Iodine vs IodideYou will often see iodine and iodide used interchangeably when referring to the biological roles of iodine. Iodine refers to the atom iodine(I) or the molecule iodine(I2) while iodide is the negatively charged ion of iodine (I-). Most of the iodine on the planet is in the iodide form as it is water soluble. Molecular iodine(I2) is toxic and should not be consumed orally. Iodide(I-) is taken up by the thyroid where it is converted to iodine (I) and added to tyrosine to make the thyroid hormones T3 and T4, a process we will discuss in depth a little later. Unless specifically noted, I will use iodine for both iodine(I) and iodide(I-) within this article.
What foods contain Iodine?Iodine is found in high concentrations in seawater, so foods that come from the ocean tend to be the best sources of iodine. This includes sea vegetables such as seaweeds as well as fish. Iodine is also found in many land foods such as broccoli, spinach, milk, and eggs. The caveat with land foods is that the soil that they are grown in or, in the case of animals, the soil that the foods they eat are grown in must be high in iodine. This is why it's important to rotate crops, if you continually grow spinach in the same soil it will deplete that soil of iodine and decrease the iodine content of foods grown in that soil. Taking it a step further, animals who eat food from iodine deficient soil will also be deficient in iodine. This is why looking up the nutrient contents of foods can be deceiving, while nutritionata may state that a food has x amount of a nutrient, there's no way they can tell what the condition of the soil the food was grown in is like.
A perfect case of this is the goiter belt, an area that encompasses the Great Lakes, Midwest, and mountainous areas where the soil is deficient in iodine. People in the goiter belt are particularly prone to goiter, an enlargement of the thyroid gland. As these people become deficient in iodine, their thyroids enlarge in order to trap more iodine from the circulation. In 1924, iodized salt was introduced successfully to combat goiter in these areas. In fact, the FDA recommendation of 150 mcg per day of iodine appears to be the dose of iodine necessary to prevent goiter. This doesn't, however, mean that this is the optimal dosage for human health.
Thyroid gland and enlarged thyroid gland (Goiter)
Iodine and thyroid functionIodine plays a significant role in thyroid function. Thyroxine(T4) and triiodothyronine(T3) are the two hormones secreted from the thyroid gland that help regulate cellular metabolism(Energy production). They get the names T4 and T3 because the number corresponds with the number of iodine atoms contained within each. While T4 is the most abundant thyroid hormone, T3 is the far more active one. T4 is broken down to T3 inside cells by selenium containing enzymes where it increases energy metabolism.
Iodide is trapped by the sodium iodide symporter of the cells of the thyroid gland from the bloodstream and imported in to the thyroid where it is converted to iodine. In the case of iodine/iodide deficiency, the thyroid lacks one of the key components it needs to make thyroid hormones. This causes an increase in thyroid stimulating hormone (TSH), a hormone that signals the thyroid to make more T4. In an effort to trap more iodine from the bloodstream, the thyroid gland enlarges and produces a goiter. In mild iodine deficiency, TSH levels may be normal because the body compensates by converting more T4 in to T3. In this instance there may be a goiter present in someone with normal TSH levels. However, the cells of the body require selenium to convert the inactive T4 in to the active T3, so if the person is selenium deficient this conversion cannot happen. Selenium's role in converting T4 to T3 make it essential in protecting the thyroid while supplementing iodine. In the acute sense, if a person with a goiter is given sufficient iodine and selenium the goiter will go away. If the goiter has been untreated for five or more years, iodine therapy may not be able to get rid of the goiter.
Side view of a person with a goiter
Taken from http://www.taytan.com/images/goiter.jpg
Given that thyroid hormones regulate energy metabolism in all of your cells, you can see how important getting sufficient iodine is. Cells that have insufficient thyroid hormones won't produce energy properly and, therefore, won't be able to perform their role in the body. Muscle cells won't contract properly, neurons won't fire properly, and all cells will not be able to replicate properly. Any process that requires energy will be affected, which is nearly every process. As such, every system can be affected including the digestive system, immune system, nervous system, endocrine system, basically every system of the body. The role of iodine doesn't end at the thyroid hormones, however, let's take a look at other roles iodine plays in the body.
Roles of iodine in the bodyWhile most people associate iodine with the thyroid hormones, iodine also has other functions outside of this role. A sodium iodide symporter (Primary transporter of iodine in to cells) has been found in the stomach mucosa and thymus gland(Part of the adaptive immune system). In addition, the mammary glands of lactating mothers and the salivary glands concentrate amounts of iodine equivalent to the thyroid gland. Finally, the skin, joints, arteries, bone, cervix, and choroid plexus all take up significant amounts of iodine(1).
Researchers have theorized that iodine may have an antioxidant effect in humans (1, 2). This hypothesis comes from studies in animals that have shown an antioxidant role of iodide in the brain of rats(3) as well as the eyes of rabbits(4). Furthermore, iodine deficiency has been show to increase oxidative damage in the thyroids of rats and mice as evidenced by an increase in the expression of antioxidant defense genes(5). In humans, the thyroid and breast both concentrate large quantities of iodide and a have a highly efficient ability to neutralize hydrogen peroxide(H2O2), a potent free radical, by donating an electron from the iodide to the H2O2. In the thyroid, the remaining iodine combines with tyrosine to make the thyroid hormones(6). It is theorized that low iodide leads to high levels of thyroid stimulating hormone (TSH) which produces higher levels of H2O2 that, since they are not being used to help produce thyroid hormones, cause oxidative stress to the thyroid gland. This could be the beginning pathology that leads to thyroid cancer(7, 8).
There is strong epidemiological evidence that points to iodine deficiency playing a role in breast (1, 8, 9, 10), gastric(8, 11, 12, 13) and ovarian cancers (10). While there is conflicting evidence showing no role of iodine deficiency with gastric cancers(14) and the role of iodine deficiency hasn't been studied often in ovarian cancer, the role of iodine deficiency in breast cancer appears to have strong support and little evidence to contradict it. People with iodine deficiency induced goiter are three times more likely to be diagnosed with breast cancer than the general population(1). Furthermore, researchers have found a significant difference in urinary iodine excretion rates and women with breast cancer(9). Urinary iodine content is a valid way of identifying iodine intake, has been used in multiple studies(10), and is a way to determine iodine sufficiency; something that will be discussed in part two of this blog.
In a study on iodine and breast cancer cells, molecular iodine induced apoptosis in 4 out of 5 breast cancer cell lines(15). Apoptosis is programmed cell death and is a necessary measure the body uses to get rid of abnormal cells. One of the hallmarks of cancer cells, along with higher levels of oxidative stress, is that they somehow manage to become resistant to apoptosis. In vitro, iodine induces apoptosis in tumor cells while providing protective effects in non-tumor cells(16). In an analysis published in the Journal of American Physicians and Surgeons in 2006, Miller provided a very thorough analysis of the association between iodine deficiency and fibrocystic breast disease(1). The two primary studies referenced in this analysis showed a 70% success rate in treating fibrocystic breast disease with iodine. All of this evidence points to an important role of iodine in breast health.
Recommendations for iodine intakeThe US RDA for iodine is 150mcg for adults, with an increase to 220mcg for pregnant women and 290mcg for lactating women. The discrepancy between the RDAs is due to the nutritional needs of the fetus during pregnancy and developing child during breast feeding. These numbers appear to be the amount of iodine required to prevent goiter in the general population and to prevent cretinism and mental retardation in the children of pregnant and nursing mothers. The problem is that these numbers do not necessarily reflect what is optimal. This is the problem with some of the RDAs of micronutrients as most were developed to prevent diseases of malnutrition such as rickets and scurvy and do not necessarily reflect what is optimal to human health. In addition, the RDAs are merely snapshots in time and do not reflect increased nutritional needs that may occur due to changes in the environment as well as the food environment. This is very important as certain environmental changes can increase or decrease your need for specific nutrients, particularly iodine.
ConclusionAs you can see, iodine has many roles in the human body. While it's role in the thyroid hormones is critical to human health, it is important not to be myopic and ignore the other roles it plays in the body. In part two of this series we will discuss environmental and food factors that may contribute to iodine deficiency as well as a new grass roots campaign forming over the internet with doctors and patients that are bucking conventional wisdom and using milligram doses of iodine to combat many of the health issues discussed above. While iodine's role in health is important, as is any essential nutrient that you may become deficient in, it is important to not look at single nutrients as a panacea for all of the worlds ills. Rather, you should look to optimize your life by living a healthy lifestyle and look to get optimal levels of all essential nutrients. The question being asked in these blogs and by iodine researchers and physicians who routinely use it is if the RDA is sufficient for optimal health and not just to treat goiter.