In Japan, seaweed is a common dietary component which gives Japanese adults an average daily intake of 5280 micrograms (µg) of Iodine each day. This is very different to the rest of the world where the average intake of Iodine is only about 209 µg/day. Now there are obviously other dietary differences between traditional Japanese people and the rest of the world, not to mention many lifestyle and cultural differences, but could eating seaweed, and in particular consuming relatively high amounts of Iodine, be an important protective factor against breast cancer?
You can see in the following figure whose data has been taken from a collaborative study between Australian and Japanese researchers, published in 2020[i] that although the rate of breast cancer has increased dramatically in both countries between 2006 and 2015, that the age-specific rates are approximately double in Australia when compared with Japan.
It’s likely that you know about the importance of Iodine to the health of the Thyroid gland. You might even be aware of the pioneering work in public health performed by the Australian Dr Basil Hetzel (1922-2017), who improved thyroid health worldwide through the addition of Iodine to household salt. I was lucky enough to know Basil as a colleague and friend and I’m sure he would have been extremely excited to know that Iodine’s critical roles are not just limited to the thyroid but that it also has important roles outside the thyroid gland as an ANTIOXIDANT, DIFFERENTIATION FACTOR AND IMMUNE MODULATOR [ii] However, there is a major difference in the chemical form of Iodine that is critical to thyroid function with that in cancer prevention: The thyroid gland uses ‘Iodide’ salts whereas cancer suppression occurs with ‘elemental Iodine’. In fact, elemental Iodine probably plays a preventative role against all cancers, but few have been studied. Nevertheless, there are several well-controlled scientific studies that demonstrate Iodine’s role in controlling breast cancer.
The studies on Iodine and cancer prevention are mostly very new and it will probably be a long time, if ever, before we see Iodine being recommended as a ‘cancer cure’. Furthermore, there is other strong evidence that Iodine itself is not enough! For example, in a well-designed ‘prospective’ Italian study, the researchers found that Iodine intake alone did not reduce breast cancer risk and that Iodine needed to be combined with adequate Selenium to be effective. Indeed, women who had intakes of each of Iodine and Selenium that were above average, reduced their breast cancer risk by at least 25%. This supports an hypothesis linking Iodine and Selenium to cancer prevention that was first proposed in 2000[iii].
The study result fits with the protection offered against breast cancer by dietary seaweed because seaweed is high in both Iodine and Selenium, and it also fits what is known about the biochemistry of Iodine in the breast in that it requires the activity of Selenium-dependent enzymes for its function. Selenium is indeed a very important trace element that is essential to the function of several of our critical ‘defense enzymes’ in most cells. But – a WARNING – it must always be remembered that Selenium is a ‘trace element’ and too much is as bad as or worse than too little!
Another nutrient that is strongly associated with breast cancer is Vitamin D. Almost all studies show that higher levels of Vitamin D confer a lower risk of breast cancer and you’ve probably guessed that seaweed is a nutritional source of Vitamin D – although you probably need more than you will consume in your seaweed! You can synthesize your own Vitamin D very effectively from sunshine (when your skin is exposed to it) but nowadays because of the number of hours most of us spend indoors together with our use of sunscreen and clothing to protect ourselves against skin cancer, most of us are deficient in Vitamin D (for some or all the year). Having a dietary source of Vitamin D or taking a supplement is thus often necessary.
How does Vitamin D intake fit with Iodine and Selenium? Well, without going into the details there is plenty of biochemical evidence to show that Vitamin D plays a role in ‘up-regulating’ the Selenium-dependent enzymes that are involved in many defense and scavenging activities in our cells. So, although there is some Vitamin D in seaweed, sufficient Vitamin D is important to in addition to the protection offered by Iodine and Selenium.
Take-home message! Either eat seaweed on a regular basis or find other reliable sources of Iodine and Selenium and make sure your intake of Vitamin D is sufficient. I strongly suggest that if you are intending to eat seaweed on a regular basis that you research your seaweeds carefully. Different seaweeds contain very different amounts of Iodine and Selenium and you do want to make sure you have the ‘goldilocks’ amounts – not too little and just as importantly not too much!
[i] Mizukoshi MM et al (2020) Comparative analyses of breast cancer incidence rates between Australia and Japan. Asian Pacific J of Cancer Prevention 7: 2123-2129
[ii] Acerves C et al (2021) Molecular Iodine has extrathyroidal effects as an Antioxidant, Differentiator and Immunomodulator. Intl J of Molecular Sciences 22: 1228-1243
[iii] Cann SA et al (2000) Hypothesis: iodine, selenium and the development of breast cancer. Cancer Causes Control 11: 121-127