DEAR DR. BLONZ: My best friend, his roommate and I eat the same basic foods. The three of us are similar in size, age and ethnic background, and we exercise using the same routines. In the past, we measured our intake and checked food composition tables and found that we eat just about the same number of calories every day. The reason for this question is that, despite these similarities, our weights and body shapes have diverged over the past years. How is it that people with such similar caloric intakes can end up with such different weights and shapes? -- F.S., Newark, New Jersey

DEAR F.S.: Food composition databases are helpful because they provide useful information about what's in the foods we eat. There is marginal precision for given food items as individual nutrients and calories are averages that blend variations between varieties, growing regions and practices, seasonal weather, and production and packaging methods.

On labels or databases, calories per serving are theoretical calculations of potential energy for an average individual. The efficiency with which a fixed number of calories gets used, however, varies from one body type to another. It's analogous to the way miles per gallon can vary among different types of automobiles driving the same routes.

One's basal metabolism, which is the number of calories used while the body is at rest, tends to rise as the amount of muscle in the body goes up. This is because muscle is active tissue. Fat (adipose) tissue, by contrast, is inactive. Going back to an automobile analogy, this would be similar to how a car with an eight-cylinder engine burns more gas than a six- or four-cylinder engine when idling and during acceleration. Bodies with different genetic makeups, compositions and fitness levels also behave differently.

Other factors that help to explain individual variations include age, sex, level of physical activity and the efficiency with which one digests and absorbs food. All these can affect how a given number of calories gets handled by a particular body. The key contribution of exercise -- especially that designed to support muscles -- is that, in addition to burning calories, it can increase the amount of muscle in the body, which will burn calories even when you are at rest. For more on fitness and different types of exercise, see b.link/sfhbd8h.

As is true with most aspects of life, some people have to work harder to accomplish the same fitness goals. Where weight is concerned, though, one's value system should not get bogged down with comparisons to others. Rather, we should remain focused on doing the best with the hand we've been dealt.

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: What is understood about how the role of the kidneys changes during pregnancy and how this affects what we should be eating? -- P.O., Scottsdale, Arizona

DEAR P.O.: Our blood not only carries nutrients, it also serves as the fluid medium for transporting metabolic wastes on their way to elimination. The kidneys and the liver are organs that serve as cleansing filters to regulate the concentration of various substances in our bloodstream. This can also include holding on to substances in short supply for recycling.

During pregnancy, there is a marked increase in specific nutrient demands at certain points in the process, as well as an increased production of metabolic wastes that are byproducts of the amazing construction underway. Our kidneys regulate substances based on their concentration in the blood, and one of the body's adaptations is to increase its blood volume to boost its capacity to transport nutrients and waste products.

Following conception, one of the first parts of the fetus to develop is its nervous system. Folic acid plays a key role, but the critical element is that sufficient folate must be present during the first few weeks after conception -- a time at which most women may not yet be aware they're pregnant. Spina bifida is a type of birth defect in which one or more of the vertebrae of the spinal column fail to develop properly. As much as 75% of all cases are attributable to a folate deficiency during those first few weeks of pregnancy. The U.S. Public Health Service recommends that all women of childbearing age who are capable of becoming pregnant consume at least 400 micrograms (0.4 mg) of folic acid per day to reduce the risk of a pregnancy complicated by a neural tube defect. The recommended daily allowance (RDA) rises to 600 micrograms during pregnancy.

There is also a special need for iron as the developing fetus requires sufficient iron stores to make its own blood supply during the first months of life. A newborn's main source of nourishment is often breast milk, which, while rich in vital elements, is not a good source of iron. This helps to explain why the RDA for iron rises from 18 milligrams for non-pregnant, menstruating women to 27 mg per day during pregnancy.

Calcium and vitamin D are also standouts, as they are needed for bone development. Calcium absorption doubles during pregnancy, and vitamin D is needed for calcium absorption and utilization. The RDA for calcium is 1,200 milligrams and the RDA for vitamin D is 10 micrograms.

The final standout is water. A pregnant person should drink 8 to 12 cups of fluids per day, with water as the primary source. Throughout her pregnancy, a woman's blood volume increases by 50%, and the extra water helps to transport nutrients and oxygen to the developing child and to carry waste products away. For a more extensive discussion of nutrient requirements during pregnancy, see b.link/hn5jfm4.

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 am interested in the health benefits of plant phytochemicals, and you include chocolate among them. Are the phytochemical antioxidants in the cocoa powder or in the cocoa fat? If they are in the fat, does that mean the only way to get the benefits is to eat full-fat chocolate? -- M.D., Raleigh, North Carolina

DEAR M.D.: Successful plants evolved the ability to capture energy from the sun, using it to fuel the construction of their complex structures and substances and power all aspects of their metabolism. (We rely on the foods we eat to provide our energy.) But, aside from serving as an essential source for energy, the sun's rays can have devastating effects on living tissues. Similar to the visible effects of sunlight bleaching colors, excessive exposure to the sun's oxidizing rays can damage tissues and cause cancer.

While animals develop skin adapted to their habitats, they can move and find shade and shelter. Plants remain anchored by their roots; successful ones have evolved appropriate defenses for every stage of their life cycle. The most straightforward protections are shields in the form of opaque barriers. Examples in nature include leaves and seed coverings. We rely on hats, umbrellas and sunscreen applied directly to protect our skin, but critical are the antioxidants that can inhibit oxidative damage to body tissues and substances. We find antioxidants and other protective phytochemicals in the fruit and leaves that are exposed to the environment. Likewise, plants that tend to have higher levels of fats and oils will usually have lipid antioxidants on call.

Most basic to this discussion is an appreciation that phytochemical protectants are there for the plant's benefit, not ours! Every plant has evolved its own protection strategy, underscoring the wisdom of including a variety of plant-based foods in the daily diet. It is only when such foods are a regular part of what we are eating that they can be there to work their magic for us when needed. For more on antioxidants, see b.link/tb55h26.

As you pointed out, research on chocolate has identified its variety of beneficial compounds. The primary antioxidant phytochemicals in cocoa beans are the flavonols, a type of flavonoid found in the solid fraction of chocolate. These are in the cocoa powder and, of course, in the whole chocolate confection. Dark chocolate tends to contain more than milk chocolate, with white chocolate containing little, if any. As for whether having cocoa powder is a better way of getting the benefits of the chocolate, that, I think, is a matter of personal preference. For more on the benefits of chocolate, see b.link/m7wkbxs.

There are plenty of foods with wonderful attributes. Eat chocolate, not because it is prescriptive, but because you enjoy it.

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.