QUINN ON NUTRITION: Understanding percentages | Get Healthy –


In response to a recent column about diabetes, reader F.D. sends this question: “Question on (A1C) test: Please explain how does this test know to take an average of three months and not two months or one week? I check my blood level every day, and that gives an answer at that precise moment of time, which I understand. No one has been able to answer my question.”

Dear FD: I remember hearing about this amazing blood test years ago when I worked at the University of New Mexico Medical School. A1C is also referred to as hemoglobin A1C or HbA1C.

Hemoglobin is the oxygen-carrying part of our red blood cells. The A1C test — through a small sample of blood — measures the percentage of sugar molecules (glucose) that are attached to the hemoglobin portion of our red blood cells. The higher the number, the higher one’s average blood glucose levels have been during the past three months.

Why three months? That’s the approximate lifespan of our red blood cells. If your day to day blood sugar values are improving, that means less excess sugar in your blood is sticking to hemoglobin. As your body sloughs off old red blood cells that are more saturated with sugar, it makes new ones that have less sugar attached. So your A1C should show improvement after about three months.

Reader I.B. writes this: “Hi Barbara, I’ve been reading the nutrition labels on the different packages of the food that I eat and noticed that a lot of the values are listed as percentages, but percentages of what? And how do you translate that into mg (milligrams)? I was amazed by how iron rich raisins are. Have been putting them on my cereal in the morning and in doing so meet my quota for the day. Thank you.”

Dear I.B: Those percentages on the Nutrition Facts label refer to Daily Values (DVs) — basically your recommended quota for the day of specific nutrients.

According to the US Food and Drug Administration (FDA), the percent daily value (%DV) is how much a nutrient in a single serving of an individual food or dietary supplement contributes to your daily diet.

For example, the current DV for iron is 18 milligrams (mg). One serving (¼ cup) of raisins has 0.7 mg of iron and a percent DV of 4%. That means that you can meet 4% of your total iron need for the day in one serving (¼ cup) of raisins. You will need to get the other 96% of your iron from other foods or supplements.

Keep in mind too, that — based on new research — the daily values for some nutrients were updated for the newest Nutrition Facts label. However, the DV for iron has remained the same at 18 milligrams.

Barbara Quinn is a registered dietitian. Email her at

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