Over the past 50 years, public-health nutrition has evolved into a series of absolute recommendations that are simple and relatively easy to understand. For example, “keep sodium down to 1,500 milligrams per day” provides people a sense of security that they are doing what is best for their health if they meet that target. But, are there any unanticipated negative consequences?

Science, especially biological health science, is complicated. To completely understand how nutrition affects the body requires an understanding of physiology, chemistry, biochemistry, genetics, etc. Because of this complexity, absolute “good” and “bad” food recommendations run the risk of creating other problems. For example, restricting salt and consuming more “natural” foods (such as sea salt instead of iodized salt) has had the ripple effect of reducing iodine intake to levels that are dangerously low, especially for pregnant women and their children.

QUESTION: What are the potential effects of low iodine intake?

ANSWER: Iodine is an essential nutrient for thyroid hormone production. This hormone regulates many aspects of body metabolism and is essential for normal brain and central nervous system development in fetuses and infants. In adults, iodine deficiency is associated with conditions such as hypothyroidism, cognitive decline and fibromyalgia.

Because iodine is quite variable in foods, iodized salt was introduced in the U.S. in 1924 to ensure adequate iodine in the diet. Making salt a villain (especially iodized salt) has now caused iodine malnutrition in some individuals following the low-sodium recommendations. When people consume large amounts of cabbage family vegetables and legumes, the potential for low-iodine problems increases. These otherwise wholesome foods naturally contain chemicals that interfere with normal thyroid hormone production and function.

Q: How much iodine is recommended and what foods are good sources?

A: The present recommended adult daily iodine intake is 150 micrograms, 220 micrograms during pregnancy and 290 micrograms while breastfeeding. A teaspoon of iodized table salt has about 225 micrograms of iodine and 2,300 milligrams of sodium exceeding the present 1,500 milligrams-per-day sodium recommendation.

Foods do provide iodine, with ocean fish being among the highest food sources. Other key foods with variable amounts of iodine include milk, cheese and eggs. Some edible seaweeds are very high in iodine, and others are very low. Most processed and restaurant foods do not contain iodized salt. Some, but not all, dietary supplements include iodine.

Q: Can a person consume too much iodine?

A: Yes. Very high iodine intake has been associated with Hashimoto’s and Graves’ diseases. The Institute of Medicine set an upper limit for adults at 1,100 micrograms a day. However, for people who have had chronically low iodine intake, increasing iodine intake should be done carefully with medical supervision.

Alan Titchenal, Ph.D., C.N.S., and Joannie Dobbs, Ph.D., C.N.S., are nutritionists in the Department of Human Nutrition, Food and Animal Sciences, College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa. Dobbs also works with University Health Services.