Portion distortion: Cook by the book, and you may feed your family too much food in one sitting.

RODALE NEWS, EMMAUS, PA—The expansion of American waistlines has paralleled the changes in a popular cookbook’s recipes over the last 70 years, according to a recent study.

THE DETAILS: Joy of Cooking (Scribner) has been updated nearly every decade since the 1930s, giving researchers a way to measure calories and serving sizes for the last 70 years of published recipes. “The calories have gone up dramatically, 45 percent across most of these recipes,” says Brian Wansink, PhD, author of Mindless Eating: Why We Eat More Than We Think (Bantham) and Prevention.com blogger. He reported the trend in the medical journal Annals of Internal Medicine.

Wansink says that two-thirds of the recipe-fattening is due to increased use of higher-calorie ingredients, such as butter and oil. Of the 18 recipes that have been in all seven editions of the cookbook, the average calories increased by nearly 930 calories, from 2123 to 3051 calories—that’s a 43 percent increase. The rest can be attributed to increased serving sizes, which jumped 33 percent since 1996, as smaller families dine on the recipes. “Typically, these recipes were designed for families of 7 or 8, and now we have families of 3 or 4,” explains Wansink.

WHAT IT MEANS: We often blame restaurant portions for our big bellies, but this study shows that home cooking isn’t off the hook. Research shows that eating meals together at home has a positive effect on your whole family’s health, but keep an eye on serving sizes and calories while planning family dinners.

Here’s how to keep home meals healthy:

• Wrap it up. Don’t fry your brain doing the math to cut down the size of a recipe. Go ahead and make the whole thing, just don’t eat it all in one sitting when it’s clearly too much food for one meal. “Make the full recipe, and cut it in half and freeze the other half,” Wansink suggests. “Right when it comes out of the oven, wrap it up so you won’t be tempted to overeat, and you’ll have something good for next week.”

• Get a handle on meat. In the 1940s and '50s, people had less disposable income and were eating more potatoes and low-calorie vegetables, explains Wansink. But in the last few decades, we’ve loaded our plates with meat and heavy sauces. To save money in this economic downturn, have a vegetarian night a few times a week—it could shrink your waistline and fatten your wallet.

• Search another source. The Rodale Recipe Finder offers thousands of recipes that have been created with good health in mind. You can search by cuisine, dietary needs, health concerns, and other criteria.