Selenium has become a popular mineral to discuss. It plays an important role in maintaining health, and it helps mitigate some of the damage from harmful metals, such as mercury and cadmium. One beneficial mineral with which it has a synergistic relationship is iodine, especially in terms of thyroid health and healthy development. Let’s find out more about these two essential minerals and their contribution to your health.
Selenium: What You Need to Know
Selenium plays an important role in 25 enzymes, known as selenoproteins, that are involved in the metabolism of thyroid hormones, DNA synthesis, immune function, and protection against oxidative damage. The recommended daily amount of selenium by the Food and Nutrition board is 55 ug/day for adults, with higher requirements for pregnant and lactating women and lower requirements for infants and children.
Although deficiency is not common, it does contribute to health problems. It is directly associated with Keshan disease and Kasin-Beck disease, both of which are more common in Asia. Additionally, selenium deficiency might play a role in male infertility and could exacerbate iodine deficiency. People most at risk of selenium deficiency include those who live in regions of the world where the soil is deficient in selenium, such as specific areas within China and Europe. Those on chronic kidney dialysis or HIV also have a higher risk of developing selenium deficiency. In the U.S., selenium deficiency is rare.
As important as selenium is, too much can also cause issues, including an increased risk of diabetes, fatigue, irritability, problems in the nervous system, skin problems, nausea, diarrhea, and more. That is why the Food and Nutrition Board has set a tolerable upper limit at 400 ug/day for adults, with lower levels for children and adolescents.
Iodine: What You Need to Know
You probably recognize iodine mostly from its association with iodized salt. It plays a key role in health, mainly as an essential part of the thyroid hormones. These hormones not only regulate metabolism, they also play a key role in the development of the skeleton and nervous system in fetuses and infants. Thyroid hormones are also involved in the synthesis of protein and the activity of certain enzymes.
In addition to its key role in thyroid health, it also supports the immune response and might have other health benefits, including improving fibrocystic breast disease and mammary dysplasia. The RDA is 150 ug/day for adults, with children, teenagers, pregnant, and lactating women needing different amounts.
Iodine deficiency can cause problems, especially in developing fetuses and infants. The most common cause of preventable mental retardation is iodine deficiency, and even slight iodine deficiency can also contribute to deficits in the neurodevelopment of children, leading to lower IQs and an increased risk of ADHD. The main symptoms of iodine deficiency are hypothyroidism along with goiters on the thyroid. It is common in areas where iodine is not in the soil, which is why many countries, like the U.S., have implemented an iodized salt program. In many populations in the U.S., iodine deficiency is now fairly uncommon thanks to iodized salt. However, some populations remain at risk, especially pregnant women, those living in iodine-deficient soils, and those who do not use iodized salt.
Although the iodized salt program was implemented to improve health, some caution is necessary if you choose to obtain your daily intake of iodine through this method. Many iodized salts on the market are highly refined, which removes some of the beneficial trace minerals of the salt and might include additives. Additionally, eating excessive levels of iodized salt contributes to high sodium levels, which might increase your risk of hypertension and other health problems, as well as high levels of iodine, which can also lead to issues. Excess levels of iodine might cause symptoms similar to that of deficiency, including a goiter and hypothyroidism. In some individuals, it can also cause hyperthyroidism. The tolerable upper limit for adults is 1,100 ug/day for iodine, and that number is lower for children and teenagers.
The Health Benefits of Selenium
Before exploring the intricate relationship between selenium and iodine, let’s take a moment to further explore the role selenium plays in health.
As discussed, one of the biggest roles selenium plays in the body is supporting the antioxidant system. It can help mitigate excessive oxidative stress, which plays a role in the development of many chronic diseases, including diabetes, obesity, cardiovascular disease, autoimmune disorders, and cancer.
Selenium’s antioxidant defense might help decrease one’s risk of cancer. One prospective study using data from the NutriNet-Sante cohort found a reduced risk of digestive cancer in those with high levels of vitamin C and E, beta-carotene, and selenium. Taking supplements did not alter the risk factor in general, but certain populations experienced benefits. In those who consumed more alcohol than recommended, taking selenium supplements provided additional protection (for both dietary and selenium levels above 10 ug/day, the HR was 0.99).
Selenium has also been found to have a dose-dependent association with a decreased odds ratio (OR) of prostate cancer, especially when there is also a high level of arsenic. This is likely due to selenium’s mitigation of the oxidative damage arsenic causes. Selenium is also able to inhibit arsenic’s activation of certain carcinogenic signaling. A meta-analysis that pooled the ORs from the included studies also found an inverse association between serum selenium levels and the OR of prostate cancer, especially in smokers.
Selenium supplementation might also help mitigate inflammation and oxidative stress. In one study looking at the effect of selenium supplementation on patients with diabetic nephropathy, taking 200 ug per day of selenium yeast for a period of 12 weeks, the group taking the selenium supplementation had a significant reduction in their hs-CRP, a marker for inflammation, as well as markers for oxidative stress (MMP-2 and MDA). There was also a significant increase in the plasma total antioxidant capacity (TAC). After adjusting, selenium no longer had a significant effect on hs-CRP levels or MDA levels, but it did have a favorable impact on MMP-2, TAC, glutathione, and plasma nitric oxide (NO) levels.
Selenium is also important in protecting against mercury toxicity. Studies have found that fish that contain higher levels of selenium had a reduction in the mercury toxicity. This most likely occurs due to selenium’s antioxidant capacity, which helps to increase excretion of mercury.
Mercury has been shown to impact heart health, including increasing the risk of dyslipidemia and high cholesterol. In one study looking at selenium’s impact on mercury, those in the highest category for levels of mercury in the toenail were 4.08 times more likely to have high LDL cholesterol and 2.24 times more likely to have dyslipidemia. This distinction was greatest among those who had the lowest levels of selenium in their toenails. The researchers found that the group with the highest level of mercury in the toenail were 5.25 times more likely to have high cholesterol and 2.98 times more likely to have dyslipidemia compared to the lowest category if they had levels of selenium equal to or lower than the median level. However, those who had levels higher than the median of selenium had a 0.98 OR for high cholesterol and 1.99 OR for dyslipidemia. This result demonstrates selenium’s potential for mitigating some of the harmful effects of mercury.
Another study found a positive association between mercury and metabolic syndrome that was mitigated by selenium. The researchers looked at chronic mercury exposure through fish consumption in a Korean population. Using a fully adjusted model, the researchers found that those with the highest tertile of mercury levels in their toenails had a 2.47 times higher chance of having metabolic syndrome compared to the lowest tertile. When reviewing the effect of selenium, the researchers found that the positive association between mercury and metabolic syndrome remained in those at or below 0.685 ug/g toenail selenium, which was below the median level. In those with higher levels of selenium, the association became non-significant, with the second tertile having an odds ratio of 0.95 and the highest tertile of toenail mercury having a 1.56 OR.
It is hypothesized that selenium might help regulate glucose homeostasis, possibly by mimicking insulin and enhancing the activity of the insulin receptor kinase and stimulating the glucose transport activity. As such, this mineral might protect against insulin resistance and type-2 diabetes, especially since it also acts as an antioxidant that protects against oxidative stress. In one large cross-sectional study looking specifically at the association between insulin resistance and dietary, not supplemental, selenium intake, the researchers found a significant negative association between selenium and insulin resistance when intake was at or below 1.6 ug/kg/day, which is roughly 118 ug/day for an individual of average weight.
Another study found that taking 200 ug/day of selenium led to improved markers for diabetes in those with type-2 diabetes and coronary heart disease. In this random, placebo-controlled study, the group taking the selenium experienced decreased serum insulin, HOMA-IR, HOMA-B, and serum hs-CRP and increased plasma TAC concentrations and the QUICKI score. However, it did not impact lipid profiles, NO, GSH, MDA, or FPG levels.
It is important to note that high levels of selenium might have the opposite effect. In one study, the highest quartile of selenium levels correlated to having increased fasting blood glucose and impaired fasting glucose, particularly in men. This study was done in a population with higher average levels of selenium than most places, which might have impacted the results. Another study also found that selenium levels were significantly increased in those who had diabetes compared to those who did not.
In another study, high levels of selenium in toenails correlated with dyslipidemia in those who took dietary supplements of selenium. However, in those with the higher levels of selenium in their toenails who did not supplement correlated with a lower prevalence of dyslipidemia and high levels of triglycerides. Thus, taking high levels of supplements beyond what is beneficial might end up causing more harm than good.
In fact, one meta-analysis found that those with lower and higher selenium levels did have a higher risk of developing type-2 diabetes, demonstrating a U-shaped curve in the relationship between diabetes and selenium levels. After pooling the data, the researchers found that comparing the highest levels of blood selenium with the lowest level, the OR was 1.63 for diabetes. The researchers also found a non-linear dose-response relationship between selenium levels and diabetes. When using a reference range of 52.5 ul/L, an increase of diabetes risk began when selenium levels exceeded 132.5 ug/L and a moderate increase when the selenium levels dropped below 97.5 ug/L. However, levels in the range between 97.5 and 132.5 ug/L experienced a flattening of the curve in the positive associations.
Selenium also plays an important role in brain health, with sufficient levels potentially preventing cognitive impairment. In one study using a mouse model of Alzheimer’s disease, researchers found that selenium-yeast helped to improve cognitive impairment, enhanced neural activity, and modulated brain metabolism. Plus, it helped mitigate inflammation and oxidative stress and inhibited GSK-3beta activity, which reduced the phosphorylation of tau, a common contributor to Alzheimer’s disease.
Another mouse study found that long-term treatment using Se-Met (6 ug/mL in the drinking water for a period of 12 weeks), which is the most common form of selenium that humans ingest, helped mitigate some of the factors involved in Alzheimer’s disease. The researchers found that selenium modulated the Akt/GSK3beta pathway, which reduced the hyperphosphorylation of tau. Additionally, it enhanced the autophagy-lysosome pathway and the autophagic flux, which increased the clearance of tau. There were also benefits to memory and spatial learning.
Fertility and Pregnancy
Selenium plays an important role in reproductive health for both men and women. In men, one study found a positive correlation between serum and fertility. Compared to fertile men, the infertile men in the study had significantly lower levels of selenium. Lower levels of selenium impacted all parameters of semen health, including sperm concentration, morphology, and motility. There was also an increase in infertility as the levels of selenium lowered. There was also a positive correlation between serum and semen selenium, demonstrating the relationship between the two.
Selenium might be able to help mitigate the negative impact of cadmium on fetuses in utero. In one study, higher levels of cadmium in the mother’s toenails was associated with an increase in the odds of intrauterine growth restriction and small for gestational age births, although the number was not statistically significant. Higher levels of selenium were associated with decreased odds of intrauterine growth restriction. There were also markers demonstrating an antagonistic relationship between the two in terms of the steroidogenic expression patterns and placental TNF.
Consuming adequate levels of selenium might also reduce one’s risk of ovarian cancer, especially in African-American women. In one population-study, the highest quartile of selenium intake had an estimated 30 percent lower risk of ovarian cancer compared to those in the lowest quartile.
The Interaction of Selenium and Iodine
Selenium and iodine interact with each other in one key area: thyroid hormone. Because this hormone plays a key role in metabolism and development, it is important to have an optimal level of it. Let’s find out more about how the two minerals impact thyroid health.
The thyroid contains one of the higher concentrations of selenium per gram than other tissues, making sufficient levels important for thyroid health. This is likely due to several selenoproteins being involved in thyroid health, including glutathione peroxidase, thioredoxin reductases, and the three deiodinase enzymes.
On a physiological level, it makes sense why these two minerals are essential for thyroid health. Thyroid hormone is made of a compound of iodine and tyrosyl residues in thyroglobulin, which are coupled together to create T3 and T4. T4 is converted into the active form, T3, when the deiodinase enzyme, which is a selenoprotein, removes one of the iodine compounds. Type I and II deiodinase enzymes are stimulated by TSH to convert more of the active form of the thyroid hormone. The other selenoproteins help to protect against the generation of H2O2, which in excessive levels can lead to oxidative damage.
Therefore, for a healthy functioning thyroid, you must have sufficient levels of both of these minerals. With both of these minerals, it is key to keep the right balance; too much can be just as disastrous as too little. Both situations can contribute to dysfunction in thyroid hormone production, as well as thyroid autoimmune disorders. Additionally, there must be a balance between the two minerals. An excess of one might contribute to or exacerbate a deficiency in the other.
The recommended daily value and tolerable upper limit, both of which are listed above, are helpful guides to plan your optimal consumption of both minerals. You can measure your individual status of iodine and selenium using a urine test, and you can measure your selenium status through a blood test, to determine if you are at risk of a deficiency or excessive intake. One study found evidence to support the WHO’s recommendation for urinary iodine between 100 to 200 mug/l for thyroid health. Another study found a reduced risk for thyroid disease with serum selenium levels between 69 and 90.99 ug/L compared to the bottom and top quintiles. Discuss your test results and any symptoms you might have with your doctor or health practitioner to determine if you require a different intake level to meet your individual needs.
Some studies have found that supplementing with selenium can help with and might even reverse thyroid problems, including autoimmune thyroid. In one study, researchers looked at the effect of supplementing with 83 mcg selenomethionine per day for a period of four months in 192 participants, of which there were 96 supplemented and 96 control patients. Out of all participants, 33 or 17.2 percent restored normal thyroid activity. Those who were supplemented had a higher success rate, with 31.3 percent of treated patients restoring euthyroid compared to just 3.1 percent.
In another study reviewing patients with goiter in Pakistan, researchers reviewed the impact of selenium status and its impact on iodine deficiency. Compared to the controls, both males and females with goiter had a lower serum level of selenium and a higher urinary selenium. These patients also had lower levels of iodine compared to age-matched controls. The researchers concluded that the selenium levels might help to determine the severity of the hypothyroidism experienced by those deficient in iodine, but that more research was needed.
In a study on rats reviewing the impact of independent and simultaneous zinc, selenium, iodine deficiency on thyroid activity, the serums levels of both total T4 and free T4 were significantly lower and the TSH was higher in all groups that were iodine deficient. Serum T3 levels were lower in selenium and iodine, selenium and zinc, and zinc only deficient groups. Additionally, the groups that were deficient in just selenium or selenium and zinc had lower levels of thyroid glutathione peroxidase activity. However, when there was also a concurrent iodine deficiency, this enzyme had a greater activity than the controls.
Sufficient levels of both iodine and selenium are essential for the healthy development of fetuses and young children, in part due to their importance to the creation of thyroid hormone. The thyroid hormone plays a key role in brain development in the womb. In the initial trimester, the fetus relies on thyroid hormone from the mother. Although the fetus starts to create its own hormone in the second trimester, even at birth, roughly 20 to 40 percent of T4 in cord blood is from the mother.
In one study reviewing data from the Generation R Study, the researchers compared the level of maternal thyroid hormones and the risk of verbal and non-verbal cognitive function and executive function in the children. The researchers did not find a connection between TSH and free T4 with any impact on language function, except for high levels of free T4 did predict a lower risk of having a delay in expressive language at 2.5 years old. However, having mild hypothyroxinaemia (a condition where TSH levels are within range but free T4 levels are low) prior to 18 weeks of gestation was correlated with having expressive language delay at 1.5 and 2.5 years of age (an OR risk of 1.44). Severe hypothyroxinaemia had an even greater risk, with an OR of 1.8. There was also a higher risk of delay in nonverbal cognitive function.
Maternal hypothyroxinemia might also contribute to the development of ADHD in the child. In one population-based cohort study, the researchers found that children born to mothers with hypothyroxinemia had higher ADHD scores at 8 years old compared to children not exposed to it, even after adjusting for confounders. There was no association with subclinical hypothyroxinemia.
Because of the importance of iodine to thyroid hormone production, severe iodine deficiency can cause developmental problems in the fetus, especially to the brain, and contributes to brain damage and cretinism. Even mild iodine deficiency might contribute to some cognitive dysfunction. In a longitudinal follow-up of an Australian cohort at 9 years old, the children who were born to mothers who had mild iodine deficiency (urinary levels under 150 ug/L), performed worse in spelling, grammar, and English-literacy compared to those who were not. There was a 10 percent reduction in spelling performance, 7.6 percent reduction in grammar, and 5.7 percent reduction in English literacy. Even after adjusting for certain other factors, the association remained. The cohort included children born prior to the implementation of an iodine fortification, so they experienced iodine insufficiency during gestation but not during early childhood.
Although most studies pointing to this issue look at iodine deficiency, some studies have started to also look at the impact of selenium levels in the neurodevelopment of children. Sufficient selenium levels during pregnancy are also important to proper development of the child. In one prospective cohort study, there was a positive association between the level of selenium during the first trimester of pregnancy and the motor skills and language skills of the child at one year old. Another study also found a positive association between maternal levels of selenium and the development in their children at 1.5 years old. This was most evident in language comprehension and psychomotor development, especially in girls.
How to Get Sufficient Levels of Selenium and Iodine
Food Sources of Selenium
Food sources are always a great place to start to ensure you have sufficient levels of selenium and iodine. There are several selenium-rich foods, especially if they come from soils that are rich in selenium. The richest sources of selenium from greatest to least include:
– Brazil nuts (2,549.6 ug for 1 cup of whole Brazil nuts)
– Turkey breast
– Sunflower seeds
Food Sources of Iodine:
Although many people only think about iodized salt for their iodine, you can find it in other food sources. Fish and dairy products are some of the richest sources of iodine. For those who are vegetarian or vegan, or otherwise avoid these foods, seaweed is also a rich source of iodine, with some varieties having almost 2,000 percent of the daily recommended value. Fruits and vegetables also have some iodine, especially if they are grown in areas that have iodine-rich soil. If you do rely on iodized table salt, it is important to be careful that the salt levels do not increase your sodium levels too much.
Supplements: What You Need to Know
Supplements can be a great source of selenium and iodine if you are at risk of a deficiency and cannot consume sufficient levels through your food, especially if you live in an area deficient of these minerals in the soil. As discussed above, if you do choose to supplement, care should be taken to not end up with excessive amounts of selenium or iodine. With both, some of the same problems that occur with deficiency occur with excess levels. When determining whether you need to supplement with iodine, do not forget to accurately calculate how much you get through your salt if you choose iodized salt.
Pregnant women, and those who plan to become pregnant, should discuss with their doctor whether they are at risk of an iodine and/or selenium deficiency, as well as a thyroid problem, to reduce the risk of problems with cognitive development.
Selenium plays many key roles in the body, but one key role is its synergistic relationship with iodine to ensure thyroid health. If you live in an area with low levels of either mineral in the soil, or you are otherwise at risk of a deficiency, then talk with your doctor, nutritionist, or another medical professional about ways to increase your consumption and/or supplement so you benefit from the health benefits discussed above.