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.

DEAR DR. BLONZ: How much does the grade of olive oil (extra-virgin, virgin, etc.) have to do with the fatty-acid proportions and other health values? -- F.M., Walnut Creek, California

DEAR F.M.: The most prevalent component in olive oil is oleic acid, which is an 18 carbon-long monounsaturated fatty acid. Most of the oleic acid will be present as part of a triglyceride, where there are three separate fatty acids attached to the compound glycerol. (This is where the word "triglyceride" comes from.)

Higher grades of olive oil have only a small amount of free fatty acids -- those not attached to a triglyceride -- which is referred to as "acidity" on the product label. The lower the acidity, the better. The International Olive Oil Council states that extra-virgin olive oil must have no more than 0.8 percent acidity, and virgin olive oil can have no more than 2 percent acidity.

In the big scheme of things, though, this type of grading has little to do with the proportion of monounsaturated, saturated and polyunsaturated fatty acids. The health value of olive oil goes beyond its high percentage of monounsaturates. The olive contains active phytochemical substances, which are nature's way of protecting the olive's fats from attack. These substances, which include polyphenols and tocopherols, are present primarily in extra-virgin oil, which is the first oil to come out during pressing.

DEAR DR. BLONZ: Why is alcohol present in so many cough and cold medicines? Does it have any role in the effect of the medicine? -- D.S., San Diego

DEAR D.S.: I have written in the past that there is evidence of a beneficial health effect from a moderate consumption of alcohol. A moderate intake is considered to be one to two drinks per day, with a drink defined as a 4-ounce glass of wine, 12-ounce glass of beer or 1-ounce shot of hard liquor. But the presence of alcohol in some medications has nothing to do with health. The alcohol is there because it's an excellent solvent, and it serves as a vehicle to keep certain types of medications in solution.

DEAR DR. BLONZ: Are less expensive domestic wines as good for the heart as fine French red wines? -- S.C., Lafayette, Louisiana

DEAR S.C.: Research studies on the benefits of wine have come from both sides of the Atlantic, and I am not aware of any study performing a head-to-head comparison. I'd think it safe to assume that the wine made from similar grapes in both areas would have comparable effects. An important point not to lose sight of is that a person's overall diet and lifestyle carry more weight in determining one's state of health than any possible contribution alcohol might make.

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.