DEAR DR. BLONZ: Please explain the change in nutrient demand, and how the kidneys change, during pregnancy. -- P.O., Arlington Heights, Illinois

DEAR P.O.: Nutrient requirements increase before and during that marvelous construction project, but that also includes an increased production of metabolic waste products. It's all part of making the mother's body ready to house, nurture and protect the developing fetus, and have tissues prepared to create and provide nourishment for the baby after birth. There are also pre-pregnancy nutrition advisories, so be sure to check with your physician, as you may be advised to begin a prenatal supplement a few months before even trying to get pregnant.

Blood is the body's primary transport network, carrying nutrients, raw materials, control substances where needed and energy substances on their way to or from storage. The blood is also the conduit for metabolic wastes needing transport on the way to elimination.

The kidneys serve as the blood's cleansing filter, and are also responsible for fine-tuning the concentration of various substances in the bloodstream. The body adapts to its increased needs during pregnancy by increasing its blood volume; this allows greater amounts of the nutrients and waste products involved in pregnancy. The hormonal changes during pregnancy raise the blood flow through the kidneys and increase their size to handle the greater responsibilities. Staying well hydrated is essential, but the developing fetus does mean less room for the urinary bladder, leading to the need to relieve oneself more frequently.

DEAR DR. BLONZ: I would like to know the benefits of biotin. Would everyone get some benefit when taking this vitamin? -- E.B., Palo Alto, California

DEAR E.B.: Biotin, also known as vitamin B7, plays an essential role in the metabolism and synthesis of fats, amino acids and carbohydrates. Although essential, biotin is needed only in small amounts. The recommended dietary allowance for adults is 30 micrograms (mcg) per day. (Check the National Institutes of Health fact sheet on biotin at b.link/xd27x7.) Foods that contain biotin include organ meats, oatmeal, egg yolk, milk, soybeans, peanuts, whole grains, fruits and vegetables. Biotin can also be synthesized by bacteria in our microbiome, but it is unclear how much of this we absorb.

An interesting side note: Egg yolks are a good source of biotin, but a compound in raw egg whites, named avidin, prevents biotin from being absorbed. Avidin is destroyed by boiling and other forms of cooking. Biotin deficiencies have been found in individuals who consume large quantities of raw egg whites. Raw eggs are a favorite food of foxes, but they pay the price of overindulgence with a biotin deficiency, which is nature's way of keeping the fox out of the hen house.

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 favorite catalog advertises a "fat freezer" to reshape your body by destroying and dissolving fat cells. You apply fat-freezing pads on your problem area and select a 30- or 60-minute session. The cooling process decreases fat cell temperature and will naturally eliminate the fat cells from the body. I haven't ordered it yet, but wanted to know if it could be real. -- S.F., Charlotte, North Carolina

DEAR S.F.: Glad you asked first, as it's a dubious approach that comes with risk. Most on point, this type of treatment does not eliminate fat from the body. The cold treatment, called cryolipolysis, can affect fat cell structure and liberate stored calories. But this is liberation, not destruction; the fat will travel through the blood and find a home elsewhere. Understand that there is no app to tell the fat where to go, so what if it ends up in the wrong neighborhood or leaves you with a misshapen body?

There have been some legal issues with this as a commercial technique administered by trained individuals (see b.link/Pudbhx), but you are asking about an at-home device where all you would have is an instruction booklet. I know it's corny to end this with a note to "cool it," but that does reflect my advice.

DEAR DR. BLONZ: Fast-food chains and processed food manufacturers often project caring and wholesomeness in their marketing messages, but this remains a source of puzzlement for me, given they sell foods that shorten the lives of their customers. Why not only promote healthy, nutritious foods that will have customers living longer so they can buy more of their products? -- T.T., Phoenix

DEAR T.T.: The business logic here is not linked to the long-term health of their customers; these are companies selling products with business models based on profits. To succeed, they need to attract customers and provide what they want. It is wonderful to imagine people being more interested in sustainably produced foods that contribute to their long-term health as well as the health of the environment. If that were the case, companies would line up to please those customers, but that's not our reality.

As we make multiple daily decisions about what to eat, occasional consumption of less-healthful fare should not be a significant issue. It becomes problematic when such foods dominate one's diet. Change can be difficult; a better approach would be education that begins in childhood. Even better, have it begin with parents-to-be. There is a need to have an appropriate model in place in the home; remember that it will likely fail if it becomes a "do what I say, not what I do" situation. We should also encourage nutrition and health education courses to be offered throughout the school years, including growing vegetables in school gardens, cooking classes, and healthful, tasteful foods offered at school.

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: Is it true that raw foods still have their enzymes intact, and that when we eat raw foods, the body uses those enzymes to aid digestion? If so, does this mean that our body doesn't need to rely as much on its own enzymes, giving raw foods a net benefit to our digestion as we age? I have read this idea on food blogs, but it does not make sense, given what I recall from your columns. -- S.F.

DEAR S.F.: Enzymes are compounds that change other substances without being changed themselves. In our bodies, enzymes facilitate the disassembly of complex foods into their component parts before absorption. It's different in plants, where enzymes help make substances available for growth, reproduction or defense.

While plant enzymes don't help our digestion or absorption, they continue to act after the plant's harvest and hasten food breakdown. Refrigeration can slow this action, so many foods benefit from a chill during storage. It is not always a good thing; fresh tomatoes, for example, do not benefit from refrigeration, having a cold-activated enzyme that makes them mushy and less flavorful. (Visit b.link/ub6tzw for more about foods and refrigeration.)

A healthy human body has a cadre of enzymes tailored to our needs. Most enzymes act in the small intestine, where digestion and absorption occur; plant enzymes are not essential to these processes.

The "raw" food idea relates to the fact that cooking can deactivate plant enzymes. But this is not the only risk plant enzymes face: After being swallowed, foods go into the stomach, exposing them to a highly acidic environment that prepares complex foods for enzymatic disassembly in the small intestine. Enzymes are typically proteins, and are denatured by stomach acid like other proteins. Another benefit to stomach acid relates to the fact that foods are not sterile, and the acid helps destroy microorganisms before they can cause harm.

While there can be exceptions for specific foods, sensitivities and health conditions, the general rule is that a healthy human body makes all the necessary enzymes to digest and absorb food. Enjoy raw foods, but not because of any special enzymatic powers.

DEAR DR. BLONZ: I think the newspaper edited your recent column on olive oil to remove the answer to the reader's specific question. The question was about the differences in health benefits -- not flavor -- between extra-virgin olive oil and standard olive oil. Please consider a follow-up column on this. -- A.M.

DEAR A.M.: That was my gaffe, not that of the editors. Please allow me to follow up now.

Extra-virgin olive oil (EVOO) is a healthful adjunct to our foods. The less processed the oil is, the greater the amount of phytochemical assets it contains: substances that evolved to protect the olive and its seed from potential assaults such as the oxidizing rays of the sun. Plentiful evidence suggests that these substances can be beneficial for us, as well.

EVOO is the first oil pressed out during processing, and as such will have higher levels of these assets. For more on olives and EVOO, see b.link/7ea4n4.

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.