DEAR DR. BLONZ: There is a history of macular degeneration in my family, and I understand that lutein and zeaxanthin are nutrients needed to slow its progression. What is your view on supplements as a preventive? My father and both of his siblings were diagnosed with this condition by age 75. -- M.F., Charlotte, North Carolina

DEAR M.F.: The macula is the part of the eye where the images are focused. At the center of the macula is the "fovea centralis," the point of the eye where our vision is at its best. It is the macula where the electrical impulses that correspond to what we are looking at are initiated. Milliseconds later, after the signals have made their way to the brain via the optic nerve and have been interpreted, we understand what we are seeing.

An unfortunate condition associated with aging is a breakdown of these tissues. Macular degeneration is the leading cause of blindness in older Americans. The precise cause of macular degeneration is not yet known, but the retina -- and the lens -- are thought to be susceptible to oxidative damage.

Antioxidants, including zinc, vitamins C, E and A and selenium, all have reactions that prevent unwanted oxidation. However, the key is to have antioxidants that distribute to all vulnerable tissues, which points to the need to maintain a diet that includes a variety of healthful, whole foods that provide a range of compounds to protect all tissues.

The carotenoids, particularly the antioxidant compounds lutein and zeaxanthin, are known to concentrate in the macula. In fact, they are the only carotenoids known to be in the retina's macular region, and are referred to as macular pigments.

Spinach and corn are good sources of lutein and zeaxanthin. A study in the journal Investigative Ophthalmology and Visual Science from August 1997 reported how increased intake of these foods effectively increased blood levels of the key macular pigment compounds. A study in the same journal in October 2000 reported increased macular pigment levels after 12 weeks of supplemental lutein, or 10 mg per day. In the June 2000 issue of the American Journal of Clinical Nutrition, another study reported how servings of corn and spinach effectively raised blood levels of these macular pigments.

Other good food sources of lutein and zeaxanthin include kale, collard greens, cress leaf, Swiss chard, chicory leaf, mustard greens, beet greens, red pepper, okra, endive, celery, romaine lettuce, leek, broccoli, leaf lettuce, green peas, pumpkin, iceberg lettuce, Brussels sprouts, yellow pepper and green beans. Fruits contain lesser amounts, but some of the best include avocado, plums, kiwis, pears and grapes.

Supplements are a reasonable option, but they do not replace the need to eat healthful foods. When taken as a supplement, lutein is best absorbed when taken with meals that contain fat. Be aware that while many multipurpose supplements claim to contain lutein (or zeaxanthin), the amounts can vary. Always check the label and compare it with the 10 mg per day used in the study cited above.

Read more about macular degeneration at b.link/xu9pj8j, which includes a section on treatments and therapies.

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: I would like your help to settle an argument at a recent dinner. There seems to be such a push to restrict salt consumption in general, but the only hazard I am aware of is that salt can raise blood pressure in some people. Is there any other risk associated with salt? -- H.T., via email

DEAR H.T.: The main issue with excess salt intake is, as you mention, its association with high blood pressure (hypertension). But this is nothing to be dismissive about. High blood pressure currently affects roughly half of adults. It's called "the silent killer" because there are no warning signs until problems such as heart disease, stroke or kidney disease have already developed. The only reliable way to determine whether you have hypertension is to check your blood pressure regularly.

Salt and sodium are not automatically antagonistic for everyone, even those with high blood pressure. But there is no health benefit from excessive salt -- only liability. For those at high or uncertain risk about their blood pressure, it is best to keep salt to a minimum. Another point to consider is that one's sensitivity to salt, and ability to consume it safely, can change over time.

Finally, saltiness is one of our basic tastes. Salt helps to bring out the flavors in food. But the point should be to enjoy the natural flavor of foods, rather than that which comes from the salt shaker. Consider it a warning sign if you routinely reach for the shaker before you taste the food. (See b.link/2qkdb4 for more on high blood pressure.)

DEAR DR. BLONZ: I work with second- and third-graders as a student teacher. We would like to know: How do scientists determine the levels of protein and other nutrients in fruits, vegetables, meats or processed foods that contain them all? And where can I take my students to show them scientists analyzing foods? -- T.T., Chicago

DEAR T.T.: To analyze foods, representative samples are put through a series of laboratory tests that measure the amounts of macronutrients -- such as protein, fat and carbohydrate -- and micronutrients in the item. ("Macro" means larger amounts of the nutrient, whereas "micro" means smaller amounts. Read more at b.link/33chvc.) Each nutrient has its own test. Independent laboratories around the country perform these analyses, although food companies often have their own labs, as well.

You should check in your area for food manufacturers with in-house labs that offer tours. Consider creating a list of options and allowing the students to vote for their selection. I grew up in the Chicago area, and a trip like that was one of my most memorable class outings. I predict you will find companies willing to open their doors to your students.

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: Since the pandemic, I haven't been able to find good old-fashioned bagged barley in any grocery store. This is a product that arguably has been around since the early days of man -- now, not available. This doesn't make sense. My dad said I should ask you. Where did barley go? -- J.R.

DEAR J.R.: You have my support for the use of barley, a healthful cereal grain in the grass family that was one of the first cultivated grains. Barley has many benefits, including about three times the fiber of oats. (Read more on the benefits of barley at b.link/swvzy4ky.) Note that cereal grains -- a category that includes wheat, rye, triticale (a wheat/rye hybrid) and barley -- contain gluten, so those with a gluten disorder should avoid them.

Barley can still be found through online sources, but there has, indeed, been a decrease in its availability. While barley is a hardy crop, it is not invulnerable to climate change. Excessive heat or cold can affect plants at their flowering stage, which impacts grain yield and quality. Current shifts in temperature, erratic rainfalls and unpredictable weather conditions have had an impact in crop yields. U.S. agriculture, led by Idaho, Montana and North Dakota, produces enough for our needs. However, the war in Ukraine has had an impact, as Russia is the largest producer of barley, and Ukraine is the third-largest exporter. There may be a trickle-down effect on worldwide barley production, resulting in a drain on availability from our domestic crop.

Keep looking, and you can find it. And thanks to your father for the referral.

DEAR DR. BLONZ: I read that rhubarb contains large amounts of oxalate, which is a natural toxin. Should I be concerned about rhubarb pie, which is a holiday favorite at our home? -- M.M., Chicago

DEAR M.M.: Oxalate is found in rhubarb, spinach, soy and other plants. Oxalate can form an insoluble mineral salt with mineral elements, such as iron and calcium, making them unavailable for absorption. For this, oxalate it is often referred to as an "anti-nutrient."

Consumption of healthful plant foods that contain oxalate can mean less of these minerals will be absorbed. This is not necessarily an event of clinical significance, as the impact is based on the amount of oxalate present, and cooking and digestion can break it down. (Some individuals are more sensitive to oxalate than others.)

Back to your question. Since the main source of the oxalate in rhubarb is in its leaves, and not the stalks, your family favorite should not pose a problem. (For more on issues relating to oxalates, see b.link/jkbw848.)

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.