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.

DEAR DR. BLONZ: You have addressed the use of vinegar to help blunt the impact of carbohydrates on blood sugar levels. I want to know what white distilled vinegar is made from. And how is it made? Is it healthy or harmful? -- F.X., Los Angeles, California

DEAR F.X.: Vinegars are made from fermented carbohydrates and have been used as flavor enhancers, preservatives and folk remedies since 5000 B.C. There are many varieties, with their flavors depending on the type of carbohydrate used and other ingredients in the blend. (For a food-focused article on vinegar types, see b.link/Gqcapma.)

White distilled vinegar comes from the distillation of grains -- usually corn, but also rye and barley. The acid in vinegar is acetic acid, and it is usually present at about four parts per hundred (also expressed as 4% by volume). The main issue with using white distilled vinegar is that it has the strongest, most acidic taste of all the vinegars. It can be used in foods, but it is perhaps best suited for pickling. Vinegar does not provide any needed nutrients. While excess can overwhelm other flavors, vinegar is not unsafe or harmful when used appropriately.

DEAR DR. BLONZ: A friend told me how to figure out the percent of calories from fat. I am not a big math person, but I recall it was to multiply the grams of fat per serving by 9 (number of calories per gram of fat), then divide this amount by the calories per serving. I tried this on frozen hamburger meat that was 10% fat, but the calculation showed the fat was a whopping 52%. My confusion comes from my thinking that if it said 10% fat, only 10% of what you were eating was fat. -- M.S., Seattle, Washington

DEAR M.S.: Both your math and your friend's are correct. Ten percent of that meat is fat, but only if it's 10% of the weight. The confusion arises because the percent of calories from fat differs from the percent of fat by weight. We rely on the percent of calories from fat in dietary recommendations, where the totality of what's eaten is considered. With meat products regulated by the USDA, labels can declare the percent of fat by weight. (For more on foods under USDA jurisdiction, see b.link/p2y6f2c.)

You can calculate the percentage of calories from fat for an individual food by following your friend's instructions. There is a shorthand method for any food with a Nutrition Facts panel. Check the calories and grams of fat per serving. Every gram of fat per 100 calories equals 9% of calories from fat. So, if there are no more than 3 grams of fat for every 100 calories in a serving, the food derives no more than 27% of its calories from fat -- just below the recommended level. This method works regardless of serving size and is valid for both single items and meals.

When it comes to fats, the focus should remain on your entire diet. Almonds, for example, contain a variety of nutrients. If only percent calories from fat were looked at, you might be dissuaded from this healthful food when learning that 80% of its calories are from fat.

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.