I had never thought of our water supply as being a potentially important source of sodium. But recently my husband's doctor suggested that we switch to low-sodium bottled water to reduce our sodium intake. Is it usual for drinking water to be regarded as a major source of sodium?

It is not uncommon. The Environmental Protection Agency has recommended that sodium levels of 20 milligrams per liter (about a quart) or less be considered optimal. In nationwide studies the U.S. Public Health Service conducted more than 20 years ago, 58 percent of 2,100 water supplies contained from 20 to 49.9 mg./liter; 9 percent had levels between 50 and 90 mg./liter; and 9 percent between 100 and 249.9 mg./liter. Five percent contained more than 250 mg. per liter. Newer studies have uncovered similar results.

The sodium in water comes from various sources. Some are natural phenomena. Sodium is derived geologically from leaching of underground salt deposits and from the breakdown of sodium aluminum silicates and other minerals. Evaporated ocean spray, as rainfall, contributes some. Sea water seeping into aquifers (or underground formations of sand, gravel or rock that send water into wells or springs) represents another source. During periods of drought, when water levels sink, sodium concentration can increase.

Salt can also enter drinking water through human interventions, such as the road salt that may be carried into water supplies by rain or melted snow, or during water treatment.

You can learn the sodium content of your water supply by asking the local water supplier. If you have a well, have it analyzed by the local or state health department or environmental agency, or by a private laboratory or water-conditioning company.

Some time ago I read that low-serum cholesterol was associated with an increased incidence of cancer of the colon and thus, lowering serum cholesterol might be a bad idea. What is the latest thinking on the subject?

A recent study supports findings that suggest that low-serum cholesterol is the result, not the cause, of the disease. The new study was reported by researchers from Columbia University College of Physicians and Surgeons and the School of Public Health.

The investigators compared a group of individuals with cancer of the colon with two other groups. One had non-cancerous so-called "adenomatous" polyps of the colon, which occur in an estimated 20 to 50 percent of adults over 40 years old and can become cancerous. The second group had medical conditions unrelated to the bowel. The group whose cancer was in early stage, those with colonic polyps and those with no bowel disease at all had similar cholesterol levels. However, the individuals whose bowel cancer was at a more advanced stage had significantly lower serum cholesterol levels. This finding supports the hypothesis that the lower levels are a metabolic consequence of the tumor as the disease progresses, and that lowering serum cholesterol does not lead to increased risk of colon cancer.

In answering a reader's question about dried beans, you commented that dried-bean consumption in this country was dropping. You said that increased income was a factor, but that the decline was also related to the fact that there are more single-parent households and women in the work force. I am one of those women who has given up on dried beans because I never get them cooked. I do use canned beans to make chili or a quick soup, and am curious to learn how the cost compares.

Dried beans are considerably less expensive. In checking prices, we found that a cup of cooked dried beans, which provide the same amount of protein as two ounces of meat, costs as little as eight or nine cents. On the other hand, a cup of unseasoned cooked dried beans from a can costs between 25 and 30 cents. That is quite a difference, even taking into account that fuel must be added into the total cost of preparing dried beans.

Much of the work involved in preparing beans from the dried state is not labor-intensive. The beans can be soaked during the day while you are at work, and simmer for later use in the evening while you relax. Beside price, there is another advantage to the do-it-yourself approach. Adding seasonings like onion, garlic, carrots, celery and spices allows the beans to absorb flavors as they cook and makes for tastier results.