Guide to Ideal Iodine Nutrition
For general information in nontechnical language about IDD and iodine nutrition. This article offers answers to some of the more frequent questions.
A recent report by the Food and Nutrition Board, Institute of Medicine, National Academy of Sciences, USA, offers similar recommendations. It calculates an "Estimated Average Requirement" and from that derives an RDA (Recommended Daily Allowance). However, occasionally sufficient data are not available and instead an "Adequate Intake" is calculated, which may be set higher than the RDA would be, for safety. The recommendations for daily intake are as follows: the AI for infants 0-6 months is 110 mcg iodine and 7-12 months, 130 mcg; the RDA's are: 1-8 years old, 90 mcg; 9-13 years, 120 mcg; 14 and older, 150 mcg; pregnancy, 220 mcg; lactation, 290 mcg. The Food and Nutrition Board also sets the tolerable upper limits of the daily iodine intake as 1.1 mg (1100 mcg) for adults, with proportionately lower levels for younger age groups.
5. Where do we get iodine from? - Most of it comes from what we eat and drink. Seafood is usually a good source because the ocean contains considerable iodine. Freshwater fish reflect the iodine content of the water where they swim, which may be deficient. Other foods vary tremendously in iodine content, depending on their source and what may have been added. Plants grown in iodine-deficient soil do not have much iodine, nor do meat or other products from animals fed on iodine-deficient plants. Because the breast concentrates iodine, dairy products are usually a good source, but only if the cows get enough iodine.
Iodized salt is a special case. With only a few isolated exceptions, edible salt (sodium chloride) does not naturally contain iodine. Iodine is added deliberately as one of the most efficient ways of improving iodine nutrition. The amount added varies widely in different regions. In Canada and the United States, iodized salt contains 100 ppm (parts per million, same as 100 mcg/gram) as potassium iodide (equals 77 ppm as iodide), so two grams of salt contains approximately the daily recommended amount of 150 mcg iodine. In the United States, you can buy salt that is either iodized or not iodized, and the price is the same; about 50% of all the salt sold in the U.S. is iodized. In Canada, all table salt is iodized. Most other countries add from 10 to 40 mcg iodine per gram of salt (10-40 ppm). Daily salt intake varies greatly in different parts of the world, ranging from two to five grams in many western countries to 20 grams in some others. An average figure may be 10 grams per day.
These statements apply only to table salt. Most edible salt is added at cooking. If present as potassium iodate, as in most countries, little iodine is lost during cooking, depending on how pure the salt is. Many people get most of their salt from processed foods, especially in developed countries, and commercial practices vary as to whether these contain iodine or not.
Iodine exposure can come from many other sources. Certain food colorings (e.g., erythrosine) contain iodine, although it is only partially bioavailable. Some iodine from skin disinfectants, such as povidone iodine, is absorbed and reaches the bloodstream. Certain health foods, such as some types of kelp, contain large amounts of iodine. Other sources are dyes used for contrast in X-ray procedures and medicines, such as Amiodarone (used for heart failure and abnormal heart rhythm). People also get iodine from its use in farm animals, for cleansing udders or as part of iodine-containing medicines. Iodate has been used as a bread stabilizer in commercial baking, although this practice is less common now. Many other environmental sources of iodine exist; most of them are unrecognized or unpublicized.
6. What happens if we don't get enough iodine? - The most damaging consequences are on fetal and infant development. Maternal iodine deficiency causes miscarriages, other pregnancy complications, and infertility. Thyroid hormones, and therefore iodine, are essential for normal development of the brain. If the fetus or newborn is not exposed to enough thyroid hormone, it may have permanent mental retardation, even if it survives. Low birth weights and decreased child survival also result from iodine deficiency. Cretinism is a very severe degree of this brain damage; it includes permanent dense mental retardation, and varying degrees of additional developmental defects such as deafmutism, short stature, spasticity, and other neuromuscular abnormalities.
The most visible consequence of iodine deficiency is goiter. This word means "an enlarged thyroid." The process begins as an adaptation in which the thyroid is more active in its attempts to make enough thyroid hormone for the body's needs, despite the limited supply of raw material (iodine), much as a muscle gets bigger when it has to do more work. If this adaptation is successful and the iodine deficiency is not too severe, the person may escape with only an enlarged thyroid and no other apparent damage from the iodine deficiency. Older individuals with goiters may develop nodules (lumps) in their thyroids, and sometimes these can begin making too much thyroid hormone when suddenly exposed to iodine. This result occurs because these nodules are independent of usual controls; they make thyroid hormone at their own rate, and may over-produce it when given more iodine. Also, the nodular goiters in iodine deficiency have an increased rate of one type of thyroid cancer, called "follicular cancer." Goiters can sometimes enlarge enough to produce compression of other neck structures and may need surgical removal for that reason.
In addition to these effects on the individual, iodine deficiency has adverse consequences for the community. The mental retardation can cover a wide range, from mild blunting of intellect to cretinism, and a large part of the population may have some intellectual impairment. The mean IQ of the deficient community is decreased by about 13.5 IQ points, according to one review. Individuals in these communities have lower educability and lower economic productivity, and the output of the whole community suffers. Dramatic improvement typically occurs after appropriate addition of iodine.
7. What happens if we get too much iodine? - Most people who have previously been iodine sufficient can safely tolerate fairly large amounts. As mentioned above, some individuals have thyroid nodules that escape the body's usual controls, and they can start making too much thyroid hormone when their dietary iodine increases, to produce a condition called iodine-induced hyperthyroidism.
Iodine excess can also cause thyroid underactivity, because large amounts of iodine block the thyroid's ability to make hormone. Individuals vary widely in their tolerance to iodine. Most people can handle large amounts satisfactorily, but there are exceptions. People with a tendency towards so-called autoimmune thyroid diseases, such as Graves' disease or Hashimoto's thyroiditis, or who have family members with these problems, may be more sensitive to iodine. In fact, high iodine intakes in a population are associated with an increased incidence of these autoimmune thyroid diseases. Also, high levels of iodine in the population may increase the incidence of papillary thyroid cancer, although this is not well established. Fortunately, papillary thyroid cancer is usually a mild form of cancer and rarely causes death.
Most people can tolerate at least 1 mg (1000 mcg) of iodine daily without adverse effects. People with underlying autoimmune thyroid disease or who have previously been iodine deficient, may tolerate less. Iodine excess is undesirable, but its consequences are not nearly so severe as those of iodine deficiency, because the latter affects human development and can produce permanent brain damage. Properly iodized salt will rarely add more than about 300 mcg iodine daily to the diet. Therefore, concern about iodine excess is not a reason to stop or avoid consumption of iodized salt.
8. How can we tell if we are getting the right amount of iodine? - Usually you as an individual won't know how much iodine you are getting, particularly in countries like the United States, because iodine appears in commercial preparations without notice. You can make a rough calculation, based on (1) whether you use iodized salt; (2) how much salt you eat; (3) whether you take vitamins that contain iodine (many contain 150 mcg); and (4) whether you eat much meat, dairy products, or seafood.
For populations, a better way to learn the iodine intake is to measure the amount of iodine in representative urine samples. Most (more than 90%) of the iodine you ingest eventually comes out in the urine. Thus, the concentration of iodine in the urine, even in casual samples, is a good marker for iodine nutrition. Urine iodine concentration varies with fluid intake, so these ranges have limited use for casual samples from an individual, but they are good for assessing a population group, because individual variations tend to average out. A median urinary iodine concentration between 100 and 200 mcg/L is ideal.
Routine laboratory tests of thyroid function are not as helpful as the urinary iodine concentration in assessing a population. The serum TSH (thyroid stimulating hormone or thyrotropin) is a test that is widely used clinically to assess thyroid function in individuals. TSH is released into the blood stream by the pituitary (an endocrine control gland at the base of the brain) in response to the amount of thyroid hormone in the blood stream. TSH tells the thyroid how hard to work. It bases this decision on how much thyroid hormone is present in the blood. When blood thyroid hormone is low, serum TSH increases and this can be used as a measure for inadequate thyroid hormone production. The serum TSH is the most widely used test for diagnosing thyroid disease in individuals. However, the serum TSH is less valuable in recognizing iodine deficiency, because it can increase but still remain within the normal range.
An exception is the blood TSH in newborns. Most developed countries have a system of newborn screening, either with the blood TSH or thyroxine, to recognize congenital hypothyroidism. About one out of 4,000 newborns has congenital deficiency of thyroid hormone production, usually because the thyroid is absent or fails to develop properly. This newborn screening is important because prompt treatment with thyroid hormone can prevent most or all of the consequences of inadequate thyroid hormone on brain development. Newborn TSH screening in areas with iodine deficiency shows an increase in the number of infants with transient hypothyroidism. Usually, this transient hypothyroidism corrects itself, and most of these infants do not show brain damage, but there remains a potential risk for this result, particularly if they continue iodine deficient during nursing.
Thyroid size is also a useful marker of iodine nutrition, because the iodine-deficient thyroid enlarges, as described above. One way to recognize goiter is to feel the thyroid and make an estimate of its size; this maneuver is quite simple but fairly crude and not very accurate in detecting slight enlargement, particularly in children. Use of an ultrasound machine is simple and can be done in remote settings; this measurement provides another good index of the degree of iodine deficiency.
9. How do we adjust iodine nutrition? - Salt is the best vehicle for adding iodine. Everyone needs salt, everyone eats it, usually in daily amounts, and the technology for iodization is straightforward. It can be added as potassium iodide or potassium iodate; the latter is preferred under conditions of humidity or impurities because of its greater stability. Most countries have regulations calling for 20-40 mcg iodine/g of salt (20-40 ppm); thus if an individual eats 5 g of salt iodized at 30 ppm, she gets 150 mcg iodine from this source alone. The amount to be added varies for particular populations, depending on the amount of salt ingested, the purity of the salt (and therefore, the amount lost between production and consumption), and the amount of iodine ingested from other sources. In some countries, when daily salt intake decreases, the health authorities raise the amount of iodine in the salt, to provide a constant adequate daily amount.
Another method for providing iodine to a community is through the administration of iodized vegetable oil. These preparations, e.g., Lipiodol, were developed as an X-ray contrast medium. One milliliter (one-fifth of a teaspoon) contains about 480 mg of iodine. A single administration orally provides adequate iodine for about a year, and if given by intramuscular injection, is satisfactory for about three years. Iodized oil is most useful when the iodine deficiency is severe, when immediate correction is important, and when iodized salt is not yet available.
Drinking water is another occasional vehicle for iodine nutrition. Some systems slowly release iodine from a porous basket (containing a concentrated iodine solution) into well water. Another approach adds an iodine solution to water in a well or flowing through a pipe. A simpler version merely adds a few drops of a concentrated solution manually to vessels containing drinking water in a school or home. If iodine (I2) is added, it can also sterilize the water; this property is useful because many regions with iodine deficiency also have contaminated food and water.
Occasionally, iodine is given as tablets of potassium iodide, from 100 to 300 mcg per day or about 1 mg per week. Some vitamin/mineral preparations for daily use contain 150 mcg iodine, the recommended amount.
Other vehicles like sugar, bread, and tea have been occasionally used for iodine delivery. However, the many advantages of iodized salt make it the overwhelming favorite for most countries.
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