Ever wondered why table salt usually contains iodine? Iodized salt first appeared on US grocery store shelves in the 1920s to combat the rising number of Americans suffering from Iodine Deficiency Disorder, or IDD. But iodine deficiency isn’t just an American issue. Like many other nutrients, iodine deficiency ties back to the nutritional content of our food. A September 2020 study published in Environmental Geochemistry and Health, investigates a potential way to address iodine deficiencies without the fortified salts found in the United States.

IDD has serious health consequences globally. The human body needs iodine to produce hormones, and without it there are body-wide effects. The presence of a swollen thyroid gland, or goiter, is a classic sign of IDD. In severe cases, this goiter can make it difficult to breath and swallow. IDD is especially detrimental during pregnancy where it can cause lifelong thyroid disorders and impaired brain development in developing fetuses.

Distribution and Severity of IDD Globally (Image Source: Bevis et al., 2014)

With health consequences this severe, it’s alarming that IDD remains widespread. IDD impacts over 2 billion people globally. While some countries, including the United States, have addressed this problem with their iodized salts, fortified salts are not available globally, and their use varies widely by socioeconomic status when they are available.

Previous research has indicated that the supply of nutrients in soil can mirror the nutrients in the food grown in it. Adding the missing iodine directly back to agricultural soil might make crops nutritious enough to prevent IDD. This iodine is available to everyone who eats these crops, not just those with the resources to buy fortified salt.

To tackle this problem, a team of scientists investigated biofortification of plants to increase iodine content. The study conducted at the Bunda College of Agriculture at Lilongwe University in Malawi could play a major role in reducing iodine deficiencies in soils, crops and even humans. Biofortification is a futuristic sounding word, but it actually describes a fairly simple concept—if you apply iodine directly to the soil, some of that iodine might end up in plants grown in that soil.

The September 2020 study, led by local researcher Ivy Sichinga Ligowe, looked at biofortification of crops when they are grown in different kinds of soils. This research has important implications for biofortification in Malawi, as adding iodine to soils has been explored experimentally in temperate regions, but never in tropical soils like those in Malawi. Rapeseed and Bonongwe, two commonly eaten green vegetables, were used as the research subjects in Ligowe’s work.

The crops used in Ligowe’s study. Rapeseed (left) and Bonongwe (right) (Image Sources: Encyclopedia Britannica; A Veggie Venture)

Ligowe and her team began their research by collecting three different types of local soils to serve as the canvas for their experimental treatments. Using a scientifically rigorous experimental design, the team considered not just if biofortification would work, but under what circumstances it would work best.

The data collected from this experiment show the vegetables containing the highest amounts of iodine receive more frequent iodine treatments. This application strategy also leads to larger amounts of soil-to-plant iodine transfer overall for both crops. Results also suggest that regardless of how the iodine is applied, crops that are harvested more than once need to receive biofortification treatments between harvests to ensure that the iodine concentration in the vegetables stays high. How much iodine ended up in vegetables also tended to vary by the combination of soil types used in a treatment, suggesting that the impact of iodine biofortification needs to be tailored to the farm where it’s used.

This complex approach to evaluating biofortification treatments allowed Ligowe and her team to examine more than just how much iodine ended up in the vegetables. Results showed that the overall harvest size for both vegetables was not affected by iodine application. This suggests that farmers may not have to choose between iodine-rich vegetables that people need and high yields that turn a larger profit. This win-win scenario could make it easier to promote iodine biofortification to farmers more broadly.

Before this biofortification technique can be implemented on a large scale, further research will have to be done. There are lingering questions about if the higher iodine concentrations in leaves stay put through cooking, and how much of the iodine can be physically processed by the human body. Still, these conclusions show the potential of iodine biofortification in areas with low iodine concentrations and high rates of IDD. This study proves a point accepted by soil scientists, farmers, and epidemiologists alike—soil health is human health.