DEAR DR. BLONZ: I would like to know the need and benefits of taking biotin as a supplement as it is always recommended for hair and nails. Would everyone benefit from taking this vitamin? -- E.B., Palo Alto, California

DEAR E.B.: Biotin, also known as vitamin B7, plays an important role in the metabolism and synthesis of fats, amino acids and carbohydrates. Keratin is a protein that is integral to the structure of hair, skin and nails. As a water-soluble nutrient, it's not effectively stored, which is why it is a dietary essential, albeit in small amounts. The adequate intake of biotin for adults aged 19 and over is 30 micrograms per day (one microgram is one millionth of a gram), and deficiencies are rare. Foods containing biotin include oatmeal, egg yolk, milk, organ meats, soybeans, peanuts, whole grains, fruits and most vegetables. Biotin can be synthesized by the bacteria that live in our large intestine, but it's unclear how much of this biotin we actually absorb.

Unless there is a dietary issue, medication interference or genetic disorder involving one of the enzymes needed for its use, there is no evidence that taking more than needed can strengthen hair, skin or nails. For more on biotin deficiencies, see b.link/hp677pr.

If taking a biotin supplement, be aware that high doses of biotin (5,000 mcg or more per day) can affect blood test results. Be sure to check with your physician to see how many days before your tests you need to stop. More importantly, have this and other supplement information in your medical record in case tests are needed in an emergency situation.

Finally, one of the more interesting aspects of biotin concerns eggs. Egg yolks are a good source of biotin, but the egg also has avidin, a compound in the raw egg white that binds the biotin and prevents its absorption. Biotin deficiency has been found in individuals who consume large quantities of raw egg whites. Cooking the eggs breaks down the avidin and nixes that effect. Some think it was nature's way of keeping the fox out of the hen house.

DEAR DR. BLONZ: My iron levels are a little low. At breakfast, I have milk on cereal; some of it has iron. For lunch, I have fruit topped with yogurt and high-iron granola. I also have a mug of milk along with it. For a mid-afternoon snack, I have a handful of mixed nuts. For dinner, I usually have some chicken with veggies or beans, though sometimes I don't have any meat or beans. I have orange juice to drink. Thank you. -- C.M., via email

DEAR C.M.: Much depends on the routine monitoring of your blood levels. If you need to increase your intake of dietary iron, it is best to avoid consuming high-iron foods with calcium-rich foods. You also want to keep the iron away from caffeine-containing foods. The most efficient absorption of iron comes when it is in the heme form. For more on iron-rich foods, see b.link/r5hrdrs.

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: 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.