Sunscreen and insect repellent both have expiration dates

Hello, dear readers, and welcome to our first letters column of the summer. We’ve had a lot of mail -- thank you! -- so we’re diving right in.

-- Let’s start with a question regarding two essential tools of outdoor summer living -- sunscreen and insect repellent. A number of you have written to ask how long these products remain effective. It’s true that most don’t include an expiration date on the packaging. However, the Food and Drug Administration requires sunscreen to maintain its original efficacy for at least three years. (Of course, that means you’ll have to remember when you made your purchase.) Most manufacturers say their insect repellents remain effective for two to three years. If you need a more precise answer, you can call or email the manufacturer’s customer service department and provide them with the lot number of the product you’re asking about.

-- A column about the unhealthful nature of processed foods puzzled a reader from Iowa. “Can breakfast cereal, which is extruded, molded and milled, be considered a healthy processed food?” she asked. The answer is that it depends on the specific cereal. Those that are made from whole grains, have little-to-no added sugar and salt, and are high in fiber can be considered healthy-ish. (Emphasis on the “ish.”) Eat them with a serving of fresh fruit, watch portion size and don’t dip into the sugar bowl. Cereals fortified with vitamins, minerals and iron can contribute to daily nutritional goals.

-- A reader in Virginia thought we left out an important detail in our discussion of preventing deep vein thrombosis, which is the formation of blood clots, on long plane flights. “Never once did you mention wearing compression socks on the flight,” he wrote. “Doesn’t that help prevent DVT?” You’re correct that wearing compression socks or stockings can help reduce the risk of developing deep vein thrombosis. According to the National Institutes of Health, about 2 out of every 10,000 passengers develops DVT on a long flight. For those at high risk, knee-high compression stockings with compression strength between 15 and 30 mmhg are a good idea.

-- We received a lot of mail in response to a column about the importance of vitamin B12, an essential vitamin the body needs but doesn’t produce. The answers to your varied questions are: B12 is manufactured by bacteria living in the guts of a variety of mammals, fish and poultry. It’s available in animal products, including fish, meat, poultry, eggs, milk and milk products. It’s also available as a supplement, both overthe-counter and through prescription. OTC products, although not regulated by the FDA, are considered to be a reliable source of the vitamin. Studies have found no advantage of sublingual (under the tongue) B12 over tablet form. Most B12 supplements exceed the recommended dietary allowances (RDA) for adults of about 2 to 3 micrograms per day. However, the body absorbs only as much as it needs, and any excess of the water-soluble vitamin is excreted in the urine.

Thank you to everyone who wrote with kind words. We’re thrilled that you continue to find this column both interesting and useful. We look forward to seeing you again in this forum next month.

Eve Glazier, M.D., MBA, is an internist and associate professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and assistant professor of medicine at UCLA Health.