DEAR DR. BLONZ: What does the "L" stand for in L-arginine? Is it the same thing as plain old arginine? I have also seen this letter on other amino acids such as ornithine. -- L.T., Santa Rosa, Calif.
DEAR L.T.: The "L-" or "D-" before a chemical substance reveals something about the compound's three-dimensional structure. To be more specific, it refers to the optical rotation of the substance, and while this may seem like a subtle structural difference, it has definite effects on how the compound can be used chemically and in the body.
We use "L" amino acids in the synthesis of our proteins, and they are found in the protein foods we eat. "D" amino acids have no nutritional value for humans. As you indicate, the "L" might be left out in some writings. It is a bit less precise, but it saves the need for this explanation. On the "D" side of things, we have D-alpha-tocopherol (vitamin E); in this case it is the "D" version that's the active substance. One final point: If both a "D" and an "l" are noted together, as in "Dl-alpha-tocopherol," this is usually a sign the compound has been made synthetically.
DEAR DR. BLONZ: I take a calcium supplement that contains calcium carbonate along with magnesium and vitamin D. I have been advised by a close friend that calcium carbonate is the least absorbable form of calcium, and that I need to switch. My question is whether the presence of vitamin D makes up for this and helps with the calcium absorption issue. Are the other forms of calcium still more absorbable than calcium carbonate, even with vitamin D? Thanks for any light you might shed on this issue. -- N.N., Phoenix
DEAR N.N.: First, on vitamin D: Having it in the same pill with calcium doesn't enhance the absorption of the calcium in the pill, but our body definitely needs adequate vitamin D to produce its "calcium-binding protein." That's the protein made by the body that attaches onto dietary calcium in the small intestine and facilitates its absorption. This makes vitamin D an important part of the big picture, but it doesn't need to be in your stomach at the time as the dietary calcium.
We do find vitamin D in calcium-containing foods (it is added to many dairy products), and as you indicate, it's not uncommon to find it as an ingredient in calcium supplements. The body makes its own vitamin D when it is exposed to sunlight (about 15 minutes per day on the face and hands is plenty). Seniors, particularly the homebound elderly, are at risk for a vitamin D deficiency because they don't often get out into the sun.
As regards your supplement, don't be that concerned about it being a calcium carbonate product. People always seem to yearn for "The No. 1 Best Supplement," but in the case of calcium, the degrees of improvement from one form of calcium to another are not significant enough for you to toss your pills. You will be fine as long as you take it regularly -- ideally at mealtime or with an acidic beverage, such as orange or tomato juice.
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