DEAR DR. BLONZ: Is there any valid concern about soy products affecting thyroid hormones? I need to take them, and have read that soy can cause problems. -- K.K., Chicago

DEAR K.K.: Thyroid hormones regulate the metabolism of every active cell in our bodies. If, for any reason, a person does not produce a sufficient amount of thyroid hormone, they can be given a medication to stimulate increased production by the thyroid gland, or they can receive a replacement for the hormone itself. (A thyroid deficiency can be diagnosed by your physician with routine blood tests.)

The essential element iodine plays an integral role in the synthesis of thyroid hormone. Isoflavones, a class of phytochemicals found in soy, have an ability to make iodine less available to the body.

Does this mean that having soy could be a problem for you? There are a couple of studies I like to turn to on this topic. The first is from the March 2007 issue of the Journal of Medicinal Food. This study reported that adding soy protein isolate, at a level of 9 milligrams of soy isoflavones per 10 pounds of body weight, did not significantly affect the thyroid function in a population that had an adequate iodine intake. (Isoflavone content can vary from product to product, but 1 cup of soy milk contains about 20 milligrams of isoflavones. The amount in this study would be the equivalent of 2 quarts of soy milk per day for a 180-pound person.)

Next we have the June 2008 issue of the Journal of Nutrition, which contained a review article on soy protein, isoflavones and thyroid function. This paper concluded that soy may reduce the efficiency of thyroid hormone function due to an effect on iodine utilization, but it does not appear to be a problem if soy is consumed as a reasonable part of a mixed diet.

There could be a concern with infants and children if soy products represent a large proportion of their diet. Regarding iodine, the Daily Value is 150 micrograms. There is additional information on iodine at tinyurl.com/27g35fq.

Be sure to follow the directions that came with your thyroid medication, as some need to be taken on an empty stomach to avoid potential interactions with foods, dietary supplements or other medications like antacids. Consult your family physician or a pharmacist if you have further questions.

DEAR DR. BLONZ: How many days after it is opened can sour cream be safely eaten? -- G.T., San Diego

DEAR G.T.: Assuming it was refrigerated properly and opened before the freshness date stamped on the carton, your sour cream could last from one to three weeks. When it's being used, the container should be resealed and returned to the refrigerator after any contents are removed; the fewer the openings, the better. Avoid returning leftover portions to the container, and use a clean utensil every time you take some out. These are general guidelines. It is outside our control whether the product was refrigerated properly along the way from production to your store.

You should toss the product if you find any strange growth or pinkish color, off flavors or odors. Be alert if you begin to discover variability in the quality and shelf life of refrigerated goods from a particular store; there may be issues with that store's equipment or handling practices.

In general, a grocery run to purchase refrigerated/frozen foods should be the last errand before returning home, in order to minimize these foods' time away from the fridge, and cold foods should be unloaded and stored promptly upon arrival home.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: Please provide some background about the use of chicken soup as a cold remedy. -- M.M., San Francisco

DEAR M.M.: Chicken soup has been used for generations, but a tradition can only go so far if it doesn't deliver. There is science here, but it will help to first understand what happens when you get a cold.

The immune system goes on the attack whenever it detects that a cold virus is attempting to take up residence in the upper respiratory system: the nose, sinuses and throat. A battle ensues, with one side effect being the inflammation that causes swelling of the mucous membranes lining the nose and throat.

At first, the immune system gets trounced, but over time, it learns to make the elements needed to squelch the invading virus. This can take hours, a few days, a week or longer, all depending on the tenacity of the virus and the state of your immune system. The "common cold" normally causes congestion, a runny nose, sore throat, a minor cough and a headache. If the virus persists and spreads down to the lungs, there is an increased risk of laryngitis (inflammation of the larynx/voice box), acute bronchitis and even pneumonia.

Stuffiness and discomfort are side effects, and they don't help the healing process. Congestion actually slows things down; if you rid yourself of the nasal secretions and mucus, you assist your immune system by allowing it to focus on making the weapons to win the battle. Blowing your nose is one obvious step, but care must be taken not to inadvertently force the bad stuff deeper into your sinuses (more info at tinyurl.com/pnfhjcn).

A group of physicians at Mount Sinai Hospital in Miami found that chicken soup was able to help clear the congestion that often accompanies a cold. This was discovered using a controlled experiment that measured the velocity with which mucus congestion was cleared through the nasal passageway. Dr. Irwin Ziment, a pulmonary specialist at the University of California at Los Angeles, attributed some of the soup's powers to the fact that chicken soup contains the amino acid cysteine, which is chemically similar to acetylcysteine, a prescription medicine used to combat congestion.

A cold is not the only instance where chicken soup can lend a hand: It can also help with asthma, where there is a narrowing in the bronchial tubes that carry the air in and out of the lungs. Coughing and mucus production can accompany the labored breathing of asthma, and one of the aims of therapy is to open up the airways and clear the lungs. Anything that helps move the mucus up and out of the body would certainly help.

Dr. Stephen Rennard at the University of Nebraska reported that chicken soup's effect might be due to its ability to block the inflammation and congestion caused by the movement of white blood cells (neutrophils) in the windpipe. In his experiment, the medicinal effect wasn't noticeable until the chicken soup was made with carrots, onions, sweet potatoes, turnips and parsnips.

Chicken soup doesn't have the same abilities as a modern antibiotic, but its powers are more wide-ranging, in a way: Antibiotics are only effective against bacterial infections, whereas soup can aid congestion brought on by either bacteria or viruses (like colds).

When thinking of making chicken soup, there are plentiful recipes to choose from. The basic recipe involves putting pieces of chicken (cooked or uncooked) into a large stock pot, then adding a couple of carrots, a large onion, a few stalks of celery, salt, pepper and enough water to cover. Then turn on the heat. Mimi Sheraton, in her book "The Whole World Loves Chicken Soup" (Warner 1995), supplies over 100 recipes from every corner of the globe.

With chicken soup, you get a satisfying, low-calorie dish that gives your body needed hydration and a potential vehicle for vegetables, protein and fiber. All this, along with an ability to help your body overcome that unwanted visitor. Not bad.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: What is the comparison between the vitamin, mineral and lycopene content of sun-dried tomatoes versus cooked and fresh tomatoes? -- F.F., San Jose, California

DEAR F.F.: Sun-drying will reduce vitamins C and A, and, to a lesser degree, some of the other vitamins, but the mineral content is comparable. As for lycopene -- a reddish phytochemical that contributes to the tomato's characteristic color -- there isn't any significant difference between the levels in cooked, fresh or dried tomatoes. Lycopene is chemically bound inside the plant-cell matrix of the tomato. The cooking process breaks apart the intact cells and actually makes the lycopene more bioavailable, which means it gets absorbed with greater efficiency.

Sun-drying isn't the same as cooking. In the drying process, the cells shrivel and break as the water is removed, especially if heat is used. If lycopene absorption is your only consideration, the bioavailability of the lycopene will be greater with sun-dried than with fresh tomatoes.

Eating sun-dried tomatoes requires a bit of a chew; this not only helps release the flavors, but it helps mix the nutrients in the meal to facilitate absorption. All the better if there is a little olive oil (or any other oil) in the meal: Lycopene, like other carotenoids, is fat-soluble, so oil present at the same meal will help with absorption. And any lycopene that does not get absorbed still helps our bodies by contributing to the healthful environment in our intestines.

But we miss the point of healthful eating when we get overly focused on the individual substances in the whole foods we eat. The point is that you should enjoy healthful foods like tomatoes in whatever form you like best.

DEAR DR. BLONZ: Please tell me what is so bad about bananas. I have seen online ads saying things like "The 5 Foods to Avoid," and they always show a banana. I clicked on one of these and listened to a long spiel that never mentioned bananas, but had a pitch for a "bargain" $49 offer. I start my day with a banana. Most of my mother-in-law's diet was bananas. I had a dietitian once who didn't want to allow me bananas at all, but reluctantly let me cut one in half! I'm puzzled. -- R.P.

DEAR R.P.: What's bad about bananas? Perhaps bad PR, in that their commodity group does not pursue those who characterize the banana as less than "appealing" (pun intended). There is no science (or logic) behind the banana being touted as a poster child for "foods to avoid." One rare exception might be a person taking medication to control their potassium levels. Concern about the sweet taste in a ripe banana is also misplaced; it is the sugar added to processed foods that should be our concern, not that which is naturally present in a healthful whole food.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.