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.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 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 there more vitamin A in dark-colored vegetables than lighter-colored vegetables? There is a debate in our class. -- M.H., San Jose, Calif.

DEAR M.H.: There is no pre-formed vitamin A in fruits and vegetables; rather, plant foods have compounds that the human body can convert to vitamin A as needed. The key players are the carotenoids, a group of colorful compounds that range from yellow to deep red. Any food's vitamin A potential depends on the carotenoid present, as not all can end up as vitamin A. For example, lycopene (red), lutein and zeaxanthin (yellow-orange) do not get changed into vitamin A.

Let's look at a group of vegetables to see how much vitamin A they provide per serving, first on a weight basis (vitamin A per unit weight), then on a calorie basis (vitamin A per calorie). Our eight representatives are all examples of healthful foods and excellent dietary sources of carotenoids. They are: fresh spinach, basil, kale, carrots, and leaf lettuce; baked sweet potato; canned pumpkin; and dried pasilla peppers.

Taking portions of identical weights, they rate, from highest to lowest: dried pasilla peppers, sweet potato, carrot, pumpkin, kale, spinach, lettuce, basil.

Viewing this same list on vitamin A per calorie, the highest to lowest are: lettuce, pumpkin, carrots, spinach, kale, sweet potato, basil, dried pasilla peppers.

In both cases, there is a slight straying from any conclusion that dark equals more vitamin A. Perhaps the issue is that your discussion was limited to one known nutrient. Eating foods rich in natural colors is a great goal, and their presence tends to signify a richness of natural, healthful phytochemical compounds.

You can get additional information on the nutrient content of foods by visiting the USDA database at ndb.nal.usda.gov.

DEAR DR. BLONZ: I just read an article about potatoes, and I am a bit concerned. I just bought a bag of organic Russet potatoes, and they sprouted very quickly. I have been pulling the sprouts off, and the potatoes don't appear terribly green. Are these still harmful to consume? I would not prefer to waste five pounds of organic potatoes! -- K.S., via email

DEAR K.S.: You might consider returning them, especially considering that you "just" bought them. But if there is no green on the potatoes, and they are still firm, you can remove the sprouts and a bit of the sprout base and it should be fine. One issue with the alkaloids present in green potatoes (solanine) is the dose, so as long as they are only for a side dish, there should be no problem. Potatoes should be stored in a cool, dark place and you shouldn't get more than you would expect to eat in a week or so. One issue is that you don't know how long the potatoes were at the store before you bought them. Always give produce a good inspection before you put it into your cart.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 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: We recently had a close call with botulism and are lucky enough to be able to tell the tale. I have learned about its causes, symptoms and progress but have one unanswered question: How long is it safe to leave covered food in the refrigerator? I tend to cook one day a week, preparing soups and vegetables for later in the week. They are covered and refrigerated until it's time to eat them. Would it make a difference if they were uncovered? What other precautions should we take? Also, what risk is there to our dog from botulism? -- W.O., Albany, N.Y.

DEAR W.O.: First, some background for readers who are not as familiar with botulism as you now are. Botulism is a very severe type of food poisoning -- much worse than the flu or E. coli that we hear of periodically in the news. An amount of botulism toxin the size of a single crystal of salt can kill several people in less than hour.

To appreciate how botulism works, it helps to understand its three facets: the bacteria, its spores and the deadly toxin. The bacteria's name is Clostridium botulinum, it reproduces by giving off spores, and these spores are what produce the deadly toxin. Botulism is caused when an individual has the misfortune of ingesting foods contaminated by the toxin.

The bacteria are found everywhere and are usually present in soil. What is particularly insidious about these bacteria is that the spores can remain in a vegetative state for extended periods until they are either destroyed, or until conditions become right for them to grow.

The botulinum bacteria can grow in low-acid, oxygen-free environments, and they tend to be a particular problem with improperly processed canned foods, such as vegetables, fish and meat. One saving grace is that the toxin is not heat-stable. Food preservation techniques designed to control these bacteria must include a heat treatment. The botulism toxin can be inactivated when exposed to a temperature of 180 degree F for 10 minutes or longer. Eliminating the Clostridium botulinum bacteria entirely takes exposure to 212 degrees F for 10 minutes or longer. The spores are the most heat-tolerant, requiring exposure to 240 degrees F to destroy them.

The botulinum bacteria are also inhibited by an acidic environment, which is why acidic foods such as tomatoes, citrus and pickled foods tend to be safe. The bacteria are also thwarted by nitrite preservatives, which is why sodium nitrite is added to cured meat products.

In your situation, it becomes essential to follow good kitchen hygiene, scrubbing all foods before use and avoiding any cross-contamination with uncooked foods. Prepared foods should be immediately refrigerated (under 38 degrees F) in covered containers and used within three to five days.

Relying on a once-a-week cooking system is pushing it. You might also consider storing the later-in-the-week portion of your vegetables in the freezer, as toxin production from botulinum bacteria is inhibited at freezer temperatures (0 degrees F). As for your dog: Dogs are less at risk due to their more acidic digestive system. This allows them to drink, eat and lick items that are a bit less than sanitary.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 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.