Not getting enough iodine is rarely a concern in the U.S.

I notice that the specialty salts I now buy, such as Himalayan or sea salt, are not iodized. Should I be concerned about getting enough iodine in my diet?

— P.M.

Iodine is necessary to make the thyroid hormones. In the United States, the areas with the least iodine are the Great Lakes, Appalachians and Pacific Northwest. Iodized salt, and the availability of food grown in areas with high iodine concentration, has made iodine deficiency much less common in the past 100 years. Most people do not need to worry about low iodine now: Severe deficiency is rare in the U.S. and Canada, and moderate deficiency is found in 1% of Americans and 6% of Canadians.

Sea salt has many trace minerals, but only miniscule amounts of iodine. Himalayan pink salt (it’s the iron in the salt that gives it the pink color) also has very little iodine in it. Fish and shellfish from the ocean are naturally high in iodine. So are dairy products and vegetables grown in iodine-rich areas. Nonpregnant adults are recommended to take in 150 micrograms of iodine daily. If you use iodized salt, that’s the amount in just over half a teaspoon, but most people get most, or all, of what they need from food and do not need to consciously take in iodized salt.

Pregnant women are particularly important because severely low iodine during pregnancy can have terrible effects on the developing fetus, and that’s why prenatal vitamins contain all the iodine a pregnant woman needs.

Most people get far more sodium than they need, so I recommend going easy on your salt intake. If you like the taste of your specialty salts, keep using them sparingly.

I am 91. Literally overnight, I went from eating a very high-fiber diet (which had agreed with me for many years) to requiring a low-fiber diet with two probiotics a day. What could have caused this complete change to my system?

— R.C.

Sudden changes in bowel habits can occur from many different causes. Some common causes, like irritable bowel syndrome and abnormal thyroid levels, tend to change gradually. A sudden change like the one you are describing is very concerning for an anatomical change in the colon, and that is very worrisome for colon cancer. The risk of colon cancer increases over the lifespan, and both men and women over the age of 85 are at the highest risk.

There are certainly many other potential causes, but a sudden change in bowel habits, especially in a person over 45, is highly suspicious for colon cancer and should be evaluated.

I often recommend that colon cancer screening is unlikely to be helpful for people over 85, but I just want to be clear: This is not a colon cancer screening. This is an evaluation of new symptoms, and it’s important to find out what is causing it, so please see your gastroenterologist or regular doctor.