DEAR DR. BLONZ: What is the difference between a “serving,” as used on the USDA food pyramid, and the “serving size” on food labels? Why is there a difference? -- J.L., San Leandro, California

DEAR J.L.: It can get confusing if you don’t get the distinction. The serving sizes on the food pyramid are consistent with dietary recommendations, while food packages have the burden of dealing with the size of the container and providing correct nutrition information for the amount to be consumed.

Let’s use vegetable juice as an instructive example. According to dietary recommendations and the USDA Pyramid, one serving is 8 ounces. In the store, however, vegetable juices are often sold in 12-ounce cans, where the Nutrition Facts panel indicates that the serving size is the entire can.

The purpose of the Nutrition Facts label is to inform the consumer how many calories, grams of fat, sugar and protein, etc. are present in the portion they are most likely to consume. If the 12-ounce can had a serving size of 8 ounces, there would be 1 1/2 servings per container. Then, having the whole can, as individuals tend to do, would require math to get an accurate count of the nutritional contents. To extend this example, check the label of a large bottle of the same vegetable juice. On that large container, the Nutrition Facts reverts to the 8-ounce standard.

It makes sense when you consider that the priority with the Nutrition Facts label is to tell consumers what’s in the foods, allowing them to compare brands and options.

Before recent upgrades, the Nutrition Facts labels’ serving sizes were consistent with USDA recommendations. But this caused confusion when a small bag of chips, for example, would contain more than one serving. Health professionals voiced concern because it made the calorie/fat numbers on the container out of sync with the amount consumed.

So while food label serving sizes often differ from the USDA recommendations, they now provide information about the amount that people actually eat. This approach is now required, and can facilitate comparisons between brands.

For reference, here are some serving sizes used with dietary recommendations.

-- Vegetables: 1 cup of raw leafy greens, 1/2 cup of cooked or chopped vegetables

-- Breads, cereals and grains: one slice of bread; 1 cup dry cereal; 1/2 cup cooked cereal, rice or pasta

-- Fruits: a medium apple, orange or equivalent-sized fruit; 1/4 melon or grapefruit; 1/4 cup dried fruit; 1 cup berries

-- Dairy: 1 cup of milk or yogurt, 1 1/2 ounces natural cheese, 2 ounces processed cheese

-- Fats and oils: 1 teaspoon of oil or a spreadable fat, 1 tablespoon of an oil-based dressing

-- Meat, poultry and fish: 3 ounces, about the size of a pack of playing cards

-- Other proteins: 1/2 cup cooked beans, 4 ounces tofu, 1 ounce nuts, 1 egg

-- Wine/alcohol: 4 ounces of wine, 12 ounces of beer, 1 ounce (a shot) of hard liquor

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 old would a person have to be to be considered an “adult” when it comes to vitamins and supplements? My daughter is 14, and she has terrible eating habits, with her own set of quirks and refusals. I would really like her to take a supplement. I don’t know if she would be considered a child, and to take children’s vitamins, or an adult. She is active and not overweight. -- S.T., Broken Arrow, Oklahoma

DEAR S.T.: There isn’t any specific age at which a child magically becomes an adult. During the adolescent years, ages 13 through 17, the child transitions into their adult body, but this can vary from individual to individual. Some medications direct you to use adult dosing from age 12 on, while other medications might have separate guidelines up to age 18. That is why it is important to always follow the directions for each product. After all, a child is not a miniature adult; pharmaceutical dosages are based on the way a drug acts in each given age group.

As far as vitamins and minerals are concerned, the “daily values” on the labels are set for adults and children over the age of 4, which would indicate that your daughter is most definitely in adult status.

Do your best to educate her regarding the importance of good food and good nutrition, especially as regards the need for calcium and other essential minerals. Teens and young adults need to appreciate that the first 35 years of life is the critical period during which their bones can put on more mass than they lose, but this only happens when there is an adequate intake of calcium. The fourth decade of life is when we begin to shift gears, and any bone mass present serves as a “savings account” for the slow but incessant drain that occurs in the decades ahead. An active lifestyle is also important, but healthful dietary habits during the first half of life are critical. If your daughter is not going to be getting all she needs in her diet, a supplement can serve as a reasonable alternative, but it’s not a long-term solution.

Childhood and adolescent food quirks and refusals may come across as a challenge to your authority, but mealtime should not be allowed to degenerate into a power struggle. Missing an occasional meal, failing to eat from all the food groups on a daily basis, or never touching the spinach or broccoli are not signs of impending malnutrition. The body has amazing powers to conserve needed nutrients and make the most of them when they finally appear.

There’s no way to predict how individual tastes develop; they are facts of a child’s life that tend to ebb and flow over time. Talk with other parents. What a child’s peers enjoy can hold sway over their preferences.

In the end, aim to instill a sense of appreciation for all that’s involved in bringing food to the table in a supportive family setting. Add a nudge toward a healthy food selection, be consistent with the example you set, and you’re on track for promoting long-term healthful habits.

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 times during the previous year, I took St. John’s wort for periodic bouts of mild depression. I have had little success with the prescription antidepressants I have tried. Does the fact that I have not had any problems with my St. John’s wort indicate that I can continue taking it? -- S.C., Tulsa, Oklahoma

DEAR S.C.: We all need to appreciate that the human body has sophisticated systems designed to protect it from potential dangers posed by unfamiliar substances. These systems try to detoxify, break down or eliminate “foreign” compounds that enter the body. Foreign compounds can include everything from herbal products and prescription or over-the-counter (OTC) drugs to pesticides and environmental pollutants. It doesn’t matter that some of these might be thought of as beneficial; foreign compounds get identified as such, and processed for elimination.

The complexity starts because there can be multiple compounds processed at any given time. This gives rise to interactions between medications and herbs that can affect the way they work and the rate they get eliminated from the body. Our liver is the focus of this process. The method typically involves a reaction with the substance that’s tantamount to putting metabolic handcuffs on it, preventing it from being active while it’s in queue for elimination.

A specific family of enzymes plays a key role in metabolizing, or breaking down, unknown substances. Compounds broken down via this enzymatic system include herbs; drugs used to treat heart disease, HIV infections, seizures, depression and cancer; drugs used to prevent the rejection of transplanted organs; and oral contraceptives. Different substances can either compete with each other for enzyme attention -- resulting in slower decreases in their respective blood levels -- or they can stimulate the enzymes, which results in a more rapid decline. This is significant for health conditions where reliable medication blood levels are a key part of the treatment.

St. John’s wort is metabolized by this enzyme system, and it acts as a stimulant. That means that if St. John’s wort is being taken, the blood levels of other drugs can be lower than expected -- lower, even, than needed to achieve that drug’s therapeutic effect.

I don’t know if you are taking other medications or substances, but this is definitely a discussion you should have with your physician or pharmacist without delay.

Before prescription drugs are approved by the Food and Drug Administration, they have to undergo rigorous testing for safety and efficacy. Tests are also done to determine how a drug works, the effective dose, any potential side effects or adverse reactions, how long the drug lasts, and how it is eventually eliminated from the body. All this helps to provide the information used by health professionals when dispensing the drugs. For OTC drugs, this info is available for you either on the package or in the package insert.

Herbs have been in use for centuries, and some may have health-boosting potential. Our knowledge, however, about side effects and interactions with other medications -- even with other herbs -- is still in its infancy. That is one of the reasons why people should approach the use of herbs with caution, especially when also using other medications. There are websites providing interaction information, including drugs.com and rxlist.com, but these should not be considered a substitute for that talk with your health professional.

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.