DEAR DR. BLONZ: In my 20s and 30s, I was able to maintain a reasonable weight with diet and exercise. Now, at 43, I have maintained my activity, but have been gradually gaining weight. I now find myself about 15 pounds overweight. I have researched diets and am considering going very low-carbohydrate, but I want to know how this diet is supposed to work and whether it's worth it. -- F.T., Chicago

DEAR F.T.: When you eat a diet that is very low in carbohydrates (under 20 grams a day), you have threatened your body's access to glucose. A chronic shortage of available glucose creates the situation in which your get-up-and-go energy will have, in effect, gotten up and gone.

The fat stored in our body is our major energy reserve, but the body needs small amounts of glucose to help it burn the fat correctly. The incomplete combustion of fats produces ketone bodies, and these can be toxic if allowed to accumulate. The body starts to eliminate them through the urine. The resulting condition, called ketosis, also occurs with uncontrolled diabetes; there, the body is unable to produce the insulin needed to let glucose into the energy-producing (fat-burning) cells of the body.

Certain tissues -- including the red blood cells, the kidney medulla, the lens of the eye and even the brain -- rely on glucose as a primary fuel. When there isn't enough glucose, the body begins to scavenge around for potential sources among its own tissues. The liver has glycogen, a source of stored carbohydrate, and there is a small amount in the muscles, but the amount stored is small in relation to total bodily needs.

Protein can be used, because some of its amino acids can be turned into glucose. The muscles represent the body's largest reserve of protein, and like all protein tissues, they are about 80 percent water by weight. When the body begins to break down muscle tissue to get amino acids to turn into glucose, the water portion of muscle tissue gets released and eliminated. Like magic, the numbers on the scale begin to go down -- but it is water weight, not excess body fat, you are losing. This type of lost water weight is quickly regained once carbohydrates are back on the plate.

The bottom line is that a low/no-carbohydrate diet is a questionable way to lose weight. And besides, who wants to live life without carbohydrates?

It is important to accept that it's not unusual to gain weight as we pass through our 30s, 40s and on up. It's a natural side effect of an age-related slowdown in our metabolic rate, coupled with a trend toward less physical activity as we age. While the former is beyond our control, we can definitely do something about the latter.

A good strategy is to avoid quick-fix fad diets and strive to be the healthiest person you can be. I am certainly impressed by your attempts to stay active, and I hope you are able to keep it up regardless of which dietary path you choose. I wish you the best.

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: Assuming I will eat the same number of calories from the same types and amounts of foods, is it better for me to have those calories earlier in the day, or later? -- A.C., Dallas

DEAR A.C.: There is a bit of evidence that a calorie may have varying effects when consumed at different times of the day. A classic 1976 study by professor Franz Halberg had volunteers consume only meal per day, either as their breakfast or their dinner. The participants adapted to the new routine and reported hunger pangs prior to their scheduled meal.

All things considered, those who had their meal at dinnertime were more likely to gain weight than those having their calories in the morning. The body, it seems, is more likely to use more calories when they're consumed earlier in the day.

There is some logic here, given that the one meal represents the entire day's energy intake. The full-day calorie supply entering the body after a morning meal will need to be portioned out to the energy-requiring activities of the day, as well as toward storage. Contrast this with a full-day's calorie supply being absorbed after an evening meal, with the evening's tapering energy demands. This creates more of an uninterrupted flow toward calorie storage.

The science that studies such time-related issues was first named chronobiology (chrono meaning "time"), but was more recently dubbed chronomics. It began in the early 1700s when botanists discovered that plant behavior adhered to a daily cycle. Since then, scientists have identified rhythmic behaviors in animals, and research continues on rhythms in the human body.

It has been learned, for example, that bodily events such as blood pressure, body temperature, heart rate and urine excretion all have distinct daily rhythms. Cycles that occur on a 24-hour basis are called circadian rhythms, but there are also those that are weekly, monthly or yearly. It helps explain hunger pangs, jet lag or even a ritual daily visit to the bathroom. Research is telling us that the patterns may provide a clue as to how nutrition could best serve us in the prevention and treatment of many ailments.

Optimizing meal timing can have implications for numerous groups of people: the elderly who find it difficult to eat large meals; pregnant or lactating women with limited food resources; athletes in training, seeking to foster muscle growth; or dieters continually baffled by their bodies' resistance to giving up excess weight. Other possibilities would include the most efficient timing of meals in relation to radical treatments, such as chemotherapy.

There's little question that research in this area may unlock many of the body's mysteries. Science once tended to gloss over the time factor, but an awareness of chronomics might provide tools to better understand the body's elusive rhythms.

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 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.