DEAR DR. BLONZ: I eat a vegetarian diet (no fish) and have been relying on flaxseed as my source for omega-3 fatty acids. I do this by sprinkling the flax on my cereal or using it in baking. My concern relates to whether there is something toxic in raw flaxseed. I read that one should avoid, or at least limit, flax intake until it has been heated. Does this mean I should stop eating it raw? Should I stick to pure flaxseed oil? -- S.F., Dallas

DEAR S.F.: Let's address your "toxic" concerns first. Flaxseed contains very small amounts of compounds that can produce cyanide, a metabolic poison. But the mere presence of these compounds does not make flaxseed dangerous. With cyanogenic (cyanide-producing) and other potentially dangerous compounds, it comes down to the dose, and it also depends on the nutritional status of the consumer.

Such compounds are widely distributed in nature. A book on my shelf since graduate school, titled "Toxicants Occurring Naturally in Foods," was published by the National Academy of Sciences in 1973. It is an academic text that is now available online (tinyurl.com/lqlrtgu). Cyanogenic compounds cause problems primarily in individuals who are malnourished, particularly those with an inadequate intake of protein. The cyanogenic compounds in flaxseed are a greater concern for livestock, where very large amounts are consumed. Heat, or processing, does cause a breakdown of these substances, thus reducing the risk, but it is questionable whether this is a valid food safety concern.

The fatty acids in flaxseed are highly unsaturated, more so than most other vegetable oils. This makes flaxseeds more susceptible to oxidation, a reaction that destroys the nutritive value of an oil and turns it rancid. This doesn't make it toxic, but rancid fats are not what you want in your food or in your body. Exposure to air (oxygen) and heat can speed up the oxidation process.

The intact flaxseed has a protective coat that keeps the oil safe inside. The seed coat is so strong that most intact flaxseeds tend to pass right through our digestive system. Inside the flaxseed are also a number of antioxidants, this being nature's way of helping assure the viability of the seeds once planted. The healthful components of the flaxseed become available to us once the seeds are cracked or ground, but this process also increases the susceptibility to oxidation. This is why ground flaxseed should be stored in airtight containers and kept in the refrigerator once opened.

If you were to take pure flaxseed oil, you would get its omega-3s, but not the fiber and phytochemicals naturally present in flax. You would avoid the cyanogenic compounds, but the risk of rancidity would remain. If you are interested in using flaxseed oil, consider a brand that contains all the beneficial compounds found in the intact seed, including the phytochemicals known as lignans. Flaxseed oils, particularly when purchased as liquids, need to be stored in the refrigerator or freezer.

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 explain a bit about marinades, and how the enzymes they contain work. Is it true that they are heat-activated and need temperatures around 170 to work? If that is the case, how could they possibly work in the refrigerator, or even at room temperature? -- D.S., Sun City, Arizona

DEAR D.S.: Marinades are used to infuse or enhance flavors and/or affect the texture of food. Depending on their makeup, they can also help slow spoilage. When used with meats, they can reduce the risk of carcinogenic compounds forming during high-heat grilling and getting deposited on the food. This takes place when a fatty food is cooked directly over hot coals; carcinogenic substances can form when the drippings hit the hot coal surface, and can then get carried back to the food in the resulting smoke. (It's a good reason not to inhale the smoke from your grill, and to arrange fatty foods so that they are not directly over the coals.) Marinades that are acidic and contain herbs such as rosemary, garlic, thyme, oregano and basil tend to be the most effective in lowering carcinogenic formation.

Marinades are usually acidic, using a citrus, wine or vinegar base, but these days any flavorful substance into which a food is immersed might be referred to as a marinade. It is the acidic nature of a marinade that can help break down muscle tissues or plant fibers to help with the penetration of flavors. All of this is an effect of the pH, or acidity. Heat does not activate or inactivate the process, but it can proceed more rapidly at warmer temperatures, or if the food is cut or pierced to increase the available surface area.

Marinading is usually done at refrigerator temperatures in non-aluminum containers. Avoid overdoing it, as you could end up with mushy results. Another bit of advice: Don't use any leftover marinade that has touched your uncooked meat, poultry or seafood as a sauce later. The only exception would be if it were brought to a boil first, to lower the risk of food poisoning. If you want to use your marinade as a finishing sauce, set a portion aside ahead of time.

The topic becomes a bit more complex if the marinade contains tenderizing enzymes. Enzymes are substances that change other substances without themselves being changed. Our body relies on enzymes to digest many of the foods we eat, but enzymes can also be used in food preparation to help tenderize meats.

Often coming from fruits such as papaya or pineapple, such enzymes will slowly break down muscle fibers at cooler temperatures, but their action speeds up as the temperature rises. This is not an "activation" so much as the fact that different enzymes have different ideal circumstances under which they work. Our own bodily enzymes tend to work best around 98.6 degrees F, which makes perfect sense. The enzymes from fruits, such as papain (from papaya) or bromelain (from pineapple) have their peak activity level at 120-140 degrees F. Plant enzymes can be inactivated by high heat, which is the reason that fruits or vegetables, prior to being put away for storage, are often blanched (dipped briefly in boiling water) to inactivate their breakdown enzymes.

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 controls the amount of cholesterol the body manufactures? Can it make more cholesterol than it gets from food? -- D.D., Sun City, Arizona

DEAR D.D.: Cholesterol is an essential structural element in every cell of the body. In addition, it serves as a base material for the body's manufacture of a number of hormones, including estrogen and testosterone. Most of the cholesterol in the body is manufactured in the liver. If there wasn't any cholesterol in your diet, your body would have to make some. When cholesterol is present in the foods we eat, the liver is programmed to make less.

There are a number of rare genetic conditions in which the body makes more cholesterol than it needs. More common, however, is an unbalanced diet that's heavy on fats and carbohydrates and light on whole foods like greens, grains and fruits. Add to this a stress-filled, unhealthy lifestyle, and you end up with blood cholesterol levels that are not ideal.

DEAR DR. BLONZ: Is it true that the body can maintain itself better with a more alkaline diet? Can a pH test of saliva be an accurate determination of the body's acid/alkaline levels? -- G.W., San Francisco

DEAR G.W.: Degrees of acidity or alkalinity are expressed in terms of their pH. Distilled water has a pH of 7.0 and is considered neutral. When the pH is higher than 7.0, the substance is a base (i.e., alkaline), and when the value is lower, the substance is an acid. The greater the numerical distance from 7.0, the stronger the acid or base. Coffee is considered weakly acidic with its pH of 5.0, but battery acid, a strong acid, has a pH of 0.8.

The pH of the human body is slightly alkaline and it is kept within a very narrow range: between 7.35 and 7.45. There are overlapping systems set up to maintain the correct pH because essential chemical reactions won't take place if the pH is out of whack.

The pH of food varies (for examples, see tinyurl.com/lfdmusr). The human body is adept at dealing with variances; it makes its adjustments while foods are in the digestive system, and also after they are absorbed. The kidneys play a key role: The pH of urine will change according to our state of health and/or disease, and it reacts to what we eat, drink or do. In addition to urine, our perspiration and breath also help to keep the body's pH in its optimal range.

Relying on a saliva pH test is of questionable value as a general diagnostic, or as an indicator of your body's pH. Our salivary pH varies, being affected by ongoing dental issues as well as by the flora that live in the mouth. Certain disease states, anxieties and medications can also influence pH. The systems in our body are designed to compensate to keep things on track. Healthful, whole foods from plants tend to be more alkaline, but we don't need these foods just for their alkaline nature: They are assets to health because they provide nutrients and naturally occurring phytochemicals honed by nature over the millennia to support the vitality of the plant. We do best with an assortment of healthful foods coupled with an active and limited-stress lifestyle.

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.