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Iodine and selenium--'trace' minerals in New Zealand.

Abstract

The trace minerals iodine and selenium have many important functions during pregnancy and lactation, including a vital role in fetal and neonatal brain development. Although these nutrients are only required in small amounts, the dietary intake of these trace minerals in New Zealand has been reported to be suboptimal. There is a limited supply of iodine and selenium available in the New Zealand food supply. Therefore strategies, including regular consumption of foods containing these minerals and possibly supplementation, are required to ensure pregnant and lactating women consume adequate amounts of these minerals.

Introduction

Iodine and selenium are important nutrients required during pregnancy for a variety of functions including normal immune function and for the production of thyroid hormones and essential compounds such as glutathione peroxidase. Most importantly, iodine and selenium are necessary for normal brain development in the fetus and neonate. Worldwide, iodine deficiency is the leading cause of preventable brain damage (Delange, 2001). This is of potential concern for pregnant women particularly in countries such as New Zealand where the availability and intake of iodine and selenium are low (Thomson, 2004). The following article provides an overview of the general functions and major food sources of iodine and selenium followed by the effects of inadequate intakes of these trace minerals in pregnancy. I focus on the importance of these trace minerals during pregnancy and highlight the difficulties in attaining adequate iodine and selenium status in New Zealand.

What are trace minerals?

Trace minerals are defined as nutrients required by the body in small amounts, typically less than 100 mg/day, for good health. Iron, zinc, copper, iodine and selenium are examples of trace minerals.

Iodine

Iodine is a chemical element found in relatively constant amounts in seawater; however its distribution on land and in fresh water varies geographically (International Council for the Control of Iodine Deficiency Disorders, 2001). The primary function of iodine is in the production of two hormones, tri-iodothyronine (T3) and thyroxine (T4), by the thyroid gland (Figure 1). These hormones have an essential role in growth and development throughout life including the fetal and neonatal stages. Thyroid hormones are involved in stimulating the basal rate of metabolism, oxygen consumption, heat production, and in the production of key proteins, some of which are involved in brain development and linear growth (Groff & Gropper, 2000).

Selenium

The selenium concentration in foods also varies worldwide, with the New Zealand food supply having relatively low levels (Thomson, 2004). Selenium has a variety of roles in the human body including its role in iodine metabolism. Selenium is present in a variety of proteins referred to as selenoproteins. These proteins are essential for the formation of the thyroid hormones, the importance of which has already been described (Groff & Gropper, 2000). Therefore selenium, like iodine has an important role in ensuring normal growth and development early in life.

Consequences of inadequate iodine and selenium intakes

Iodine

A spectrum of conditions called iodine deficiency disorders (IDD) have been reported in the literature. These conditions include mental retardation, hypothyroidism, goitre, cretinism, and a variety of other growth and developmental abnormalities (Institute of Medicine Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, 2000).

Low dietary intakes of iodine can result in the reduction of circulating thyroid hormones referred to as hypothyroidism. There are also other causes of hypothyroidism however these will not be discussed here. Over time low iodine intake results in an increased thyroid volume, a slightly enlarged thyroid gland detected through palpation or ultrasound, a condition referred to as goitre. Goitre is the earliest clinical feature that appears with inadequate iodine intake and is the thyroid gland's attempt to sequester more iodine for synthesising thyroid hormones (Hetzel & Clugston, 1999). This condition in pregnancy, if not corrected, can lead to serious conditions that have detrimental effects on the fetus and newborn. A major target organ for the thyroid hormones is the developing brain (fetal and neonate) where they ensure normal myelination of the central nervous system. Iodine deficiency in pregnancy, resulting in a reduction of circulating thyroid hormones, can therefore lead to mental retardation and cretinism, as well as increased perinatal death and infant mortality (Delange, 2001).

Selenium

Selenium deficiency is associated with Keshan disease, a fatal form of cardiomyopathy. Keshan disease has been reported in areas of China where selenium soil levels are extremely low. Although selenium soil levels in New Zealand are also low, Keshan disease has not been reported here. The reason for this is unknown. Nevertheless the selenium status of New Zealanders is considered by nutrition experts to be suboptimal (Thomson, 2004).

As with iodine deficiency, a deficiency in selenium can lead to abnormal thyroid function and subsequently altered brain development due to its role in thyroid hormone production (Figure 1). More recently selenium deficiency has been associated with certain forms of cancer (Combs, 2005). Glutathione peroxidase, and other selenium-containing compounds, appear to have antitumorigenic properties, however not all clinical trials of selenium supplementation have reported a reduction in cancer rates (ibid). Selenium deficiency has also been associated with an increased prevalence of asthma in children (Rubin, Navon & Cassano, 2004). It is not known whether low intakes of selenium and iodine during pregnancy or lactation are in any way related to cancer or childhood asthma.

Trace mineral recommendations for pregnancy and lactation

To prevent the clinical manifestations that arise from iodine and selenium deficiency, nutrient recommendations have been established. Recommendations for trace minerals during pregnancy are derived from studies that examined the effect of supplementation in pregnancy as well as the level and status of trace minerals in infants (Table 1). The recommendations for lactation meet the needs of an adult female and take into consideration the amount of trace minerals needed to replace that which is secreted into breast milk (Australian National Health and Medical Research Council, 2005). Recommendations for pregnancy and lactation are higher than for non-pregnant and non-lactating women to accommodate the needs of the fetus and neonate who are dependent on the mother to obtain these trace minerals.

Do pregnant and breastfeeding women in New Zealand consume adequate intakes of iodine and selenium?

While there is an increased need and recommendation for both iodine and selenium in pregnancy and lactation, pregnant and lactating women in New Zealand do not appear to achieve these recommendations. It is difficult to obtain accurate dietary intakes for iodine. This is due in part to incomplete nutrient databases, varying and inconsistent practices by food manufacturers to use iodised salt and discretionary use of iodised table salt that is often not captured by dietary records (Thomson, 2004). Therefore, in place of dietary records urinary iodine excretion is considered the gold standard for assessing iodine intake (ibid). The majority of iodine consumed is excreted by the kidneys; therefore, the greater the urinary iodine excretion the greater the iodine intake. Skeaff, Thomson & Gibson (2003) measured the urinary iodine concentration in a group of pregnant New Zealand women, predominantly of New Zealand European descent (n=50), and found that despite the use of iodised salt by participants the women were not consuming adequate iodine intakes. Despite a relatively small and homogenous sample size, with respect to ethnicity, it remains the only evidence to suggest that pregnant New Zealand women have poor iodine intakes. Clinical indicators of iodine deficiency, namely goitre, are not widespread in New Zealand and were not reported here, however Skeaff, Thomson and Gibson (2003) suggest that based on urinary iodine concentrations, mild to moderate iodine deficiency has re-emerged in New Zealand. New Zealand breast-fed infants have also been reported to have suboptimal iodine status suggesting that lactating mothers likewise do not consume adequate intakes of iodine (Skeaff et al, 2005).

Using standard nutrient intake methods such as diet records, the mean selenium intake of New Zealand females over the past three decades has ranged from 11-55 micrograms/day (Thomson, 2004). Although this data does not include lactating women it does include data from one study of pregnant women (McLachlan, Thomson, Ferguson & McKenzie, 2004). There is no evidence to suggest that dietary intakes of lactating women are significantly different than those of pregnant women therefore similar intakes for lactating women might be expected. The data reveals low intakes of selenium, far less than the recommendations of 60 and 75 micrograms/day for pregnancy and lactation, respectively.

Despite limited data on the iodine and selenium intakes of pregnant and lactating New Zealand women, it appears from data available that recommendations for both iodine and selenium are not being met by these groups.

How to ensure adequate intakes of iodine and selenium

Iodine

There are few foods rich in iodine, however due to the constant level of iodine in seawater; reliable food sources of iodine include fish, shellfish and seaweeds. During pregnancy however women should limit their intake of large predatory fish such as shark, swordfish, barramundi, orange roughy and southern bluefin tuna to four serves (of 150g) per week due to their high mercury content (Food Standards Australia New Zealand, 2001). The level of iodine in soil and freshwater however is more variable. In New Zealand the soil concentrations of iodine are relatively low compared to other countries such as Australia and the United States; therefore fruits, vegetables and grain products grown in New Zealand are not good sources of iodine (Thomson, 2004). In contrast, animal foods such as dairy products, eggs and meat are good sources of iodine in New Zealand because animals concentrate iodine and livestock are usually supplemented with iodine (ibid). It is of no surprise therefore that vegetarians, in particular strict vegetarians whose diet is devoid of animal products, are at increased risk for iodine deficiency (Krajcovicova-Kudlackova, Buckova, Klimes & Sebokova, 2003).

The other source of iodine in the New Zealand food supply is iodised salt. Iodisation of salt however is not mandatory in New Zealand and all salts with iodine added must be clearly labelled as 'iodised salt'. Although processed foods often contain high levels of salt, food manufacturers do not routinely use iodised salt. Finally, natural salts including sea salt and rock salt are poor sources of iodine because much of the iodine evaporates during the drying process (Aquaron, 2000). The New Zealand Ministry of Health nutrition guidelines for all adults including pregnant and lactating women do not recommend increasing salt intake as a strategy to increase one's iodine intake, in part because high salt intakes are associated with elevated blood pressure, a risk factor for heart disease. Instead the guidelines state that if individuals use salt they should choose iodised salt (Ministry of Health, 2003; Ministry of Health, 2006).

In addition to foods, supplements containing iodine can be used to meet iodine requirements. However, in New Zealand iodine-containing supplements including seaweed or kelp supplements are not recommended for pregnant and lactating women because these supplements tend to have excessive levels of iodine that may result in iodine toxicity to the mother, fetus or neonate (Ministry of Health, 2006). Iodine toxicity may result in a variety of conditions ranging from fetal and neonatal hypothyroidism and goitre, iodine-induced hyperthyroidism, thyroid underactivity, papillary thyroid cancer and an increased incidence of autoimmune thyroid disorders such as Hashimoto and Graves Disease (Dorea, 2002; International Council for the Control of Iodine Deficiency Disorders, 2001)

There are also several non-conventional means of obtaining iodine during pregnancy and labour. An indirect means of obtaining rather large doses of iodine is through the use of iodine-containing antiseptic solutions such as those used for cesaeran sections. Although these solutions are absorbed and passed into breast milk, they are eliminated relatively quickly with no adverse effects on the infant. The use of iodine-containing medications such as potassium iodate throughout pregnancy however must be monitored as chronically high intakes of iodine may result in fetal and neonatal hypothyroidism and goitre (Dorea, 2002).

Selenium

The main dietary sources of selenium in New Zealand are seafood, poultry, eggs and muscle meats. Other sources include organ meats, Brazil nuts, legumes and cereals such as wheat that are grown in selenium-rich soil (Thomson, 2004).

[FIGURE 1 OMITTED]

Although the number of selenium-rich foods is limited in New Zealand, the amount of imported foods containing selenium and the use of supplemental selenium in New Zealand animal feeds has increased over the years (ibid). However, as highlighted earlier, selenium intakes in pregnant and lactating women are low, less than the recommended intakes.

Other than consuming selenium-rich foods pregnant women may consider selenium supplementation to ensure intakes that meet but do not greatly exceed the recommendations. Selenium supplements in New Zealand must be labelled with a recommended daily dose of no more than 150 micrograms/day, well below the safe upper intake level of 400 micrograms/day (Dietary Supplements Regulations 1985).

Conclusion

The levels of iodine and selenium in the New Zealand food supply are low relative to other countries. Although this has not resulted in widespread goitre, hypothyroidism, thyroid disorders or Keshan disease, intakes of iodine and selenium are below recommended intakes. This is of concern for both pregnant and lactating women due to the important role both iodine and selenium have in brain development and growth. Pregnant and lactating women should therefore make a concerted effort to choose iodine and selenium-rich foods on a daily basis.

Strategies to ensure optimal iodine and selenium status for pregnancy and lactation

* Consume iodine and selenium rich foods on a daily basis.

* Consider using iodised salt in place of noniodised salt.

* Consider the use of selenium supplements.

Accepted for publication: February 2007

Elias, S. (2007). Iodine and selenium 'trace' minerals in New Zealand. New Zealand College of Midwives Journal, 36, 25-27.

References

Aquaron, R. (2000). Iodine content of non iodized salts and iodized salts obtained from retail markets worldwide. Summary of Proceedings of the 8th World Salt Symposium. The Hague, Netherlands 2, 935-940.

Australian Government National Health and Medical Research Council. (2005). Nutrient Reference Values for Australia and New Zealand including Recommended Dietary Intakes. Australia: National Health and Medical Research Council.

Combs, G. F. (2005). Current evidence and research needs to support a health claim for selenium and cancer prevention. The Journal of Nutrition 135, 343-347.

Delange, F. (2001). Iodine deficiency as a cause of brain damage. Postgraduate Medical Journal, 77, 217-220.

Dietary Supplements Regulations (1985). New Zealand.

Dorea, J. G. (2002). Iodine nutrition and breastfeeding. Journal of Trace Elements in Medicine and Biology, 16, 207-220.

Food Standards Australia New Zealand. (2001). ANZFA issues advisory statement on mercury in fish for pregnant women. Accessed February 2, 2007 from http://www. foodstandards.govt.nz/newsroom/mediareleases/

Groff, J. L., & Gropper, S. S. (2000). Advanced nutrition and human metabolism (3rd ed.). Belmont, California: Wadsworth Thomson Learning.

Hetzel, B. S. & Clugston, G. A. (1999). Iodine. In M. E. Shils, J. A. Olsen, M. Shike & A. C. Ross (Eds.). Modern nutrition in health and disease (9th ed.) pp253-264. Baltimore, Maryland: Williams & Wilkins.

Institute of Medicine Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. (2000). Dietary reference intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington: National Academy Press.

International Council for the Control of Iodine Deficiency Disorders. (2001). Ideal iodine nutrition: A brief nontechnical guide. IDD Newsletter 17, 27-30.

Krajcovicoca-Kudlackova, M., Buckova, K., Klimes, I. & Sebokova, E. (2003). Iodine deficiency in vegetarians and vegans. Annals of Nutrition & Metabolism, 47, 183-185.

McLachlan, S. K., Thomson, C. D., Ferguson, E. L. & McKenzie, J. E. (2004). Dietary and biochemical selenium status of urban 6- to 24-month-old South Island New Zealand children and their postpartum mothers. The Journal of Nutrition, 134, 3290-3295.

Ministry of Health (NZ). (2003). Food and nutrition guidelines for healthy adults. A background paper. Wellington: Ministry of Health.

Ministry of Health (NZ). (2006). Food and nutrition guidelines for healthy pregnant and breastfeeding women. A background paper. Wellington: Ministry of Health.

Rubin, R. N, Navon, L. & Cassano, P. A. (2004). Relationship of serum antioxidants to asthma prevalence in youth. American Journal of Respiratory and Critical Care Medicine 169, 393-398.

Skeaff, S.A., Ferguson, E.L., McKenzie, J.E., Valeix, P., Gibson, R.S., & Thomson, C.D. (2005). Are breast-fed infants and toddlers in New Zealand at risk of iodine deficiency? Nutrition, 21, 325-331.

Skeaff, S.A., Thomson, C.D. & Gibson, R.S. (2003). Iodine deficiency disorders (IDD) in the New Zealand population: another example of an outmoded IDD control programme. Asia Pacific Journal of Clinical Nutrition, 12, S15.

Thomson, C.D. (2004). Selenium and iodine intakes and status in New Zealand and Australia. British Journal of Nutrition, 91, 661-672.

Sandra Elias MSc

Sandra is a Senior Lecturer at the School of Midwifery Otago Polytechnic. She completed an MSc in the area of Maternal Nutrition from the University of British Columbia, Canada. Contact for correspondence:

Email: sandrae@tekotago.ac.nz
Table 1. Selenium and iodine recommendations for pregnancy and
lactation in New Zealand (1)

 Iodine Selenium
 (micrograms/day) micrograms/day)

Non- Non-
pregnant Pregnancy Lactation pregnant Pregnancy Lactation

150 220 270 60 65 75
150 220 290 55 60 70
140 140 140 60 60 75

(1) Reference: Australian Government National Health and Medical
Research Council, 2005
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Title Annotation:PRACTICE ISSUE
Author:Elias, Sandra
Publication:New Zealand College of Midwives Journal
Article Type:Medical condition overview
Geographic Code:8NEWZ
Date:Apr 1, 2007
Words:2845
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