Iodine is vital for good thyroid function, which in turn is essential for good health. Iodine deficiency was prevalent in the U.S. before the iodization of salt became a common practice in the 1920s [Journal of the American Dietetics Association 79 (1981): p.17].
Some otherwise healthful foods contain goitrogens – substances which can interfere with iodine uptake or hormone release from the thyroid gland. These foods are generally only a concern if iodine intake is low. Consumption of brassicas, such as cabbage, Brussels sprouts, broccoli and cauliflower, increase the requirements for iodine, especially if consumed raw. Soybeans, raw flaxseed, cassava (used in tapioca), sweet potatoes, lima beans, maize and millet also increase the requirements for iodine.
Some individuals deliberately take kelp to try to lose weight by over stimulating the thyroid. This is a dangerous practice.
An iodine intake of less than 20mcg per day is considered severe deficiency; 20-50mcg per day is considered moderate deficiency and 50-100mcg per day is considered mild deficiency.
In the US, iodised salt is widely used and some other foods are fortified with iodine. In Canada all table salt is iodised. The UK has no iodine fortification strategy for plant foods or salt.
Iodine deficiency during pregnancy and early infancy can result in cretinism (irreversible mental retardation and severe motor impairments).
Hypothyroidism can manifest as dry or scaly or yellowish skin.
Geographic differences in the rates of breast, endometrial and ovarian cancer appear to be linked to iodine intake, suggesting that low intake may produce increased gonadotrophin stimulation leading to a hyperestrogenic state characterized by a relatively low estriol : (estrone + estradiol) ratio which may increase the risk of these cancers. [Lancet, pp.890-1, 1976]
Consumers of dairy products generally receive sufficient iodine due to the use of iodine-containing disinfectants for cleaning dairy equipment, which leach into the milk.
Iodine is typically undesirably low (about 50mcg per day compared to a recommended level of about 150mcg) in vegan diets unless supplements, iodine-rich seaweeds or foods containing such seaweeds are consumed. The low iodine levels in many plant foods reflect the low iodine levels in the soil. About half the iodine consumption of omnivores comes from dairy products.
In October of 2003, one of many articles appeared on iodine deficiency in vegetarians and vegans [Annals of Nutrition and Metabolism 47 (2003): p.183]. Quoting from the paper: "One fourth of the vegetarians and 80% of the vegans suffer from iodine deficiency..." Only 9% of the meat-eaters were deficient. The milk drinkers were protected in part because iodine-containing disinfectants are used to clean the milk processing equipment. None of the vegetarians and vegans were eating sea vegetables, and none were using iodized salt – they were all using "natural" sea salt, which has significantly less iodine.
The amount of iodine in bladderwrack is highly variable, probably as a result of different amounts of iodine in the water where it grows. A reasonable portion of bladderwrack may contain the U.S. adult recommended dietary allowance (RDA) of iodine (150mcg).
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