration in China, eating 150-300 micrograms as recommended by WHO is about 6-12 grams of salt. The WHO recommends that a daily iodine intake of less than 1000 micrograms is safe, and the Chinese Nutrition Society recommends a maximum safe daily iodine intake of 800 micrograms.
In short, there is a large margin of safety between the above dose and the amount of salt iodized: according to the upper limit of 30 mg/kg of iodine added in China, you need to eat 20 grams of salt to reach 600 micrograms, and the recommended salt intake is only 5-6 grams, and the average salt intake of Chinese people is about 12 grams, so you don’t have to be so heavy, do you? If that’s the case, you need to consider the risk of cardiovascular disease rather than thyroid problems.
6. Which people need to reduce the intake of iodine? Which groups of people should pay extra attention to the adequate iodine supplementation?
According to the current state of iodine nutrition and salt iodization in China, the vast majority of the population is extraordinarily concerned about iodine intake, and choosing non-iodized salt is actually a blind and anti-intellectual tendency for most people.
At present, it is considered that certain people who have been living in high iodine areas (note that high iodine areas are not coastal areas) for a long time, patients with hyperthyroidism and significantly increased T3 and T4, and certain patients who need isotopic iodine treatment or examination should be on a non-iodized or low iodine diet, but of course, patients whose hyperthyroidism has been controlled and whose T3 and T4 have returned to normal can still consume iodized salt normally.
In particular, women of childbearing age, pregnant women, lactating women, fetuses, infants, school-age children and adolescent children are the main victims of iodine deficiency, as these people are at a special physiological stage and are most vulnerable to iodine deficiency in the same living environment.
In conclusion, the current state of iodine nutrition in China is basically reasonable and there is no need to worry too much. Iodized salt is the main source of iodine in the diet of the current population and can be consumed normally.