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.

DEAR DR. BLONZ: Our daughter has eaten mostly vegetarian throughout her teens. She is not a vegan -- she eats seafood and dairy -- and she has been happy and healthy with her diet. Now comes an ideal opportunity to study in Chile, a country not known for its vegetarian proclivities. She will live and work in a rural area, and will need to conform to the normal dietary patterns of her host family.

Is there a "best" way to reintroduce meats such as chicken, beef and pork into a body that hasn't been exposed to them in years? We are concerned that initially she will be unable to digest this food, and it may leave her feeling quite sick. We'd like her to adjust her diet before she leaves home. Do you have any idea how long we should allow for her system to readjust to meat? Should we supplement her diet with protease enzymes? If so, for how long? Is there any possibility that her genes for producing proteases are permanently turned off?

Our daughter is quite enthusiastic about this opportunity and will do whatever it takes. We've got a few months until she leaves, and are most anxious for her transition to be as comfortable as possible. Any advice you can give us would be most appreciated. -- S.T., Seattle

DEAR S.T.: You need to give the human body a bit more credit. I do not believe that your daughter will have much difficulty making the transition -- at least not from a physiological standpoint. If she has been consuming fish and dairy products, or any food containing protein, she has already been using the relevant digestive "machinery." This includes the acid environment in the stomach and the protein-digesting enzymes needed for meats. There is no basis to believe that enzyme supplements would provide any advantage. She may get exposed to some novel fruits, vegetables, spices and herbs, and caution may be needed with public water in some areas, but it is unlikely that your daughter will have much difficulty digesting the meat in her diet, or that it will make her sick.

Chileans do eat meat, but there is a great variety of foods to sample. The basic Chilean diet focuses on bread, beans, potatoes and meat, but there is a long coastline and fish are plentiful in many cities. There are many soups and stews that combine corn, meat or chicken, and a variety of vegetables. Much will depend on the family your daughter lives with, but it is very likely that they will be interested in helping her feel at home.

Your daughter's attitude seems totally appropriate and on target. You could always slowly reintroduce meat as a condiment, say as a part of a sauce, to help her out. Then, over time, gradually have more meat-centered meals. Have her chew her food thoroughly and drink appropriate fluids. I expect her time in Chile will be a positive and enriching experience that will last a lifetime.

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: Omega-3 fats are touted when naturally present, and are now being promoted when added to other foods. Are all omega-3s the same? In terms for a non-science major, please explain what the "omega" represents and why these fats are being promoted. -- J.S., Phoenix

DEAR J.S.: To explain the meaning of the "omega," we will need to consider a bit of structure. I will do my best to make it digestible.

First, we need to consider that fats and oils are similar in structure, the main difference being that fats are solid at room temperature (around 68 to 72 degrees F), while oils are liquid. For simplification, I will use the term "fat" for both. Think of fats as long chains of carbon atoms. In addition to being bound to each other, carbon atoms along the chain can have up to a pair of hydrogen atoms (though there are different rules for the carbons at the chain's end). If all the carbons in the chain have a pair of hydrogens, the fat is said to be "saturated," i.e., saturated with hydrogens.

When hydrogens are missing, adjacent carbons form a double bond. When this happens, the fatty acid is "unsaturated." When there is one double bond between adjacent carbons, the fat is "monounsaturated." Olive oil is an example of a food fat that's rich in monounsaturates. (Check tinyurl.com/zzbxdpz for a picture of a saturated and monounsaturated fatty acid.) When there is more than one double bond, the fat is "polyunsaturated." (On a side note, you may now better understand the terms "hydrogenation" and "partial hydrogenation," as these refer to the adding of hydrogens to unsaturated fats to change their performance characteristics.)

The term "omega" is used to signify the location on the carbon chain where the first double bond occurs. An omega-3 fatty acid is an unsaturated fat where the first double bond involves the third carbon atom from the end of the chain.

The significance is that our body requires omega-3 fats, but cannot make them on its own. We also require omega-6 fats, where the first double bond is on carbon number 6. Both types are considered "essential fatty acids," so we need to get them from our diet.

Linoleic acid is an omega-6 fat found in vegetable oils such as corn, safflower, soy and sesame, and it is also found in many nuts and seeds. Fish oils contain EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), both omega-3 fatty acids. Other omega-3s can be found in flaxseed, canola, soy and walnuts, but the EPA and DHA from fish are based on longer chains of carbons, which gives them unique characteristics. The body can make the longer omega-3 fats from the shorter ones, but it does not do it efficiently. This is why fish consumption is often promoted as an important part of a healthful 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.