Is Iodine Key in the Fight Against Breast Cancer?

What every woman needs to know about the iodine-breast cancer connection

Photo by Angiola Harry on Unsplash

I recently heard of yet another young woman in her thirties who has an advanced form of breast cancer.

Although breast cancer rates rise in women over 50, its incidence is rising even faster in younger women.

Between 2012 and 2022, women under 50 have seen a 1.4% per year increase in breast cancer, compared to 1% in women over 50.

Although risk is still low in younger women (11% of breast cancer diagnosis), those under 40 are more likely to have more aggressive disease. And breast cancer is the leading cause of death in women ages 20-49.

I keep returning to a 2017 study that suggests declining iodine levels may be contributing to the increasing rates of aggressive breast cancer in young women.

Iodine is important for all women, but its significance starts way before midlife.

All women should know that healthy breast tissue requires iodine, and its presence in our diets has been decreasing for decades.

Why Iodine?

Iodine is a trace mineral that is found in the soil and the sea. The thyroid gland stores most of the iodine we consume. But the second most frequent place you’ll find iodine is in breast tissue.

The sodium-iodine symporter (NIS), a protein that captures iodine into cells, moves iodine from the blood into the thyroid and breast.

Iodine plays an important role in maintaining the health of breasts, which expand after puberty and changes during pregnancy and lactation. It has antioxidant and antiproliferative properties.

Studies in rats as far back as the 60s, show that iodine deficiency results in hyper-responsiveness to estrogen which can increase breast cell proliferation.

Researchers have also found that iodine supplementation suppresses breast cancer tumors in vitro and in animal studies. It’s even being studied as an adjunct therapy for breast cancer and is an off label treatment for fibrocystic breast disease.

Adding fuel to this argument is the observational studies showing a link between thyroid dysfunction and breast cancer risk. Not to mention, women in Japan have much lower breast cancer rates (15 per 100,000) than American women (76 per 100,000), which has been attributed to their iodine-rich diet.

It’s still a theory, but an important one at that. Iodine helps protect our breasts, especially from the effects of estrogen, and deficiency could play a causative role in breast cancer. And it’s been declining for decades now.

Sadly, many in healthcare believe iodine deficiency is rare, so they don’t look out for it. But evidence shows this couldn’t be further from the truth.

The increased sensitivity of breast tissue to estradiol induced proliferative changes in the setting of dietary iodine insufficiency, provides a plausible mechanistic explanation for the increasing incidence of breast cancer with distant involvement in this age group. - Rappaport, J of Cancer, 2017

Eating fewer foods with iodine

Around the early 1900s, after discovering iodine and its role in conditions like goiter, public health experts knew they couldn’t solve the problem with food.

That’s because natural sources of iodine were limited to seafood and seaweed. So, public health officials sprayed table salt with iodine, which first became available in 1924. By the 1950s, over 70% of households cooked with iodized table salt.

Yet at this time, eating out was rare and even by the 1970s, 80% of calories were consumed at home. In 2017, this declined to 65%, and in 2023, according to one source it was 44%!

This is a problem because restaurants don’t use iodized salt and despite its often-high sodium content; it’s not in convenience foods either. It used to be in bread, but most companies have stopped using iodized dough conditioner.

This leaves dairy—mainly milk and yogurt—as a leading daily sources of iodine in the diet. This is because of iodine added to cow’s feed and iodophor cleansers in milk cans.

Yet milk intake has been on a steady decline - most dramatically during the 2010s than in the past previous six decades. This is probably because of all the food trends—no dairy, low-iodine plant milks, and popular diets.

Since 90% of iodine is excreted in urine, healthcare providers use urinary iodine levels to assess iodine status, classifying <100 mcg/L in adults and <150 mcg/L in pregnant women as deficient.

The result? Iodine levels are declining

A review of 11 studies indicates that women aged 15-44 are at risk of iodine deficiency.

Urinary iodine levels in pregnant women have decreased over time, from 294 mcg/L in NHANES I to 128 mcg/L in NHANES III.

In the LIFE study, which followed 501 women who discontinued contraception to become pregnant, 43% were found to be iodine deficient. This deficiency was associated with reduced chances of conception.

For older women, the situation varies depending on the study. In a 2015 study of 210 postmenopausal women, about half had urinary iodine levels below 100 mcg/L. Even when people aren’t deficient, there’s clearly been an overall decline in iodine levels.

For example, urinary iodine levels decreased from 320 mcg/L in 1971-1974 to 145 mcg/L in 1988-1994. Additionally, the percentage of people with urinary iodine levels below 50 mcg/L increased 5.6-fold, from 2.6% to 14.5%, during the same period.

Nobody eats the same way anymore

The way we consume media today is similar to how we approach food.

In the past, when it came to TV shows and movies, most people watched the same programs or went to the same theaters, creating a shared experience.

Now, with streaming, everyone can watch different shows and movies at their own convenience, leading to a more segmented experience.

Our eating habits have also become more diverse.

In the past, most people ate the same food gathering for home-cooked meals. But now, some follow high-protein, low-carb diets, others are vegetarians, and some prefer eating out for almost every meal. Just like with media, our food choices can vary drastically between individuals.

To see how this can play out, take a 2018 study with 70 postmenopausal women. Half followed a Paleo diet, which eliminates dairy and table salt, and the others followed a nordic nutrition recommended diet.

At baseline, 24-hour urinary iodine was 134mcg/day, but after 6 months on Paleo it went down to 70mcg. Not only that, levels of fT3 decreased after 6 months and TSH was higher after two years, showing an effect on the thyroid.

Iodine and breast cancer studies to date

Sadly, there are not a lot of quality studies examining the relationship between iodine and breast cancer.

One recent small study found that subjects with breast cancer had lower urinary iodine (80mcg) compared to those without breast cancer (144mcg). This wasn’t the case with a couple of other studies I found.

The Malmo Diet and Cancer study with 1,159 breast cancer cases and 1,136 controls showed that iodine status was protective of breast cancer only when selenium intakes were also high.

Selenium also plays a protective role in the thyroid when iodine levels increase.

This is what makes iodine so tricky. Besides the interaction with other micronutrients, excess iodine sometimes negatively affects the body, including the thyroid.

It doesn’t help that iodine recommendations which were last updated in 2001.

So, what do we do?

Our eating habits have changed, but iodine policies are outdated and don’t work anymore.

We certainly need more studies, updated guidelines and policies, and a focus on iodine in women’s health, especially breast cancer

But we can’t wait. Unless you have a health condition that prohibits it, ensuring you and your family consumes iodine sources or supplements, is a smart thing to do.

This is for people of all ages, but especially girls and women.

Until new evidence comes out, aim for recommended levels of iodine >150mcg and no more than 1g. And for pregnant/breastfeeding need a minimum of 220mcg/290mcg/day.

I’m not a fan of large doses unless it’s under medical supervision as a treatment. I like Mary’s Iodine drops, just over the RDA for two drops or you can get it in your multi.

If your curious if your diet provides enough iodine, ask your doctor about urinary iodine test or get one at a lab like Quest. Wouldn’t it be nice if we included a test when giving urine at doctor visits?

One can only dream.

For the girl going through puberty who turns her nose up to dairy or the woman in her twenties trying out Paleo, or the midlife women reducing certain foods to make room for more protein.

Please, don’t forget about iodine.

It’s not in many foods, and it may just play a role in breast cancer risk. It’s time we get the word out.

Information in this post is meant to educate and not replace medical advice. Talk it over with your healthcare provider!