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Abstract
Introduction
Prevalence of iodine deficiency and iodine-deficiency disorders in China
Government actions to control iodine-deficiency disorders
Effectiveness of use of iodized salt
Manufacture and use of iodized salt
Major problems in iodine fortification
References
Junshi Chen and Huiyun Wu
The authors are affiliated with the Institute of Nutrition and Food Hygiene in the Chinese Academy of Preventive Medicine in Beijing, China.
Iodized salt is the best means of providing iodine to deficient populations, and it has been used successfully and safely for 70 years around the world. In China about 450 million people live in iodine-deficient areas. The prevalence of endemic goitre in 7- to 14-year-old children was estimated to be 20% (7 million cases). The Chinese government has undertaken to eliminate iodine-deficiency disorders by the year 2000, and the manufacture and use of iodized salt throughout China has been compulsory since early 1995. Currently, potassium iodate is used. National regulations require the iodine content of iodized salt to be no less than 30 mg/kg in the salt-processing plant, no less than 25 mg/kg in the market, and no less than 20 mg/kg in the household. According to sporadic sample checking, however, the iodine content of salt in the market and the household is far from satisfactory. The loss of iodine during cooking is as high as 50% to 70%. The major problems in the fortification of salt with iodine in China are the use of uniodized salt in remote areas, an unsatisfactory system for monitoring the quality of iodized salt, the lack of knowledge and skill among marketing staffs, the loss of iodine during storage and cooking, and the lack of nationwide systematic studies to monitor the effectiveness of iodized salt in the control of endemic goitre.
About 1,500 million people, or nearly one-third of the earth’s population, live in areas of iodine deficiency. Its consequences, the iodine-deficiency disorders, include irreversible mental retardation, goitre, reproductive failure, increased child mortality, and socio-economic compromise. All of these results can be prevented by sufficient iodine in the diet. Eliminating iodine deficiency is recognized as one of the most achievable of the goals that the 1990 World Summit for Children set for the year 2000.
Iodized salt is the best means to provide iodine to iodine-deficient populations. It is physiological, simple, practical, and effective. It has been used successfully and safely for over 70 years in programmes around the world. The Codex Alimentarius standard for food-grade salt permits the use of sodium and potassium salts of iodides and iodates in the iodization of salt. The level of fortification that has been used ranges from 30 to 200 ppm, which will provide enough iodine to meet the requirement of 150 to 200 mg per person per day [1]. In China, although iodized salt has been commercially available for more than 40 years, it was 1995 before its use became mandatory throughout the country.
About 450 million people live in iodine-deficient areas of China, and more than 30% of the total population is at risk for iodine-deficiency disorders. The average prevalence of endemic goitre in children between the ages of 7 and 14 was estimated to be 20% in 1995. All of mainland China’s 30 provinces have reported the occurrence of iodine-deficiency disorders, primarily endemic goitre; however, there are significant differences in prevalence among geographic regions. There is at least a 40-fold difference between the county with the lowest prevalence and the county with the highest prevalence. It is estimated that in China in 1993 and 1994, there were more than 7 million cases of endemic goitre and more than 200,000 cases of cretinism [2].
It is widely acknowledged that most iodine-deficient areas are located in hilly or mountainous regions. However, it has been uncertain whether iodine deficiency is an issue of public health concern in large cities in China. A recent investigation demonstrated that subclinical iodine deficiency (urinary iodine < 100 mg/L) was quite common in schoolchildren in some large cities, such as Shanghai, Zhengzhou, Hefei, and Fuzhou (table 1) [3].
The Chinese government has undertaken to eliminate iodine-deficiency disorders by the year 2000. In 1994 the Ministry of Health and the Ministry of Light Industry jointly promulgated the programme outlines for accomplishing this. The strategic goals of this project are the following: all salt for human and animal use should be iodized; 95% of the population should use iodized salt; more than 95% of special populations (newly married women, pregnant women, lactating mothers, infants, and young children) should use iodine oil; and 95% of the counties should meet the criteria for elimination of iodine-deficiency disorders (prevalence of endemic goitre in schoolchildren < 5%).
To ensure the mandatory nationwide use of iodized salt, in August 1994 the State Council of the People’s Republic of China promulgated the Regulation on Adding Iodine to Salt to Eliminate Health Hazards Due to Iodine Deficiency. It stipulated that the health administration department in the State Council shall be responsible for the control of health hazards caused by iodine deficiency and the health inspection and supervision of iodized salt, and that the salt administrative department in the State Council shall be responsible for the manufacture and marketing of iodized salt. According to the regulation, the mandatory manufacture, distribution, and use of iodized salt throughout China began on 1 October 1994 (Order No. 163 from the State Council of the People’s Republic of China, 23 August 1994).
Although the nationwide use of iodized salt was not started until early 1995, the effectiveness of iodized salt in the control of endemic goitre was clearly shown in several observations and trials. Table 2 shows the consistent reduction in the prevalence of goitre at the provincial, city, and county levels, where the use of iodized salt was mandatory in every household, although no parallel control population was available in those observations [4-11]. However, considering the large differences in climate, dietary patterns, cooking habits, and other lifestyle factors among various parts of China, the effectiveness of iodized salt in controlling iodine-deficiency disorders needs further studies and long-term monitoring.
TABLE 1. Urinary iodine concentrations in 10-year-old school-children in 10 large Chinese cities
|
City |
N |
Median iodine concentration (Hg/L) |
|
Beijing |
1,226 |
137.3 |
|
Shanghai |
1,427 |
71.3 |
|
Wuhan |
1,074 |
178 |
|
Jinan |
1,175 |
100.3 |
|
Shenyang |
1,153 |
109.8 |
|
Xian |
1,196 |
178 |
|
Harbin |
1,178 |
95.4 |
|
Zhengzhou |
1,200 |
77 |
|
Hefei |
1,190 |
75.4 |
|
Fuzhou |
1,195 |
57 |
|
Location |
Duration of consumption |
% reduction in goitre prevalence |
|
Provinces |
||
|
Jiangxi |
1982-92 |
21.0±7.6 |
|
Shandong |
1993-94 |
20.7±11.3 |
|
Cities |
||
|
Jianou, Fujian |
1984-92 |
47.0±9.5 |
|
Shenyang, Lioning |
1978-87 |
44.8±7.1 |
|
Counties |
||
|
Shangzhi, Heilongjioang |
1983-87 |
31.4±4.3 |
|
Xide, Sichuan |
1977-82 |
31.9±2.5 |
|
Guangrao, Shandong |
1987-91 |
15.0±10.9 |
Different types of salt are used as the substrate for iodized salt, depending on the sources available in specific areas. The major types of salt in China include sea salt, lake salt (solar salt), and mined salt. There are 109 salt plants in China, with an annual production capacity of 800 million tons of iodized salt. According to the Ministry of Health, in 1995 the total production reached 639 million tons, of which 600 million tons were sold. Most plants adopted the spraying method for fortification of salt with iodine, which caused considerable loss of iodine because of the high temperature during spraying. However, the facilities in all 109 plants were renovated in 1996 with financial support from the World Bank and other agencies. Improvements in packaging technology were completed by the end of 1997.
Since 1990 potassium iodate (KIO3) has replaced potassium iodide (KI) for iodization of salt in China.
National regulations require the iodine content of iodized salt to be no less than 30 mg/kg at the production level, no less than 25 mg/kg at the market level, and no less than 20 mg/kg at the household level. In order to reach these criteria, the actual level of fortification during salt processing must be 40 mg/kg.
Preliminary studies have shown, however, that the iodine content of iodized salt decreases continuously during the whole process from the salt plant to the consumer, depending on manufacturing methods, packaging materials, and storage time. The shortest half-life was found to be 12 weeks [12]. In general practice, the storage time is about one month in the plant and three months in the marketing system (provinces and counties). However, in some remote areas, the storage period could be as long as six months.
In order to examine the quality of the iodized salt supply, the Ministry of Health recently conducted a nationwide survey after one year of compulsory manufacture and use of iodized salt. The results showed that 80% of the salt used at the household level contained iodine, but only 51% of the samples met the requirement for iodine content (20 mg of iodine per kilogram of salt). At 20 of 22 national monitoring points for iodine-deficiency disorders where iodized salt has been used for a longer period, the salt samples did not meet the required iodine concentration [13]. A case study in Shandong province demonstrated that the iodine content of iodized salt decreased progressively from 75% in the fortification plant, to 49% in county warehouses, to 8% in the local market, and finally to 7.6% in the households [5]. This decrease indicates that in addition to iodine loss, other factors may account for the low levels of iodine found in the salt purchased by consumers, including low levels of iodine added during processing, the use of cheap, uniodized salt (especially in remote, poor areas), and the use of locally produced crude salt as a substrate.
Cooking loss is another major reason for the failure of control of iodine-deficiency disorders by iodized salt. Since recipes and cooking procedures vary in different parts of the country, and the number of dishes using salt is so large, it is difficult to know the overall iodine loss during cooking, and so far very little information is available. Preliminary unpublished data from the Institute of Nutrition and Food Hygiene of the Chinese Academy of Preventive Medicine show that the loss of iodine during conventional Chinese cooking ranges from 50% to 70%.
At present the Chinese recommended daily intake of iodine is 150 to 200 mg per person per day, and the criteria for elimination of iodine-deficiency disorders in children aged 7 to 14 years are a prevalence of endemic goitre of less than 5% and urinary iodine content above 100 mg/L. According to the nationwide nutrition survey conducted in 1992, the mean daily intake of salt was 13.8 g per person, indicating that the daily intake of iodine should have been 276 mg per person based on the government fortification standard of 20 mg/kg. Taking all these factors into account, the dietary intake situation is far from satisfactory.
The Chinese government is determined to reach the goal of elimination of iodine-deficiency disorders by the year 2000 by using iodized salt as the major control measure in the general population. The national iodized salt programme has been implemented since early 1955, but there are major problems in enforcing it. These problems include the use of uniodized salt in remote rural areas combined with the lack of adequate processing technology in these areas, which results in iodized salt of poor quality that does not meet the national standard; an unsatisfactory system for monitoring the quality of iodized salt, which provides no guarantee that the salt used by consumers will meet the national standard; the lack of knowledge of the public health significance of iodine-deficiency disorders and the importance of iodized salt for its control among the marketing staffs of companies that process and sell salt; the loss of iodine during storage and cooking; and the absence of nationwide systematic studies on the effectiveness of iodized salt in controlling iodine-deficiency disorders (although its effectiveness in individual areas has been established). The appropriate concentration of iodine in salt for the Chinese population has not been established, given the characteristics of salt manufacturing, transportation, storage, and cooking. There is much to do to achieve the government’s goal of eliminating iodine-deficiency disorders in China by the year 2000.
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