DEAR DR. BLONZ: I read that the minute a jar or bottle of cooking oil -- such as safflower, canola, peanut or olive -- is opened, oxygen enters and the oil starts to go rancid. Will refrigeration stop this? I know that people won't die from using unrefrigerated oils, but are there negative consequences, healthwise? I read the label on a bottle of canola oil and it said nothing about needing to be refrigerated. And what about fish oil in capsules? -- J.S., Phoenix

DEAR J.S.: Any oil can go rancid if stored in the wrong way. Rancidity occurs when the oil reacts with oxygen. Aside from having an odor and giving foods an "off" taste, the consumption of oxidized oil does represent a health risk.

With refined oils, there is only minimal risk in leaving oils at room temperature, provided you follow some simple guidelines. Oils should be stored out of the sunlight and away from any source of heat. Since air, and therefore oxygen, gets in whenever the container is opened, keep the container sealed when not in use. This doesn't mean you need to get frantic and cap the container instantly, but it does make sense to close it when you are done, especially if it takes you months to get through the entire container. If you buy more oil than you tend to use in a few months, consider splitting the large size into smaller portions and refrigerating the unused bottles until needed.

Whenever an oil is stored in the refrigerator, keep it well-sealed to prevent it from picking up any undesirable refrigerator odors. Some oils may become cloudy when refrigerated, but this disappears when they return to room temperature, and is not a reflection on the oil's wholesomeness.

It is an oil's points of unsaturation, the double bonds in its fatty acids, which are most vulnerable to attack from oxygen. This means that the greater the degree of unsaturation, the greater the tendency to oxidize. As produced in nature, oils are blends of fatty acids with varying degrees of unsaturation, but we tend to classify them by their predominant type. Monounsaturates, such as olive oil, mostly contain one double bond. Polyunsaturates, such as soy, corn and safflower, have two. These types are more stable, but they should also be well-sealed and stored away from sun and heat. With flaxseed oil, over half the fats contain three double bonds, so it should be chilled.

Fish oils have four or five double bonds, so they are among the most susceptible to oxidation, and should always be refrigerated when in bottles. As dietary supplements, however, fish oil tends to be dispensed in air-tight capsules, which can be kept at (cool) room temperature.

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: I am hoping you will weigh in on a discussion regarding eating habits in our house. My concerns relate to my husband's reliance on nutrition bars. The brand he favors contains protein, along with a bunch of vitamins and minerals. Just about every work day, he will have one for breakfast and another for lunch, sometimes with a piece of fruit such as a banana or an apple. Dinners are more normal: We have a variety of dishes, most balanced and healthful, some less so. We often have meat, poultry or fish for dinner.

We are in our mid-40s. My husband takes supplements and works out about three times a week, though his favorite snacks are cookies and chips. He is not overweight and has no health problems.

His reasoning is that the nutrition bars are a reasonable option since he doesn't have the time or energy to make breakfast or bring "real" food to work, and he does not have time to go out to eat. They are also a way to control his overindulgence tendencies during the day so he can give in to his cravings at night. My argument is that he needs more variety, and also he could be harming his colon because of the lack of roughage. There could also be other deficiencies I don't know about. Would you please enlighten us? I have my husband's best interests at heart. -- I.P., Berkeley, California

DEAR I.P.: No question that your husband is doing many good things with his active lifestyle. Nutrition bars have a place, but they are not a balanced, whole meal to be relied on day after day. Those bars are made with recipes designed to accomplish a specific flavor and texture -- and, ultimately, a certain marketing goal. Whole foods, by contrast, provide a working symphony of healthful compounds, honed through millennia of trial-and-error "rough drafts." Everything works together with health and survival as the goal.

We continue to research which components, in what quantity, are best for us in given situations. At present, the whole-food model provided by nature is associated with the best health outcomes for us. Granted, we are not plants, but it seems foolhardy to grab a processed food from column A and another from column B and think we've got it covered. We may feel fine doing so, but chronic disease sneaks up on us like a thief in the night.

A more thoughtful approach is to rely on real, whole foods as the core of the diet. There are many healthful, flavorful alternatives to a daily reliance on nutrition bars, but they require some advance preparation. Suggestions include cereals with fresh fruit for breakfast; lunchtime can include salads or sandwiches with whole-grain breads, sprouts, greens and a rotating menu of stuff in between. Then there is plain yogurt, nuts, seeds, dried fruit and cut-up veggies.

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: How does fish farming affect the environment? Also, what are the nutritional differences between fish raised on a farm and those caught in lakes, oceans or bays? -- K.B., Springfield, Illinois

DEAR K.B.: The cultivation and raising of fish and water plants for human use, otherwise known as aquaculture, is a rapidly growing form of agriculture. Studies are generally supportive of the raising of fish in water-based pens.

I say "generally" because these systems attempt to take a natural process and reproduce it in a commercial, fixed-location, semi-enclosed system, and such endeavors never work perfectly. Quality control is crucial as farms cope with the inevitable curveballs that nature throws their way. Adherence to safety standards is essential to ensure that the fish are of good quality before they are harvested and sent to market. When things run well, the fish are safe to eat, consistent in quality, and they look and taste great.

The nutritional value of wild-caught and farmed fish tends to be comparable. One exception might be the amount of the oils in the fish, and the "you are what you eat" doctrine applies. Ocean-dwelling fish such as salmon have a leaner diet composed of smaller fish, insects and sea plants. All these can be sources of the compounds that give salmon meat its flavor and color. Those raised on farms dine on controlled diets. Because the genes of the salmon are responsible for its long-chain omega-3 fat production, these levels tend to be comparable between farmed and wild fish. Farmed fish, however, tend to contain more fat per serving.

Keep in mind that based on where they live and feed, even fish in the wild can accumulate toxic compounds. The controlled environment of the farm-raised fish can translate to lower levels of natural toxicants, and studies have borne this out.

Finally, aquaculture can be effective, but there can be no promise of an unlimited supply of fish. Care must be exercised to respect the area's ecology. Overcrowding can quickly turn the effort into a waterborne version of a factory farm. Depending on the location and breeds being raised, there are risks of escaping fish introducing an unnatural species that could interbreed with, or destroy, other area inhabitants. There are also risks of disease transfer, and the problems related to waste disposal and antibiotic use.

No question, fish farms need to operate in an environmentally friendly manner. We, as consumers, can help the system by patronizing those who do it right. Ask at your local fish market, or supermarket counter, where their fish come from. Identify your issues of concern, and let them know that you care. If are unable to get satisfactory answers, take the issue up with the store's corporate headquarters. Your purchasing power can help affect change.

Check out the Marine Stewardship Council (msc.org), Friend of the Sea (friendofthesea.org) and Cleanfish (cleanfish.com) as examples of organizations seeking to foster sustainable aquaculture. As always, it is best to eat a wide variety of foods. Whether caught in the wild or sustainably raised through aquaculture, fish are an important part of any healthful, varied diet.

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.