1. Shader, Richard I. MD

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Like others with hypertension, I try to limit my sodium intake. Although daily salt intake by adult humans is highly variable, usual daily intake ranges between 5 and 15 g.1 I do not pay attention to what type of salt goes into my limited daily intake. In a typical day, I ingest some of each of these 3 salt types: iodized table, sea, and kosher. From an informal survey of 6 clinicians I know, I learned that not one of them asks their patients about the types of salt they use.


It is a trendy thing these days to mention sea salt on packaging or in product labels and to call for kosher salt in many recipes. The former is popular because the potassium, magnesium, and calcium present give these products a more flavorful taste, and the latter is preferred because its larger and rougher granules draw out the juices from meats and poultry and they are easier to spread. Notably, the larger size of the crystals of kosher and sea salts means that a standard teaspoon contains a little less sodium.


Why is salt type important? The answer is simple. Humans do not make their own iodine, and iodine is necessary for the generation of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). Insufficient consumption of iodine can lead to hypothyroidism. I looked at the labels of 3 multivitamins (MVs) in my medicine cabinet, and not one mentioned iodine-so there is no help from typical MVs. Only some, but not all, MV mineral supplement labels include iodine content.


Iodine is found in sea vegetables such as nori and kelp and in fish and shellfish such as cod, shrimp, and oysters. Few of us eat enough of these naturally occurring sources to meet the Institute of Medicine's (IOM's) recommendation of 150 [mu]g of iodine per day.2 Most Americans ingest enough dairy products, especially via milk consumption, to provide approximately half of this daily goal.3 One study examining the relationship between salt restriction and iodine deficiency found that the restriction/deficiency relationship was meaningful only for women.4


Neither sea salt nor kosher salt contains enough iodine to promote healthy thyroid functioning. Kosher salt has none, and sea salt typically contains no more than 5 [mu]g per teaspoon.5 Iodized table salt typically contains 45 [mu]g of iodine in each gram of salt6; 1 g is equivalent to just under a quarter of a teaspoon. Unless we are getting sufficient iodine from other sources, we should all have the equivalent amount of iodine that would be present in two-thirds of a teaspoon of iodized table salt per day. This would provide us with approximately 167 [mu]g of iodine, enough to meet the IOM's suggested target.


Prevalence estimates for subclinical hypothyroidism among elderly adults hover around 7%. Could this be a function of dietary salt restriction for such common afflictions in the elderly as hypertension or congestive heart failure? I was unable to find any studies specifically examining the role of lack of iodized salt intake as a causal or risk factor for hypothyroidism in the elderly. I did find one report suggesting that higher daily iodized salt intake (>5 g per day), especially in the elderly, is an independent risk factor for thyroid nodules7-a 5-g intake of iodized table salt contains 225 [mu]g of iodine. I could find no independent confirmation of this finding; this is important because the IOM's upper limit for iodine is 500 [mu]g. Because too little or too much salt can lead to problems, clinicians need to be more aware of the types and amounts of salt their patients use.


For clinicians interested in knowing more about the iodine content of foods and supplements, a useful guide to relevant databases was recently published.8


In addition, I have recently learned about another noniodized salt, kala namak (also known as black salt), from northern India, which also contains iron sulfite (greigite).9 Black salt is thought to have antioxidant effects and comparatively lower sodium concentrations; it also contains fluoride.10


Finally, a few studies in humans provide a testable hypothesis that low salt intake (<7 g per day) may be linked with increased psychological distress and depression.11-14 In my view, these studies were not rigorous enough to be anything more than hypothesis generating.


Richard I. Shader, MD




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