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.

DEAR DR. BLONZ: One of my resolutions is to cut down on caffeine from all sources, but I'm finding that labels do not spell out how much caffeine is actually there. There are plenty of products claiming to be "99 percent caffeine-free." Does this mean the product still contains 1 percent of its caffeine, or that 99 percent of the ingredients listed are caffeine-free, or what? If the product had a bunch of caffeine and 99 percent was removed, doesn't this mean that there may still be enough to give a caffeine-sensitive person a minor jolt? -- N.O., Boston

DEAR N.O.: Let's go down your list of concerns to clear up the confusion. The "percent caffeine-free" claim relates to the caffeine in a product as a whole. Given the sensitivity of analytical devices, few ingredient-removal processes can claim to be 100 percent effective, and the decaffeination process is no exception. This means that irrespective of how much caffeine was originally present, there will always be a very small amount of it in a decaffeinated product.

A 6-ounce cup of coffee normally contains between 100 and 200 milligrams of caffeine (the finer the grind and longer the brew, the higher the number). For decaffeinated coffee, the amount drops to about 2 milligrams of caffeine -- hardly enough to cause even a minor case of the jitters.

Caffeine-free cola drinks make their claim because there was never any caffeine to start with. The caffeine gets added to the regular beverage, and is simply omitted for the caffeine-free version.

The ingredient statement on a food label will list caffeine whenever it is added to a product. This means you'll find it on caffeine-containing soft drinks, but it won't appear on the label for a product such as chocolate-covered espresso beans, where the caffeine is a natural component of the coffee bean. There is no requirement to state how much caffeine is present in a food; to get this information, you will have to consult the manufacturer or check the USDA Nutrient Database (ndb.nal.usda.gov), which lists the caffeine content for many foods.

DEAR DR. BLONZ: I am a vegetarian and would like some nutritional information on dates and raisins. Are they a good source of iron, or is it best to take a supplement? -- S.T., Chicago

DEAR S.T.: Dates and raisins do contain some iron, with a half cup containing 1 milligram and 1.5 milligrams of iron, respectively. If you enjoy dried fruits, you might consider figs, which contain just over 2 milligrams of iron per half cup. Other vegetarian sources of iron include dark green leafy vegetables, lentils and other legumes, blackstrap molasses, iron-fortified cereals and enriched breads. Assuming there are no health problems or absorption issues that would create a need to take a larger amount, a vegetarian should have no problem satisfying his or her iron requirement using foods alone. The National Institutes of Health has more information on iron in the diet at tinyurl.com/cksla.

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.