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food for thought

Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD

It’s a trace mineral you might not give much thought.

Thanks to the mandatory addition of iodine to table salt, severe iodine deficiency rarely occurs in Canada. The policy to iodize salt, in effect since 1949, was a result of the widespread presence of iodine-deficient soils in the country.

New study findings out of McMaster University in Hamilton, Ont., however, suggest that iodine deficiency may be on the rise in certain parts of Canada owing to changing cooking and eating habits.

Here’s what to know about iodine – and why salt iodization may not be as effective as it once was at ensuring adequate levels of the nutrient.

What iodine does

The body needs iodine to make thyroid hormones. As such, it’s crucial for proper thyroid functioning and controlling the body’s metabolism.

Thyroid hormones are also necessary to regulate bone and brain development during pregnancy and infancy. And they’re required for the cognitive development of children early in life.

Iodine also supports the immune system and plays a role in female fertility.

Consuming too little of the mineral is associated with intellectual disability, stunted growth, infertility, hypothyroidism, depression as well as other iodine deficiency disorders.

Those at highest risk for iodine deficiency include pregnant women, young children, people who don’t use iodized salt and vegans or people who eat few dairy products.

About the new study

The study, published in the July issue of the journal Nutrients, measured iodine levels in 24-hour urine samples of 800 adults, average age 61, living in Vancouver, Hamilton, Ottawa and Quebec City.

Overall, 12 per cent of participants were categorized as having moderate to severe iodine deficiency.

The researchers also found unexpected regional variations in iodine status. Residents in Vancouver and Quebec City had a 2.5-fold greater risk of deficiency compared with those in Ottawa and Hamilton.

In addition to having lower iodine levels, those in Vancouver and Quebec City were also more likely to be exposed to higher levels of iodine uptake inhibitors which block the body’s absorption of the mineral. This “double whammy” puts people at even greater risk of iodine deficiency.

Iodine uptake inhibitors called thiocyanates were strongly linked to cigarette smoking. Other inhibitors, nitrates, had a moderate association with intake of vegetables, especially green leafy and cruciferous vegetables.

Dairy intake and use of vitamin-mineral supplements containing iodine were protective against deficiency. It’s possible that the dairy products in Vancouver and Quebec City contained less iodine than those in Hamilton and Ottawa, contributing to a greater prevalence of iodine inadequacy.

While dairy products are considered a good source of iodine, the amount varies depending on local sanitation practices. Most of the iodine in dairy comes from iodine-based sanitizing agents used to clean the teats on cows between milking, which then leaks into the milk supply.

Changing diets impact iodine intake

According to the researchers, recent trends in cooking and eating are changing the effectiveness of universal salt iodization programs.

The trend toward eating plant-based diets, consuming non-dairy milks which are not fortified with iodine and using non-iodized salts (e.g., sea salt, Himalayan) could leave people lacking iodine. Public health messaging to reduce sodium (salt) intake to prevent hypertension may also contribute to lower iodine diets.

A diet that contains lots of highly processed foods, while high in salt, will be lower in iodine since salt used in food manufacturing is usually not iodized.

According to Philip Britz-Mckibbin, the lead study author from the department of chemistry and chemical biology at McMaster University, “it may be time to rethink how to improve iodine nutrition perhaps by fortifying certain staple foods or beverages, which has already been implemented in Denmark, Australia and New Zealand”.

Best iodine sources, daily requirements

One of the best food sources of iodine is seaweed (e.g., kelp, nori, kombu, wakame) but amounts vary widely between species. Other food sources include dairy, fish, seafood and eggs.

Commercial breads are a good source of iodine only if manufacturers use iodine-containing dough conditioners. But that’s hard to know since nutrition labels are not required to include iodine information.

Iodine is added to many multivitamin and mineral supplements, often at a dose of 150 mcg, the daily requirement for adults. Gummy multivitamins typically contain much less.

During pregnancy females require 220 mcg daily which many, but not all, prenatal supplements contain (check labels). Daily iodine needs increase to 290 mcg during breastfeeding.

Excessive intakes of iodine can cause some of the same symptoms of iodine deficiency and should be avoided.

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