Perchlorate Controversy Calls for Improving Iodine Nutrition

By David M. Crohn, PhD

Perchlorate is a water pollutant that can potentially interfere with thyroid function. Regulators across the United States are currently trying to agree on what is a safe perchlorate intake level. The chemical is of concern because it amplifies thyroid problems in people who are iodine deficient. Some scientists have concluded that vegetarians, and especially vegans, 1-2 can be more susceptible to thyroid problems as a result of perchlorate exposure because they may have lower iodine intakes than the general public. Impaired thyroid function is especially problematic for women who are pregnant or nursing a child. Fortunately, adequate dietary iodine can effectively eliminate the toxicity of commonly occurring levels of perchlorate.

Perchlorate is used in solid fuels that power high-energy devices such as rockets, flares, fireworks, and airbags. Perchlorate pollution is usually traceable to sloppy rocket fuel manufacturing or disposal, though modest amounts can occur naturally. Recent improvements in the ability to detect perchlorate at low concentrations have revealed widespread contamination of both water and irrigated vegetables. The Environmental Protection Agency (EPA) has detected elevated perchlorate levels in drinking water (>4 parts per billion, or ppb) at some 400 locations in 35 states, 3 the majority in California and Texas. 4 The highest concentration observed in a United States drinking water was 200 ppb in Duval County, Florida. Once in the environment, perchlorate is quite water-soluble so that it washes easily into rivers and moves readily through soils. Today, more than 11 million people have perchlorate in their drinking water at or above its standard detection threshold of 4 ppb; however, no national drinking water standard is in place as of yet. Also, perchlorate accumulates in fresh vegetables, such as lettuce, if they are irrigated with polluted water, so the mechanisms for exposure include both water and food. 5 Because it is so water-soluble, cooking vegetables in water removes perchlorate.

Government estimates of safe perchlorate concentrations in drinking water range from 1 ppb in Massachusetts to 6 ppb in California 1 to 24.5 ppb nationally. 4 The differences between the proposed limits are largely due to different approaches to correcting for inevitable uncertainties that result when conclusions based on human and animal studies are applied to the general population. Perchlorate appears to concentrate in breastmilk. 5 There is a scientific consensus that children of pregnant and nursing mothers with low levels of dietary iodine represent the population most susceptible to perchlorate problems, 3 but effects of perchlorate on this population have not been directly studied. 4 Corrections are intended to protect this group.

Other differences arise from assumptions about the occurrence of perchlorate in foods, about which information is limited. The presence of perchlorate in foods is currently under study by the Food and Drug Administration. 6 Only 14 of the 400 detected incidents of perchlorate contamination in drinking water exceeded 24.5 ppb, but the U.S. Government Accountability Office (GAO) believes that the federal government, particularly the Department of Defense, needs to be more proactive in looking for incidences of water contamination, particularly in areas where solid fuels are in production or active use. 4

Depending on the amount ingested, perchlorate may affect the body in a number of ways, but it is normally expected to interfere with the thyroid first. The thyroid is a small butterfly-shaped gland at the front of the neck. It uses iodine to produce the only biologically active compounds in the body that contain iodine, the hormones T3 and T4. To produce these hormones, the thyroid must collect and concentrate iodine from the bloodstream. When perchlorate is present, the thyroid will collect both iodine and perchlorate so that the amount of iodine that can be concentrated is reduced. Perchlorate is seldom a significant problem for people with adequate iodine in their diets because the thyroid still absorbs enough to meet the body's hormone production needs. If a person is already iodine deficient, however, perchlorate can further impair their ability to produce T3 and T4.

Adequate T3 is absolutely vital for fetal and infant health. During early development, T3 links the central nervous system's genetic blueprint to its actual construction. Insufficient T3 permanently impairs learning ability and neuromotor skills. T3 is also needed for bone development. Extreme cases of thyroid impairment are associated with cretinism, a severe birth defect characterized by both mental and physical retardation. In adults, low levels of T3 result in symptoms of hypothyroidism, which may include sluggishness, depression, dry skin and hair, weight gain, muscle cramps, or constipation. Sustained low T3 and T4 levels can lead to goiter, an enlarged thyroid gland. 3

To be fair, perchlorate is not the only chemical that interferes with the thyroid's iodide collection. At least two other compounds common in the American diet, nitrate and thiocyanate, can also interfere with iodine collection. Nitrate, the most widely occurring groundwater pollutant, results from over-fertilization with nitrogen fertilizer or manures. Nitrate concentrations are regulated in drinking water because it can contribute to a type of anemia in infants known as 'blue baby syndrome,' not because of its possible effect on thyroid function. Thiocyanate occurs naturally in certain vegetables, such as broccoli, cauliflower, cabbage, turnips, and Brussels sprouts. Although perchlorate interferes more strongly with thyroid function than thiocyanate or nitrate, these last two compounds are more common in most people's diets.

Perchlorate is, therefore, a concern because it intensifies problems resulting from iodine deficiency. Iodine deficiency is already the world's leading cause of avoidable intellectual impairment. The damage that this deficiency precipitates is permanent, but most damage is mild to moderate. Nonetheless, damage can be severe, depending on the timing and intensity of the deficiency. Typical iodine consumption in the United States has fallen significantly since the early 1970s, 7 leading to concerns that iodine deficiency problems in the public at large, and particularly in nursing mothers, may be increasing. 5 This decrease in iodine intake in the United States is likely associated with changes in the manufacturing of bread and milk that have lowered their iodine contents. In addition, processed foods, which have become more popular, frequently are manufactured with non-iodized salts. 8

Perchlorate should interest Journal readers because risk assessments suggest that the babies of pregnant and nursing vegan mothers are, theoretically, at greatest risk for perchlorate toxicity. 5 Vegans are perceived to be at greater risk than the general public, including lacto-vegetarians, because plant-based diets are generally relatively low in iodine. Evidence for this comes from knowledge of the iodine content of different foods rather than from direct measurements of iodine levels in vegans. The iodine content of most fruits, nuts, and vegetables is low but can vary depending on soil iodine content, irrigation, and fertilization practices. 9

Fortunately, it is quite easy to protect your health from the impact of perchlorate on thyroid function.

At concentrations normally encountered in the diet, perchlorate does not appear to reduce thyroid hormone production in people consuming adequate iodine. How can you be sure you're getting enough iodine? Using iodized table salt will help. A teaspoon of Morton Iodized Salt contains 400 micrograms (µg) of iodine, so three-eighths of a teaspoon daily would meet the iodine needs of a typical adult. A little more than a half-teaspoon would be adequate for a pregnant mother, while nursing women could meet iodine needs with three-quarters of a teaspoon of iodized salt. Salt can lose iodine during storage, particularly in hot, humid weather, but losses are generally modest.

Many vegans also seek out less refined foods, such as sea salt, which are naturally low in iodine. Several companies make an iodized sea salt. For example, Lifesteam Iodized Sea Salt provides 45 percent of the recommendation of iodine in a quarter-teaspoon. Therefore, to get 100 percent of the iodine recommendation, you would need to use a little more than a half-teaspoon daily. Check the label since other brands may contain different amounts of iodine.

The Recommended Daily Allowance, or RDA, for iodine is 150 micrograms (µg)/day for most adults. Children, who are smaller, require less iodine, while pregnant and nursing mothers need more. (See Table 1.) It is possible that RDA values may one day be revised upward somewhat to account for our evolving understanding of the role of perchlorate in thyroid health, 5 but use of iodized salt in cooking and seasoning can meet these needs. Pregnant and nursing vegetarians and vegans need additional iodine and should check their prenatal vitamins to be sure they contain this vital nutrient. Not all prenatal vitamins do.

Table 1: Iodine Recommended Dietary Allowances (RDA), Adequate Intakes (AI) and Tolerable Upper Intake Levels (UL)
From Panel on Micronutrients (2000) 11

Age and Status RDA/AI (µg) UL (µg)
Infants, 0 - 6 months 110 (AI) Not determined
Infants, 6 - 12 months 130 (AI) Not determined
Children, 1 - 3 years   90   200
Children, 4 - 8 years   90   300
Children, 9 - 13 years 120   600
Adolescents, 14 - 18 years 150   900
Adults > 18 years 150 1,100
Pregnant adolescents 18 220   900
Pregnant adults > 18 years 220 1,100
Nursing mothers 18 years 290   900
Nursing mothers > 18 years 290 1,100

If you are on a low-sodium diet, or otherwise prefer to avoid iodized salt, a small daily iodine supplement is a good idea. Sea vegetables such as kombu or hijiki can serve as alternative iodine sources, but it is easy to get too much iodine from some sea vegetables. (Table 2 provides information about the iodine content of some sea vegetables.) To put this in perspective, a serving of kombu (a piece approximately 5 inches long) weighing the same as a nickel (approximately 5 grams) can be expected to deliver 7,270 micrograms of iodine, 12 much more than is considered safe for long-term use. Just as too little iodine can cause health problems, so too can excessive iodine. Routine ingestion of excessive iodine may contribute to hypothyroidism, goiter, or thyroid papillary cancer. For iodine, the Tolerable Upper Intake Level (UL), the highest level of daily intake that is considered to be safe, is set at 1,100 micrograms per day for adults, with lower amounts for children and teens (Table 1). Hypothyroidism in newborn and nursing infants in Japan has been linked to their mothers' consumption of seaweed containing from 2,300 to 3,200 micrograms iodine per day. 10

Vegetarians, and particularly vegans, may be at increased risk for perchlorate toxicity, but making sure that your iodine intake is sufficient can easily fend off this threat. If you are susceptible to perchlorate, your iodine intake level is already too low. Therefore, drink your water, eat your vegetables, and check to see that your salt is iodized (or choose a low-dose supplement or prudent amounts of sea vegetables). Iodine is important for health, perchlorate or no perchlorate.

Table 2: Iodine Content of Some Sea Vegetables

Sea Vegetable Iodine (µg) in a serving Amount needed to supply 150 µg of Iodine (Adult RDA) Avoid frequent use of more than this amount per day*
Arame   732/Tablespoon 0.6 teaspoon 1.5 Tablespoons
Hiziki   786/Tablespoon 0.6 teaspoon 1.4 Tablespoons
Kelp12 1,986/Tablespoon Less than
    1/4 teaspoon
0.6 Tablespoon
Kombu 1,454/1-inch piece 1/10-inch 3/4-inch piece
Nori     40/sheet 3¾ sheets 27½ sheets
Wakame     82/Tablespoon 1.8 Tablespoons 0.8 cup
Data from reference 12. All measures are dried (uncooked) sea vegetable. These amounts will vary greatly depending on where the seaweed was gathered and how it was processed and stored.

* Based on Upper Limit of 1,100 micrograms of iodine per day. These amounts assume that your diet does not contain other sources of iodine.

David M. Crohn is an associate professor of biological engineering in the Department of Environmental Sciences at the University of California, Riverside.


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