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Low carb and no dairy diets carry a hidden danger of iodine deficiency

Pretty pink Himalayan salt has a fraction of the iodine we need. Photo/Getty Images

Pretty pink Himalayan salt has a fraction of the iodine we need. Photo/Getty Images

Cutting back on carbs and eating more rock salt may expose you to the risk of an age-old nutritional deficiency. Listener investigations reveal that a popular rock salt has only a fraction of the iodine content the supplier claims. Meanwhile, if our consumption of bread – most of which has been fortified with iodine since 2009 – continues to decline, we may end up with widespread iodine deficiency, says Associate Professor Sheila Skeaff from the University of Otago’s Department of Human Nutrition.

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Our national bread intake appears to be in slow decline. In the 2008/09 New Zealand Adult Nutrition Survey, adults reported consuming less bread than in the previous survey in 1997. Since then, legions of followers of low-carb regimes such as the paleo diet have removed all bread, grains and dairy products from their diets.

That’s unfortunate because iodised salt was added to bread to halt the re-emergence of widespread iodine deficiency. A national problem in the 1920s and 1930s, it was largely eradicated after table salt was iodised in 1938, but it has re-emerged in the 21st century as our discretionary salt intake has declined and the dairy industry has moved away from sanitising milking equipment with iodophors, which probably caused iodine residue to leach into the milk.

Associate Professor Sheila Skeaff.

Associate Professor Sheila Skeaff. Photo/Sharron Bennett, University of Otago

Thanks to the use of iodised salt by the bread industry, bread now contributes about 47% of iodine to a New Zealand adult’s diet, followed by milk and milk products at 23%. For children, the figures are 51% and 28%.

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Fortification makes bread a useful transport mechanism for iodine, but dairy products are a naturally rich source, because the mammary gland, whether of a cow or a breastfeeding woman, concentrates iodine, says Skeaff: whatever iodine comes into the body gets concentrated into the milk produced and any products formed from that milk.

It’s no accident that breast milk is a concentrated source of iodine: iodine is critical for the healthy growth of the developing brain. “That’s why you want children, pregnant women and women planning a pregnancy to make sure they’re getting enough iodine,” says Skeaff.

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Without adequate iodine, the thyroid gland gradually enlarges – forming a swelling called a goitre – in an attempt to produce more thyroid hormone. A serious iodine deficiency during pregnancy can cause stillbirth or major abnormalities, and globally iodine deficiency is the leading cause of preventable mental retardation.

In 2011, Skeaff and colleagues assessed the iodine status of New Zealand children to determine the impact of bread fortification. Their urinary iodine indicated they were consuming enough iodine, but further tests revealed the children had raised blood thyroglobulin levels – an indication their thyroid glands were still enlarged.

“It takes a couple of years for thyroid gland size to reduce to normal,” says Skeaff, and retesting in 2015 proved her right. The results, published in the journal Nutrients, showed blood thyroglobulin had dropped to normal levels.

Grown-ups are doing okay, too, according to Skeaff, whose research team published their investigations of iodine status in New Zealand adults in the April issue of the European Journal of Nutrition. But even if we regularly consume dairy products, our iodine level may drop as our bread intake declines.

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“Before we put iodised salt in bread, most New Zealanders, even on a very varied diet, were iodine-­deficient,” says Skeaff. People who don’t eat bread are “probably not eating as much [iodine] as they think they are”.

Nutrition32For women who are breastfeeding, pregnant or planning to get pregnant, that’s a problem. The Ministry of Health re­commends pregnant and breastfeeding women take a 150mcg iodine-only tablet daily and eat foods rich in iodine: seafood, commercially prepared bread, milk, eggs, meat, cereals and seameal custard.

Similarly, those who don’t regularly eat bread and dairy products should be careful to include other iodine-rich foods in their diet, such as seafood and eggs, along with iodised salt.

Any salt used in cooking or at the table should be iodised salt. Nowadays we have a wide range to choose from, including iodised coarse salts that can be used in grinders. But beware of salts promoting themselves as a “natural” source of iodine: “You’d need a couple of kilos a day to get enough iodine,” says Skeaff.

Indeed, when the Listener approached New Zealand brand Mrs Rogers to request verified test results for the iodine levels in its popular Himalayan pink rock salt, it acknowledged the iodine level stated on the packaging is incorrect. “We rely on supplier information to generate our labels. We became aware of incorrect and too-broad information supplied by our previous Himalayan Pink Salt supplier and as a result recently changed supplier.”

So whereas current labels for Mrs Rogers Himalayan Pink Rock Salt say it provides 100mg of iodine per kilogram of salt, in reality it provides less than 1% of that amount – just 0.04mg. Product manager Jono Steven told the Listener that the company will be placing stickers on all existing stock to ensure the level stated on the label is correct. To be consistent, it should also remove the claim on the pack that the salt is a natural iodine source, since 1g of the salt provides just 0.04 micrograms of iodine – about 0.03% of our daily iodine requirement.

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Each food group provides a select range of important macro- and micronutrients. Before removing entire food groups from your diet, be sure to understand what effect it will have on your nutrient intake. What you may think is an improvement may actually be a backwards step

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