Q: What is the difference between regular corn syrup and high-fructose corn syrup?

A: Both types of corn syrup start the same way: Cornstarch is treated with an acid or an enzyme, turning it into glucose syrup. To make regular corn syrup - the kind found in supermarkets and used by home bakers for candies and frostings - manufacturers clarify and reduce glucose syrup until it's the right consistency.

High-fructose corn syrup is also made from corn-derived glucose syrup, but it requires additional chemical reactions that convert some of the glucose into fructose, creating a sweetener that contains both.

You won't find high-fructose corn syrup on your grocer's shelf or in any recipes. It's used mainly in processed foods. In the 1970s, manufacturers began to replace table sugar, or sucrose, with the less expensive high-fructose corn syrup, which also works as a preservative. This means that the ingredient isn't limited to sweet treats such as soda, jam and candy. Many savory items, such as bread, crackers, tomato sauce, salad dressing and potato chips, list high-fructose corn syrup as an ingredient.

Some health experts link the rising rate of obesity and type 2 diabetes to the prevalence of high-fructose corn syrup in the American diet. Some even say that the way the body metabolizes one of the sugars it contains, fructose, leads to greater weight gain, among other health issues.

As a result, high-fructose corn syrup has a reputation for being even more harmful than table sugar. In fact, both contain similar amounts of fructose and glucose, and the way the body processes high-fructose corn syrup and sucrose is nearly identical.

The American Medical Association has said that high-fructose corn syrup doesn't appear to contribute more to obesity than other caloric sweeteners, but that additional independent research on its long-term health effects is needed.

Of course, too much sugar in any form adds empty calories to your diet and sets the stage for potential health problems.

Q: How can I use fans to cool my house? I want to rely less on air conditioning.

A: Start by keeping as much heat out of your home as possible: Draw the curtains and shut the windows during the hottest time of day, and reduce the use of appliances, which radiate heat.

When temperatures drop in the evening, fans can blow a cooling breeze and increase ventilation. Tabletop and freestanding fans create a gentle wind wherever you place them. In a room with windows along only one wall, put a fan on the opposite side of the room from the open windows to form a crosscurrent.

Ceiling fans can make a room feel up to 4 degrees cooler, so you can cut down on your use of the air conditioner. For a room with fewer than 225 square feet, the U.S. Department of Energy recommends a fan with a diameter of 36 to 44 inches; bigger spaces call for fans that are at least 52 inches. Look for models that meet the DOE's Energy Star standards.

You can put in a more efficient cooling system by installing a whole-house fan, which can use up to 90 percent less energy than air conditioning. These large fans, which cost between $1,000 and $2,000, including installation (some utility companies offer rebates on both purchase and installation), are permanently located in the ceiling of the top floor or in an attic window. They remove hot, stuffy air and pull fresh air in through open windows. When outdoor temperatures are lower than those indoors, this system will cool the house. It works especially well in dry climates where nights are much cooler than days.

To get fans - or any other type of heating or cooling system - working as efficiently as possible, make more substantial changes: Insulate your home, and seal its doors and windows; the next time your roof needs replacing, consider putting in a reflective one.

Questions should be addressed to Ask Martha, care of Letters Department, Martha Stewart Living, 601 W. 26th St., 9th floor, New York, N.Y. 10001. Questions may also be sent by email to: mslletters@marthastewart.com. Please include your name, address and daytime telephone number.