DEAR DR. BLONZ: My brother and I did genetic tests after precancerous polyps were found during his routine colonoscopy. (Our father died of colon cancer.) Both our genetic tests came back negative for the genes that increase that risk. My brother’s polyps were found at an early stage, so he should be fine, but we are both on regular monitoring programs.

We have changed our lifestyles and eating habits -- being more active and having more fruits, vegetables and fiber. I have grown to like greens such as spinach, but have learned that these and some other vegetables contain nitrates, which can become carcinogenic nitrosamines. This was a bit of a shock. Do these foods need to be avoided, or at least limited? How dangerous are the nitrates in foods? -- S.C., Chicago

DEAR S.C.: It is difficult to learn that one is at increased risk for any cancer. It is good that you and your brother are being proactive with your diet, lifestyle and, of course, the periodic monitoring. These are all associated with decreasing your risk of colon cancer. (Read more about this at b.link/fp2mqb.)

To answer your question, we need to discuss three different compounds: nitrates, nitrites and nitrosamines. Nitrates are naturally present in many different types of foods, including vegetables and fruits. Nitrites are often used as food additives in cured meat products to decrease the risk of botulism. Nitrosamines are carcinogenic compounds that can form when a nitrite combines with an amine. Amines are naturally present in foods, but they are more plentiful when the amino-acid building blocks of a protein get separated during digestion in the small intestine.

For a nitrate to turn into a nitrosamine, it must first be reduced into a nitrite. Once a nitrite, it has to be in close proximity to a specific type of amine in an environment that fosters their combination. Bacteria usually do that conversion of nitrate to nitrite, and while there are bacteria in our saliva, they convert only a small amount of the nitrates we consume. This process is inhibited in an acid environment, so if there is vitamin C in the mix, as there often is with fruits and vegetables, it proceeds even more slowly.

Nitrates tend to be absorbed after they leave the stomach and enter the small intestine. There are bacteria down the road in the large intestine, but by that point, the dangers of nitrate and amine stragglers sidling up to each other and becoming nitrosamines represents a negligible risk.

Contrast all this with nitrite-preserved meat products, in which all the players (the nitrite and amines from the meat protein) are together in the package and throughout their travels in the digestive system. Of course, there is no guarantee that nitrosamines will form, and the use of nitrites to preserve meat products is preferable to the risk of botulism. (Read more on this at medlineplus.gov/botulism.html.)

By comparison, the natural nitrates in fruits and vegetables, including the spinach you now enjoy, represent a walk in the park. So please continue to enjoy these healthful foods.

One other side note: Chronic constipation is a risk factor for colon cancer. Having a healthful lifestyle and a fiber-rich, plant-based, whole-foods diet provides additional help by keeping things moving toward the exit.

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: At what age is a person considered an adult as regards nutrient requirements? My daughter is 14, and I want her to take a supplement because of her sketchy eating habits. I developed osteoporosis -- it is now under control, but I do not want this for her. -- F.M., Phoenix

DEAR F.M.: The answer of when to grant “adult” status for nutrient levels varies based on a number of factors. For one thing, every child grows at their own rate; a look back at class pictures will show how growth through childhood, adolescence and adulthood can vary from individual to individual.

Broadly speaking, a child is not a miniature adult. At some time during the adolescent years, ages 13 through 17, the child transitions into their adult body. In some, this occurs rapidly, as evidenced by the frequent need to buy that next-larger size of clothes. (This phase often, but not always, coincides roughly with puberty.)

Pharmaceutical companies must consider this adult/child issue quite carefully. Medication dosages are based on how the drug is expected to act in each given age group, so it is essential to test a given drug on all age groups for which it may be prescribed. Some medications direct you to use adult dosing from age 12 on, while other medicines have separate guidelines up to age 18. That is another reason why it is always important to read and follow directions on medications.

As far as vitamins and minerals are concerned, food labels’ “daily values” are set for adults and children over the age of 4, by which measure your daughter is most definitely in the “adult” category.

But rather than encouraging supplements as the answer to poor eating habits, do your best to educate your daughter regarding the importance of good food and good nutrition -- especially the need for calcium and other minerals for her bones. Young men and women need to understand that our bones are constantly made and remade, and that the first 35 years of life make up the critical period when our bones tend to gain more mass than they lose. This switches during the midlife period, so you want a healthy bone mass in place before the “drain” takes over. Physical activity can help optimize skeletal health throughout life.

If your daughter is not going to be getting all she needs from her diet, a vitamin-mineral supplement might be an alternative, but is not a true solution. Food should always be the priority. Keep in mind that the example you and other adults set is the seed that takes root as your daughter transitions to adulthood. Avoid the “do as I say, not as I do” paradigm, as kids tend to see through this.

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: Much of the advice I have received about managing my digestive issues has mentioned the need to avoid certain food combinations. This advice is based on the logic that “bad” combinations overwhelm the digestive system, preventing the foods’ nutrients from being properly absorbed and contributing to digestive upsets. Is this based in fact? -- O.C., New York City

DEAR O.C.: Foods are a complex combination of different types and sizes of nutrients and non-nutrient components. Our digestive system, by contrast, likes things simple. For our bodies to absorb foods’ components, foods must be taken apart piece by piece. Think of our digestive system as a “disassembly” line.

The digestive tract workers are enzymes: chemicals with specific abilities to pull apart the proteins, carbohydrates or fats in food. Enzymes are the epitome of specialization, in that each performs one action on one type of nutrient. For example, one enzyme splits big proteins into smaller pieces, while others break the small pieces into their amino acid parts. Similar-staged disassembly occurs with complex carbohydrates and fats.

We rely on over a dozen different enzymes to digest a typical meal. The beauty of the human digestive system is that it’s designed explicitly for that mixed diet. That means the advice you were given was off the mark. Since different food components get handled in different areas and by different enzymes, we don’t have to eat our foods one at a time.

Digestion begins even before the first bite: The cephalic phase starts when we see, smell or even think about food. We have all experienced our stomachs rumbling in anticipation of a meal. This is also connected to our tendency to eat meals at the same time most days, and becomes especially noticeable when a meal is delayed. In the cephalic phase, acid gets released in the stomach in preparation for the food about to arrive. In the mouth, chewing increases a food’s surface area and mixes it with our enzyme-containing saliva to facilitate the actions ahead.

The stomach is our high-acid, muscular churning compartment that denatures proteins. The acid environment activates certain enzymes involved in protein digestion, and it can also help destroy unwanted microorganisms hitchhiking on the food. The stomach is spared from self-digestion by a protective layer of mucus. Those with the misfortune of having gastroesophageal reflux disease, or GERD, have experienced the discomfort of the stomach acid splashing up into the esophagus, where that protection is not present. (More on GERD at b.link/7kae7.)

As the stomach completes its work, the partially digested mass of food, referred to as chyme, leaves through the pyloric sphincter. As the chyme departs, it is immediately doused with the body’s own antacid solution. Your meal is now in the small intestine, a veritable enzyme factory where the central part of digestion and absorption occurs.

Returning to your question, many of us may have found that certain foods or food combinations work best for us. Consider that we are creatures of habit, and our bodies do acclimate to the way we eat.

When making major changes in what we eat, it makes sense to do things slowly, but this advice is based more on our bodies’ habit-bred efficiency than a defect of human digestion. There appears to be no physiological reason to refrain from including a variety of foods at every meal.

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.