DEAR DR. BLONZ: I keep seeing messages that foods are now mineral-deficient, and we need supplements to prevent deficiencies. Which minerals are currently believed to be essential for good health? Are there still as many of these as there used to be, or has modern agriculture made it so foods have less? -- T.S., San Diego

DEAR T.S.: A mineral is loosely defined as anything in nature that is not animal or vegetable. Minerals are inorganic chemical elements required for specific structures or metabolic functions; "inorganic" means they do not contain the element carbon. The body cannot synthesize or change minerals; they don't provide calories and are needed only in small amounts relative to protein, fat and carbohydrate. The body can become more efficient in recycling some nutrients; still, prolonged insufficient intakes lead to a failure to thrive and the development of deficiency diseases, the expression of which depends on the nutrient.

Minerals needed in more significant amounts include calcium, chloride, magnesium, phosphorous, potassium and sodium. Chromium, copper, fluoride, iodine, iron, manganese, molybdenum, selenium and zinc are equally essential, but needed only in trace amounts. (Check the National Library of Medicine's page on minerals at b.link/5wfsdv.) All the minerals the body needs can be found in food.

Think of plants as miners, pulling the mineral elements out of their environment, unable to grow and thrive if an essential mineral is absent. It follows that essential minerals that leave with the plant at harvest need to be replaced using fertilizer or compost, or subsequent yields will drop.

Minerals that are not essential for the plant can also end up in the food; what enters can vary by mineral, soil and weather -- even among different varieties of the same fruit, vegetable or grain. The plant's overall nutrient content can also vary by the length of time the plant has had to grow. Although the differences might not be significant, a plant picked green that ripens after harvest may not have the same total nutrient content as one allowed to ripen "on the vine."

For example, whole foods grown in iodine-rich or selenium-rich soils tend to have more of these nutrients than the same type of food grown in soils with lesser amounts of these minerals. Of historical interest, iodine-rich soils tend to be close to the coast, with less iodine found in foods grown inland. The incidence of goiter, an expression of iodine deficiency, once correlated with distance from the coast and led to the introduction of iodized salt in 1924. The typical diet in the U.S. is now iodine-sufficient, so iodized salt is no longer needed.

While we may have turned to varieties better suited to transportation and storage, it is incorrect to say today's fruits, vegetables or grains will provide any fewer essential nutrients than the same varieties would have provided in the past. Nutrient data tables, such as those provided by the USDA, present the nutrients expected in a particular food (fdc.nal.usda.gov/). Finally, check the source of the "foods are now mineral-deficient" messages you're seeing; aside from being misleading, you may find they originated from a party seeking to sell supplements.

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: 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.