DEAR DR. BLONZ: I am an advocate of farmers markets. While shopping at one organic booth recently, I overheard a discussion that there are more nutrients in homegrown and market produce than in the items at larger stores. Can you tell me if this is true? -- J.E., San Diego

DEAR J.E.: This discussion usually revolves around the "field to plate" time for a given food. It's true that some nutrients in fruits and vegetables can decrease over time -- mostly certain vitamins and phytochemicals; mineral values, for the most part, remain unchanged. Assuming no form of food preservation has been used, such as freezing or drying, the shortest "field to plate" times are indeed found with homegrown and farmers market offerings.

Keep in mind, though, that it's often tough to make direct comparisons: For example, many flavorful varieties of produce aren't even grown by the higher-volume commercial agricultural operations. Many varieties are simply better suited to homegrowing or production by small, local farms.

Nutritional values are analyzed by organizations that take multiple samples. This might involve picking up products from stores around the country. As there are different varieties and growing areas, and the nutrient content may vary throughout the growing season, it's best to consider posted nutrient values as nothing more than an average.

Soil conditions can also affect the nutrient value of a crop, but it is not as straightforward as you might think. Minerals can end up in plants because they are essential for their growth, or they can end up there because they are passively taken up from the soil. A deficiency in an essential nutrient might result in a lower overall crop yield, but not a decrease of that nutrient in each individual item. Nutrient levels can vary from soil to soil and region to region. It's one of the best arguments for eating a wide variety of fruits, vegetables and grains.

So, while there can be nutrient differences between supermarket produce and that at farmers markets, there is no evidence that these differences will result in significant health impacts. The bottom line is that fresh or fresh-frozen produce will always be your best nutritional bargain -- regardless of its source. All the better if you can find products that are sustainably grown.

DEAR DR. BLONZ: Please explain why BHT is in breakfast cereals. I thought it was used to help prevent oils from going rancid, but there is little, if any, oil in breakfast cereal. -- C.C. Baton Rouge, Louisiana

DEAR C.C.: BHT stands for butylated hydroxy toluene. It is an antioxidant that helps prevent oil rancidity, as you correctly point out. In low or no-fat cereals, FDA-approved BHT can help protect the destruction of fat-soluble vitamins that may have been added to the cereal. This compound can also help preserve the ability of the packaging material to remain an effective barrier between the cereal and the outside air, which is a particularly important function in warmer climates.

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 on a weight-loss program that involves counting calories. I remember reading that the calories in carbohydrates should count for less, because they require energy to be used. If this is the case, why treat all calories as equal? Also, what is the difference between "aerobic" and "anaerobic" when it comes to food, versus exercise? -- S.F., San Jose, California

DEAR S.F.: A calorie is actually a unit of heat energy, but it can be thought of as representing our food's fuel/energy potential. Everything that happens in the body requires energy; it's needed 24/7, but we only eat at distinct times. So at mealtime, we take in more energy than we need at the moment. Any excess is changed into long chains of fat -- the most concentrated source of calories -- and stashed away for later use.

Changing dietary fat to stored fat doesn't require much energy: The process extracts about a 3 percent "tariff" from the ingested calories. In contrast, changing carbohydrate calories to stored fat is a multistep process that can use up about 23 percent of the ingested calories. From this, it might appear as though carbohydrate calories contribute less than fat calories, but it's a bit more complicated than that.

Our body doesn't like to waste energy, so when carbs are available, the body doesn't waste energy changing them to fat. It uses them right away to fuel muscular work and biosynthetic processes. Carbs also replenish our glycogen stores, glycogen being a prime source of stored carbohydrate found in the liver and muscles. Only after these process have been taken care of will the excess carbohydrates be changed into fat.

Bottom line: On a diet that counts calories, it is best to consider carbohydrate and fat calories as equal. One study actually examined this issue, shifting people between equal-calorie diets based on high-fat or high-carbohydrate foods. The study found no difference in how the diets affected body composition of the volunteers.

The characterization of "aerobic" (needs oxygen) and "anaerobic" (doesn't need oxygen) fuels relates to the body's energy-producing chemical reactions. Carbohydrate is an anaerobic fuel because any oxygen it needs is integrated into its structure; it can release its energy without needing oxygen from the bloodstream. Oxygen is, however, needed for muscles to burn (aerobic) fatty acids for fuel. This is why our breathing rate picks up when we become more physically active, even when it is something as routine as walking up a flight of stairs. As soon as the needed oxygen becomes available, the muscles shift gears to the more efficient fat-burning mode.

As far as exercise, "anaerobic" usually indicates an activity with quick bursts of higher-intensity muscular work, such as weightlifting and track events. "Aerobic" exercise, such as walking, jogging and cycling, are usually lower in intensity but longer in duration. This type of exercise typically includes a warm-up period that increases the rate of breathing and allows the body to mobilize its fats.

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 have been reading about our bodies' need for enzymes. While the companies selling enzyme supplements certainly have an interest in promoting their importance, I'm concerned that there is something to their claims. Does cooking really destroy enzymes, and do we stop producing them as we age? -- E.I., Oakland, California

DEAR E.I.: A defining characteristic of an enzyme is that it effects change in another substance while remaining unchanged itself. The enzymes in our body are specialized proteins that not only help us make use of the energy in foods, but also play a role in the building and taking apart of complex substances.

There are situations where it might be reasonable to take an enzyme supplement. For example: Many adults no longer produce a sufficient amount of lactase, the enzyme that breaks down the lactose carbohydrate found in milk. The symptoms of "lactose intolerance" can include intestinal gas, cramping and diarrhea. Taking lactase supplements with milk products can reduce the annoying side effects. Another popular enzyme supplement (Beano) is based on alpha-galactosidase, an enzyme that helps digest a type of carbohydrate found in beans and some other vegetables.

There are also medical situations in which dietary enzymes might be prescribed. The pancreas produces many of our digestive enzymes. If a medical problem develops that affects the pancreas's ability to either produce or deliver its enzymes to the digestive tract, the body will be unable to digest and absorb a variety of essential nutrients.

Problems with enzymes can also be experienced by those who have undergone a gastric bypass. Those with such issues would have a legitimate reason to take specialized enzyme supplements.

But what about those who don't have an enzyme-related malady?

There is an unsubstantiated theory that our lifespan is closely intertwined with our ability to produce enzymes. It is based on the dubious idea that we can only manufacture a fixed amount of enzymes in our life; when we run out, the game, so to speak, is over. The theory posits that fresh fruits and vegetables contain "live enzymes." By eating a diet rich in raw, whole foods (because cooking kills the enzymes, they say), or by taking enzyme supplements, we effectively spare the "drain" on our bodies' resources and help extend our lives.

There is no foundation for this enzyme theory, especially the part about the body having a fixed amount of enzymes. Fresh foods are healthful not because they contain enzymes, but because they are our richest source of nutrients and phytochemical substances. Given that enzymes are protein substances, those in foods will be inactivated in the stomach's acid environment and disassembled by the protein-digesting enzymes in our digestive tract.

By all means, eat and enjoy fresh, whole plant foods -- but don't hold them in any special esteem for their enzymatic powers.

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.