DEAR DR. BLONZ: Should I be concerned that there is a great difference in the fishy odor level in the cans of tuna that I open? I'll typically have several cans of solid albacore from the same major brand, but when I open one, it's very smelly, while another identical can will have a very mild odor. Recently I considered throwing out the contents since the fishy odor was so pronounced. Any thoughts on this issue? Any safety concerns? -- F.M., via email

DEAR F.M.: "Solid" types of tuna come from blocks cut from different parts of the fish muscle. They often rely on larger fish for the solid tuna, as the muscle mass will be larger, but this also means you are getting your tuna from an older fish. All, of course, depends on the quality control of the manufacturing process, which includes the tolerances set for what gets included as solid or chunk tuna.

Other possible issues are how the particular cans were processed and stored. If the tuna spent more time in a warmer environment between its time in the ocean and being opened in your home, this could increase the tendency to develop "off" odors that presage the actual breakdown of the tissue and the spoilage process. Fish, especially cold-water fish, have this tendency by virtue of their content of certain fats and amino acids that break down to malodourous substances. All goes well if they remain chilled, but enough time away from the cold, and you have an insult to the senses.

I am of the mind that whenever you crack a can of anything and it has an "off" smell, you should not eat it. You have the option of contacting customer support at the manufacturer and telling them your story (there should be contact information on the tin). Likely they will also tell you to toss the stuff (they might ask for the lot identifiers first), and I would predict they will send you some coupons for a replacement product. Another option is to take it back to the store where you bought it and get your replacement can that way.

As long as we are talking about tuna, you should be alert to the issue of mercury. Albacore tends to have higher levels of mercury than chunk light, which usually comes from skipjack -- a smaller-sized, shorter-lifespan breed. Tuna tend to bio-accumulate mercury and the older the tuna, the greater the risk.

The main reason for this is that albacore tend to live longer and are moderately large among the breeds used for canned tuna. As such, bigger albacore caught for large commercial canners tend to come from deep waters and will have a higher risk of mercury. There are brands, such as Wild Planet (wildplanetfoods.com), that catch younger, smaller tuna. Their fishing methods include pole-and-line or trolling, and those result in fish with a lower risk of mercury. You can do some additional research at tinyurl.com/d464hdm before you buy.

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: I am interested in your thoughts on pesticides and organic fruits and vegetables. What are the dangers of foods grown using pesticides, and what is your view of the benefits of the alternatives (such as buying organic)? -- N.T., San Diego

DEAR N.T.: Health experts consider the hazards from pesticides to be well behind other dangers in our food supply, such as bacterial contamination and naturally occurring toxins. However, pesticides still do pose a challenge. One alternative is organic agriculture, where foods are raised without synthetic chemicals, such as pesticides, herbicides or fungicides. Another, called integrated pest management (IPM), limits the use of synthetic chemicals whenever possible, although they remain a part of the farmer's arsenal to be called upon when needed.

Farmers' markets can be a great source for organically grown fruits and vegetables or those produced on farms that practice IPM. There are an increasing number of supermarkets that offer these foods as well.

The discussion of organic vs. conventional agriculture needs to go beyond the safety of the foods we eat. Such concerns must also include the risk to workers who manufacture, transport and apply these powerful chemicals, and to our environment. All this helps to explain why, despite assurances, survey after survey reveals that American consumers continue to be wary of pesticides.

We are the best-fed country in the world, but this status comes with a tremendous price tag in natural and human resources. Billions of pounds of commercial fertilizers are used annually, and the use of pesticides made from petroleum to control weeds, insects, diseases and other pests helps explain why farming uses up more oil than any other single industry.

America's preoccupation with perfect-looking produce is a factor behind the continued demand for pesticides. A 1998 report out of the National Academy of Sciences titled "Alternative Agriculture" detailed how the food industry encourages the use of pesticides solely to maintain high cosmetic standards. A survey conducted on citrus fruits by Public Voice and another by the American Farm Bureau Federation found that, in some cases, over half the pesticides used are for purely cosmetic reasons, such as to prevent minor external blemishes that had nothing to do with the taste or wholesomeness of the fruit.

To eat, or not to eat, should never be the question. No one wants to impair our ability to feed the nation. The focus should be on the direction agriculture should be heading. We all need to balance human needs with costs and environmental consequences. Agriculture cannot continue to operate using environmental "deficit spending." Today's output should not mean a loss of tomorrow's resources. There needs to be a course that guides our agricultural environment back to health without impairing our current capabilities. Ideally it would be a balanced, sustainable system that gives back as it takes.

Information is power, so I encourage additional reading. The EPA has a page on pesticides at epa.gov/safepestcontrol. I also encourage you to check out the FAQs at the nonprofit Organic Farming Research Foundation: ofrf.org/organic-faqs.

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: 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.