DEAR DR. BLONZ: I will be having a routine colonoscopy next month and just received the preparation instructions. It says that in the week before the test, I should avoid seeds and even corn (one of my favorite foods). What defines a seed? What about fruits and nuts? -- F.S., Des Moines, Iowa

DEAR F.S.: A colonoscopy is a test in which a specialist, usually a gastroenterologist, uses a device that provides a view of the lining of the large intestine. It is a method used both for routine screening and for patients with symptoms that require a closer look. The preparation during the days before the test is no fun, but this is a very important procedure that can be a lifesaver. For your practitioner to get the needed view, it is essential that the surface inside your colon be as clean as possible. Once the procedure is over, you can begin to resume your regular menu, comforted that you have taken an important step to assess this aspect of your health.

Medical practices can have different methods of preparation before the test. Our doctor’s office had us on a very low-fiber diet for three days, and then on the day before the colonoscopy, nothing but clear liquids, such as flavored gelatin (any color but red), clear juices and soups. There were no fruits, vegetables or fiber-rich foods during any of the four days. My menu during the three days consisted of cream of wheat, eggs, cheese, meat, juices (no pulp), white rice, and English muffins with butter. I read labels to make sure nothing contained more than 1 gram of fiber per serving.

A key period of the preparation is the day before the test, and the use of medications that empty out your digestive tract. I used magnesium citrate, which I found to be a bit more palatable than the sodium phosphate product that was also an option. Some physicians rely on a PEG (polyethylene glycol) solution to be consumed throughout the day. There is no real culinary joy with any of these; it is something we have to endure, with the knowledge that in a short time, it will be over. Most instructions say that you should drink lots of water that last day. Please rely on your doctor’s office and the specific instructions they provide.

As for the specifics of your question: Fruits, nuts and seeds are higher in fiber, so they might be a problem, especially on the three “modified diet” days before the test. The day before, of course, they are out of the question.

Again, good preparation is the key to a good reading, so toe that line as best you can. An inadequate preparation might make it impossible for your doctor to see what needs to be seen and draw medical conclusions. Your preparation can affect how long you are allowed to wait before your next test, so there is a benefit to doing it right! Best wishes for a clean test.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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 do not eat much fish, and my doctor suggested taking fish oil capsules. My question is whether flaxseed oil is an effective alternative source of omega-3 fatty acids. This would be preferable to me, as it does not come with the risk of containing mercury or other contaminants, and there is no fishy smell. -- S.M., Phoenix

DEAR S.M.: Our body requires certain specially built fatty acids in the same way it requires vitamins, minerals and protein. Fatty acids are long chains of carbon atoms bound together. In chemistry, the term “omega” refers to the location of the first double bond along the chain. If it begins on the third carbon, it is called an omega-3 fatty acid. If it starts on the sixth, it’s an omega-6, and beginning at the ninth carbon, it’s an omega-9 fat. The precise point of that first double bond and the length of the chain affect what the substance can do in the body. We can make double bonds in some locations along a chain (such as at the omega-9 position), but not at carbon 3 or 6. We need fats built this way for various functions, and this is what makes omega-3 and omega-6 fatty acids dietary essentials.

Omega-6 fats are found in many vegetables, such as corn, soy, sunflower and safflower. The richest dietary source of omega-3 fatty acids is coldwater fish, such as salmon. There are also plant sources, and flaxseed is among the highest. Omega-3s can be found in lesser amounts in walnut, canola, soy and other plants, but these are shorter cousins to the ones found in fish oil, and it’s that longer version that the body requires for certain functions.

That means that while fish oil omega-3s are ready for the job, our bodies need to elongate plant omega-3s before using them. This can be done, but it is not an efficient process. Eating fish rich in omega-3 is definitely the best and most efficient way of getting these essential fats; the highest levels are found in salmon, tuna, mackerel, sardines and herring.

I consider fish oil capsules to be a lesser alternative. Concerns about contaminants are legitimate, so be sure to find a provider that goes the extra distance to assure the purity of the supplement before you buy.

Regardless of the source, omega-3 fatty acids must be stored properly as they are quite reactive. If they react with oxygen -- become oxidized -- they turn rancid and develop the odor characteristic of rotten fish. At that point, they change from an asset to a health liability. Refrigeration is ideal protection for these fats.

One final note: The intake of omega-3 fatty acids can affect how blood clots. Anyone using anticoagulant medications, such as Coumadin, or those at risk for issues involving blood clotting should get clearance from their health professional before starting or increasing their intake of omega-3 fatty acids.

For more information on omega-3 fatty acids from the National Institutes of Health, see b.link/EFAs.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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 enjoy the speed and convenience of cooking vegetables in the microwave, but became quite concerned when I read that this method might be destroying nutrients. Should I steam my vegetables instead of microwaving them? -- Q.T., Austin, Texas

DEAR Q.T.: It is reasonable to want the most from what we eat. Eating should be one of life’s great pleasures, and it makes no sense to sacrifice that enjoyment on the altar of requiring every last milligram of every nutrient.

Cooking can reduce the levels of certain nutrients, but it makes others more available for absorption. The answer to your question comes down to a matter of preference, and of using the technique with which you feel most comfortable.

The difference in nutrient composition after cooking is based primarily on the temperature and time of exposure. Microwaving is among the least destructive methods. Another issue is whether the food is submerged and cooked in water that then gets discarded; in this case, the cooking water can contain some of the water-soluble nutrients.

Steaming and microwaving are similar in both respects, so run with whichever you prefer. The most important factor is that you are eating and enjoying the bounty of fresh summer vegetables. Kudos on good eating habits, and their contribution to your health.

DEAR DR. BLONZ: Salt is a straightforward compound, so why does table salt need to contain additives? I know that they make salt iodized by adding a compound that contains iodine, but why are they adding other compounds such as calcium silicate and silicon dioxide? I have absolutely no problem with blood pressure, and use salt on occasion. Would I be better off using a natural salt or a sea salt where these compounds are not used? -- T.S., Sun City, Arizona

DEAR T.S.: Crystals of table salt are composed of sodium chloride, which can stick together when exposed to moisture in the room’s air. Salt that clumps together can have a hard time making it out of the shaker. One method to avoid this is to add dried rice or crackers to the shaker to absorb any moisture and keep things flowing. Another approach is to add very small amounts of compounds such as calcium silicate or silicon dioxide. (Silicon dioxide is the same compound in sand and quartz crystals.) These anti-caking substances are harmless at the levels used.

It is totally up to you: You can opt for a free-flowing product or one that needs more of a shake to be dispensed.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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.