DEAR DR. BLONZ >> I hear cautions about cooking oils and their smoke points. What happens if an oil is overheated during oil frying compared to oven roasting? I hear that canola is worse than others.

— F.S.

DEAR F.S. >> Oil frying is similar to hot-air oven roasting in that it involves a transfer of heat to the food. A main difference between cooking with oil and roasting food in an oven is that the oil can interact with the food and lend tastes and textures. However, unlike the heated air in an oven, oils can break down and form byproducts when exposed to excessive heat, including vitamin destroyers, stomach irritants, enzyme inhibitors, mutagens and lipid oxidation products. These are not items you want on your menu or in your body.

Breakdown can happen with any oil. The issue here is the concept of “excessive heat.” One variable is an oil’s smoke point, a temperature at which heat energy begins to break down the oil’s structure, and it can begin to smoke. This is not the same as smoke from breading or skin falling off the food. This is a bluish-grey, acrid-smelling smoke from burning oil structure fragments. Food cooked in such oil tastes bad, but more importantly, it’s unhealthy to eat.

Smoke points vary among oils; having a higher smoke point reflects an oil more suitable for higher-heat cooking techniques. The smoke point can vary within the same type of oil. A highly refined version tends to be associated with a higher smoke point, while an unrefined/unfiltered version tends to smoke at lower temperatures. Other factors that lower the smoke point include using oil past its shelf life, reusing oil, and debris, water, salts or spices moving from the food to the oil. Improper storage — exposure to air and light — will also hasten the breakdown rate and lower the smoke point.

The standard frying temperature is 375 degrees. Most oils designed for frying have a high smoke point— usually above 400 degrees. If an oil starts to foam or smoke at its normal cooking temperature, it’s a sign that it has spoiled in some way or has been in use too long. (Another factor to rule out is whether you have cleaned your cookware.) Check product labels for brand-specific information. .

DEAR DR. BLONZ >> I have always wondered how airport X-ray security machines affect the vitamins packed in my checked or carry-on luggage. Does it destroy their effectiveness?

— F.T., Tulsa, Oklahoma

DEAR F.T. >> The security machines in airports have a low level of radiation, not strong enough to affect your vitamin or mineral supplements or any medications you might have on hand or in your luggage. When taking medications and other substances on your travels, it is best to have them with their original packaging and labeling for reliable access and easy identification.

Kensington resident Ed Blonz has a PhD in nutrition from UC Davis. Email him at cctimes@blonz.com.