DEAR DR. BLONZ: I have a question about minerals and absorption. Let's say that our body can only absorb 50% of the minerals in a supplement, and that supplement label shows it supplies 100% of the RDA. Does that mean you must take twice the dose to absorb the RDA? How does this apply to food labels? -- S.M.
DEAR S.M.: Dietary recommendations consider the body's absorption efficiency -- they reflect the average amount an individual should eat daily to satisfy their requirements. If, for example, science determines that the average body should have 300 milligrams of a particular mineral every day, and if the research indicates that we absorb only about 30% of this mineral in our food, then the recommended dietary allowance (RDA) for that mineral would be 1,000 milligrams per day (30% of 1,000 = 300). This allows us to take RDA figures literally. Still, an additional heads-up is needed in relation to your question.
Foods and dietary supplements make use of the Facts labels (Nutrition Facts and Supplement Facts), and this system uses the Daily Value (DV), not the RDA. The RDA is based on research data to establish requirements according to gender and age. The DV, by contrast, presents one set of numbers for all adults. The intended role of the DV was to help consumers decide between foods by providing a way to check which provides the preferred nutrient profile. DVs are based on the RDAs, but are not as precise. Read more on Daily Values at b.link/mq2sqr.
DEAR DR. BLONZ: You debunked the ice hack, but it left me wondering whether there is a potential for drinking too much water that could possibly hurt the body or the kidneys. -- P.O.
DEAR P.O.: You can overconsume most things, and water is no exception. With water, it is referred to as overhydration, which can progress to water toxicity. That's right: pure water, the toxin!
This risk is based on the fact that the body cannot eliminate pure water. An amount of electrolytes, present in all bodily fluids and essential for nerve transmission, gets shown the door with every drop of urine and sweat. Sodium, the main element in salt (sodium chloride), is our primary electrolyte in body fluids and tissues; this explains why perspiration has its salty taste. Our body requires specific concentrations of items to work. Overhydration forces the elimination of the excess to bring concentrations back into line, which can drain sodium to dangerous levels (hyponatremia).
Low sodium becomes particularly problematic for folks with cardiac issues, such as congestive heart failure, but taking in too much water also becomes a serious issue for those with existing kidney disease. Plain water is the culprit; there is less of a risk when water comes with electrolytes, such as in well-made sports drinks. For more information on drinking water, see the National Library of Medicine info page at b.link/7b55s4.
Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to firstname.lastname@example.org. Due to the volume of mail, personal replies cannot be provided.