DEAR DR. BLONZ: I am in relatively good shape for one in their 50s, and I eat well. I walk and exercise regularly, but not as much as I did earlier in life. Like many others in my group, I am annoyed by the weight around my middle. I can't improve my diet much more, and am frustrated at the bulge. Nothing has really changed except the shape of my body?! Any advice you can offer on aging, calories, and specific exercises would be appreciated. -- T.T., San Jose, California

DEAR T.T.: With aging, there is a natural decline in the energy (calories) the human body requires. There is also a tendency to decrease activity. Both contribute to changes in body shape, especially with individuals who continue to consume the same (or more) amount of calories, particularly if more processed foods with added sugars have entered the mix. There is also the fact that our body weight is packed into a smaller frame due to age-related loss of height from reductions in bone mass and increased spinal compression. (For more on the effects of aging on body shape, see b.link/2a3d3ay.)

There are three basic elements to our daily caloric demand. First and foremost is the resting metabolic rate (RMR). This is energy needed to keep the body alive, but not to fuel activities. Consider this our "cost of living," which represents about 60% to 75% of the daily caloric requirement. Next is the energy used for physical activity. For an average individual, this is about 15% to 30% of the calories we need. Finally, we have the thermic effect of food (TEF), also called thermogenesis, the energy released as heat as we eat and digest food. TEF accounts for about 5% to 10% of our daily calories, and tends to be higher in those with regular exercise habits. (More on thermogenesis at b.link/gls72y2.)

Fewer calories are burned by an aging body than its younger version, even if eating and physical activity remain the same. This age-associated decline is not solely attributable to the loss of lean body mass (muscle, organ and bone) that accompanies aging.

Muscles burn calories even when you are lying around, so do your best to maintain your muscle mass to help slow any age-related decline. (Think of the difference between the gas burned by a 4-cylinder engine idling at a stop light versus an 8-cylinder doing the same.)

The cumulative effect of stress can also contribute to that unwanted midline bulge. The body reacts to stress by bringing fuels into the blood to prepare for action. This occurs even if the stress is psychological and won't require a physical response. Ephedrine (adrenaline) and norepinephrine are the stress hormones that increase fuel levels. Cortisol is the stress hormone that helps calm things down and return blood fats to storage. Receptors in the abdominal area are more sensitive to cortisol, which can lead to greater fat accumulation in that area when stress is habitual.

Keep up with your activities. There should be elements of both aerobic workouts and resistance exercises to maintain your muscle mass. Consider speaking with a trainer to see if there are specific exercises to help with muscle tone in problem areas. Working on your core muscles is particularly helpful as it supports the back, another aging-associated issue we want to stay ahead of.

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: How risky is it to drink orange juice that is a bit fizzy? Does the fizz indicate that oxygenation is occurring? Isn't this associated with the process that creates free radicals? I know that free radicals are harmful to our health, but I also know that we can't survive long without an adequate supply of oxygen to the brain. Oxygen is obviously indispensable for our survival, but are there times when it compromises our health, as in drinking fizzy orange juice? Thanks for any clarification. -- R.L., Raleigh, North Carolina

DEAR R.L.: First, a bit about fizz when done intentionally. The "fizz" in drinks comes from carbonation, where carbon dioxide (CO2) gas is added to water. Carbon dioxide does not dissolve well in water, but it becomes more soluble as the temperature decreases. Pressure can also be used to "hold" the gas in solution. If you like your fizz, consume your carbonated beverages when cold and recently uncapped. This is why there is a louder "pop" and greater release of gas when a soda is uncapped at room temperature. Carbon dioxide is only loosely held by water, which explains why agitation, such as pouring a warm soda over ice, or shaking the bottle at any temperature, triggers a gaseous eruption.

When you consume carbonated beverages, some of the swallowed gas is absorbed and exhaled. The rest exits through belching, a process in which the esophagus opens and allows the pent-up gas to escape from the stomach. If it manages to pass through the stomach, the remaining gas can then pass through the GI tract and exit at the other end.

An interesting take on this topic relates to beer. The "proper" way to consume this carbonated beverage is to pour it so the fizz escapes and forms a foamy head, not down the side of the glass to minimize the effect. Drinking beer this way helps prevent bloating and belching, and is associated with better flavors. For more on beer carbonation, see b.link/5rb2fae.

Back to orange juice: The fizz you noticed developing is a sign of spoilage due to microbial fermentation of the juice's sugars. It is not a sign of free radical oxidation or oxygen. The fermenting organism gives off carbon dioxide gas that can be detected on your tongue, thus giving a sensation similar to that caused by carbonation. The organisms consume glucose, which results in that loss of sweetness. The fact that the orange juice in your example still tasted sweet might make it seem as though only a limited amount of fermentation had occurred. But, because the spoilage organism is unknown, it is best to discard any beverage or food that has become unexpectedly carbonated.

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 would like a better understanding of the differences between nitrates, nitrites and nitrosamines in food and the body. There are studies linking nitrates to colorectal cancer, but a chemist informed me that nitrates are present in foods such as carrots and certain beers. Is this correct? -- J.C., Hayward, California

DEAR J.C.: Nitrites and nitrates (microbes in the body and elsewhere can convert to nitrites) are naturally present in several vegetables, including turnip greens, beets, celery and spinach. Nitrites are also found in beer and cheese. The risk from nitrites in meats differs because, together with protein and high cooking temperatures, they create an ideal environment for the formation of nitrosamines, substances associated with cancer.

Nitrates represent less of a risk than nitrites. Nitrosamines can form in the stomach when nitrate-containing vegetables and proteins are eaten in the same meal. But -- and this is key -- there tend to be antioxidant nutrients and phytochemicals in those nitrate-containing plant foods, and they limit nitrosamine formation.

Contrast this with nitrite-preserved meat products such as bacon or sausage, which contain all the necessary players for nitrosamine formation. The presence of nitrosamines is a risk, not a certainty. By comparison, the natural nitrates in whole plant foods would represent a healthful walk in the park. For more on foods high in nitrates, see b.link/rwdhs93.

DEAR DR. BLONZ: One of the foods I enjoy is corn, and even when it's not in season, I go for fresh-frozen organic kernels. I understand that digestion is a multi-layered process, and our digestive system is designed to break down food and absorb nutrients. Why is it that it often seems that corn passes through the body undigested? Is it something else I eat along with the corn? Are there some varieties that are easier to digest than others, or do some people have an easier time with corn than others? -- C.L., Des Moines, Iowa

DEAR C.L.: Corn is a wholesome, digestible whole food, and I look forward to it every spring when it begins to appear in the market. Each kernel of corn is a seed, and it has a tough outer shell, called the pericarp, that protects it from the elements. The germ is the nutrient-rich live part of the seed, and the endosperm is the starch that provides food for the sprouting plant. The structure of a corn kernel has important implications for seed distribution. Corn, like many other plants, relies on birds and animals to eat the seeds and carry them to new locations, where viable seeds passing through their systems get deposited and grow into new plants.

The human body has enzymes that can digest the contents of the corn kernel, but to work, they need access to the germ and endosperm. The fibrous outer coat of a kernel must be broken through via mashing, cooking or chewing for the digestive system to have its way with the contents, but even then, some fiber can remain.

There are indeed different varieties of corn, some of which are used as animal feed, and these tend to have a tougher outer coat. This provides greater resistance to the elements, relying on the grinding and chewing that animals employ during feeding. The corn varieties used in our diets are more tender and flavorful. That said, they still need a good chew to unlock the nutrients inside. (For more on different types of corn and how they are used, check b.link/rkazxkk.)

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.