DEAR DR. BLONZ: I read an article saying that a large consumption of spinach can cause kidney stones. This is one of my favorite vegetables, so I want to know if this is something I need to be concerned about. -- S.T., Fremont, California

DEAR S.T.: Spinach is one of my favorite foods, as well. I am not sure what the article meant by a “large consumption” of this healthful food, but here is some information to put the issue in perspective.

Spinach, along with a number of other leafy greens and plant foods, contains varying levels of oxalic acid, a naturally occurring organic compound produced by these plants. There are also mineral elements in these plants, and when oxalic acid reacts with one of these, such as calcium, it becomes calcium oxalate -- an insoluble salt. There are different types of kidney stones (also called urinary calculi), but calcium oxalate is the predominant composition of these solid particles. The reason calcium oxalate stones are such a menace when they form in the kidney is that they block the ducts involved with the production and elimination of urine.

With spinach, we have a vegetable that does indeed contain oxalic acid, and it also contains calcium. One half-cup of spinach contains about 100 milligrams of calcium, but spinach is not really considered to be a good source of dietary calcium. This is because substances need to be in a soluble form to pass through the absorptive surface of the intestines. When eaten, the calcium in spinach is likely to get tied up as calcium oxalate, which means it is unlikely to get absorbed.

Compare this to kale, another leafy green. Kale contains calcium, but it only contains a fraction of the oxalic acid found in spinach. Thus, kale is considered a good source of calcium.

Spinach does have a bit more oxalic acid than calcium, so some of the oxalic acid excess can be absorbed. It is important to appreciate that oxalic acid is present in many foods, and it does not cause kidney stones to develop in an otherwise healthy individual. If, however, your body has displayed any tendency to form oxalate kidney stones, spinach would be on a “foods to avoid” list.

So, how to proceed? Assuming you are in good health, that there is no history of kidney stones in your family, and that your physician has expressed no concerns that you have a tendency to develop kidney stones, it seems dubious to avoid spinach on the mere chance that you “might” become a stone former.

For more information about kidney stones, see tinyurl.com/y7becp5d.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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: My doctor says I need more fiber in my diet, and recommends a fiber supplement. But it’s expensive, and I’m on a limited retiree budget. The affordable option (I hope) is a combination of oat bran (a soluble fiber) and wheat bran (an insoluble fiber), mixed into whatever else I eat. I have read that both types of fiber are needed. Is that true? And if so, how much of each should I consume daily? Should another kind of fiber, like psyllium seed husks -- which are the basis of the dietary supplements -- be added to the mix? If so, how much? -- F.S., Casa Grande, Arizona

DEAR F.S.: At present, the typical American diet contains about 12 to 15 grams of dietary fiber per day. We should double that, bringing our intake up to 25 to 30 grams per day. Do some checking to see where you stand.

Sources of both soluble and insoluble fiber should be part of your diet, yes; if your diet contains fruits, vegetables and grains, you have this covered.

Taking a fiber supplement may work for constipation, but why strain your budget with supplements when you can get your supply naturally, from nutrient-rich whole foods? No better way to start the day than with a high-fiber, whole-grain cereal with added dried fruits, such as raisins or dried berries.

Read more on fiber at tinyurl.com/ycje978z.

DEAR DR. BLONZ: Is it necessary to use extra-virgin olive oil to get all the health benefits? We found that the flavor of the extra-virgin oil we were getting was often too strong or bitter for our taste, so we switched to ordinary olive oil. What are we missing health-wise, if anything, by not using extra-virgin oil? -- J.N., Hayward, California

DEAR J.N.: “Extra-virgin” signifies the least processed of the oils extracted from the olive. This type of oil contains the highest amount of phytochemicals, which protect the oil -- a vital energy source for the olive seed -- and can help us, as well. But it is important to understand that there are wide varieties of flavors in extra-virgin oils; they vary according to the types of olives, and where in the world they are from, and how they have been grown, harvested and stored.

This being said, extra-virgin will always be the most flavor-intense oil from any variety or batch. Olive characteristics do vary, so I encourage you to visit a store where they do olive oil tastings. Another option is to look online for stores that offer tasting notes for their various oils. You will find more options than you might have imagined.

Bottom line, though, is that irrespective of whether extra-virgin oil has more health assets, it makes no sense to use it if the flavors are not enjoyed.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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: If I were to take herring that is sold in a wine sauce and rinse it in water to remove the salt, does it lose any of the healthful omega-3 fatty acids? I enjoy eating herring, and want the fats, but prefer not to have the salt. -- M.M., Washington, D.C.

DEAR M.M.: Herring are a rich source of omega-3 fatty acids, and these fats will still be there after a salt-removal swim.

The level of omega-3 fats in herring is impressive: A study in an issue of the Journal of Agriculture and Food Chemistry reported that the sought-after EPA and DHA omega-3s represent more than 12 percent of the fatty acids in herring. Pacific herring were found to have slightly higher levels than those swimming in the Atlantic, but both types were higher than the level of essential omega-3s in sardines, salmon, tuna and trout. Read more about the different omega-3 fats at tinyurl.com/y9vd9mbo.

DEAR DR. BLONZ: Is it safe to put raw chicken in a slow cooker and cook it on low for eight hours or so? I am very hesitant and afraid it will be full of bad bacteria. -- S.G., San Diego

DEAR S.G.: A properly functioning slow cooker, or Crock-Pot, will cook foods between 170 and 280 degrees, which is sufficient to bake bacteria out of existence. The meat should be fully defrosted before it goes into the cooker. Fill the cooker at least halfway, but no more than two-thirds full. Cut the larger pieces of chicken into small chunks to ensure thorough cooking.

Finally, steam helps the process, so keep the lid in place. If you are going to be around, cook on high for the first hour, then lower the temperature to that called for in the recipe.

DEAR DR. BLONZ: In a recent column, you equated lactose and lactate. This is wrong! Lactose is a disaccharide. Lactate is the ionized form of lactic acid, a three-carbon organic acid. Lactic acid is a fermentation product of sugars, including lactose, and is found in sour milk, yogurt, pickles, saurkraut, etc. It is used as a preservative in many foods. It’s not going to contribute to symptoms of lactose intolerance. -- S.R., via email

DEAR S.R.: You are correct, and thanks for pointing out that error. “Lactates,” used as additives in certain foods, are completely different from lactose, the carbohydrate found in dairy products that can cause problems for lactose-intolerant individuals. Thanks for the careful read.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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.