Iodine is a trace element the human body requires but cannot produce, so it must be obtained from external sources, primarily through diet. Understanding where iodine is naturally present in the environment is important for comprehending its availability in our food supply.
Iodine in the Environment
Iodine is widely distributed across the Earth’s natural systems, circulating between marine, atmospheric, and terrestrial environments. The ocean serves as the planet’s primary reservoir for iodine, holding a vast majority of this element. From oceanic waters, iodine can volatilize into the atmosphere. This atmospheric iodine then returns to land through processes like rainfall and dry deposition, completing its environmental cycle.
The concentration of iodine in soil varies considerably worldwide. Regions affected by past glaciations, flooding, or erosion often have soils depleted of iodine. However, soils near coastal areas tend to have higher iodine levels due to continuous deposition from sea spray carried by onshore winds.
The presence of organic matter, clay, and iron in soil can also enhance its capacity to retain iodine, preventing it from leaching away. Iodine concentrations in groundwater similarly reflect local geology and soil composition.
Seafood and Seaweed Sources
Marine environments are particularly rich in iodine, and organisms living there efficiently absorb and concentrate this element from seawater. Seaweed, a diverse group of marine algae, stands out as one of the most concentrated natural sources of iodine. Brown seaweeds, such as kelp and kombu, generally contain higher iodine levels compared to red or green varieties. For instance, a single gram of dried kelp can contain over 2,000 micrograms of iodine, while nori, commonly used in sushi, provides 16–43 micrograms per gram. The iodine content in seaweed can fluctuate significantly based on species, growth stage, season, and geographical location.
Beyond seaweed, various types of seafood are also valuable natural sources of iodine. Fish like cod are particularly notable, with a 3-ounce serving typically providing around 158 micrograms of iodine. Other seafood options, including shrimp, can offer approximately 35 micrograms per 3-ounce serving, while oysters provide about 93 micrograms for the same amount. Tuna is another source, yielding around 17 micrograms per 3-ounce serving. The iodine levels in fish can be influenced by the specific body of water they inhabit.
Land-Based Food Sources
The iodine content in land-based foods is directly tied to the concentration of iodine in the soil where plants are grown or animals graze. This means the amount of iodine in terrestrial food sources can be inconsistent. Dairy products, including milk, yogurt, and cheese, are significant contributors of iodine in many diets. The iodine in dairy primarily comes from supplements added to cattle feed and the use of iodine-containing disinfectants during the milking process. A single cup of milk can provide a substantial amount, ranging from 85 to 168 micrograms of iodine.
Eggs also serve as a natural source of iodine, with a large egg typically containing about 26 micrograms. The iodine content in eggs is influenced by the iodine present in chicken feed. While fruits and vegetables generally contain lower amounts of iodine, their levels are entirely dependent on the soil composition of their growing location. For example, a cup of cooked lima beans can provide around 16 micrograms of iodine. Meat products, such as beef liver, also contribute some iodine, with a 3-ounce serving offering approximately 14 micrograms.