The University of Washington School of Public Health researchers have been peeking into kitchens - via interviews - for years now.
They've just published results showing people who cook at home more often are likely to eat a healthier overall diet.
"By cooking more often at home, you have a better diet at no significant cost increase, while if you go out more, you have a less healthy diet at a higher cost," said Adam Drewnowski, director of the UW's Center for Public Health Nutrition and senior author of "Cooking at home: A strategy to comply with U.S. dietary guidelines at no extra cost," published in the American Journal of Preventive Medicine.
As part of the Seattle Obesity Study, researchers interviewed 437 King County adults, who were asked to remember their last week of eating in and eating out. Researchers supervised the adults answering a questionnaire, with detailed sections on what they ate and where.
The study found that home-cooked dinners were associated with a "greater dietary compliance," meaning the overall weekly diet met more of the federal guidelines for a healthy diet. Households who cooked at home about three times per week showed a score of about 67 on the Healthy Eating Index. Those who cooked at home about six times per week had a score of about 74.
"The differences were significant, even with a relatively small study sample," said Drewnowski, also a professor of epidemiology.
Don't feel bad if you don't have time to cook at home every night. Researchers understand. Drewnowski realizes that some people in the United States suffer from what epidemiologists call "time poverty."
Roughly half of all food dollars in the United States are spent outside the home, which suggests that cooking at home may not be feasible for a large chunk of the population.
Public health nutritionists suggests that efforts to promote cooking at home should be balanced with efforts to encourage retailers and restaurants to offer healthy, less expensive prepared foods for easy purchase outside of the home.
Other measures of food consumption use calories instead of dollars. The contribution of away-from-home food to total calories rose from 18 percent in the 1970s to 32 percent by the late 1990s, according to the study. Only one in five U.S. residents meets the dietary guidelines set by the U.S. Dept. of Agriculture.
What was surprising to Drewnowski was that the study showed there was no increase in costs for eating a healthier diet. Home cooked meals were associated with diets lower in calories, sugar and fat, but not with higher monthly expenses for food.
One other message of the study was that some common assumptions are wrong about income and education. The study showed no association between income or education and eating at home or eating out. The 437 people chosen for the study were a stratified random sample.
"People have the preconception that a lower income leads to eating more fast foods, but that was not true in our study," Drewnowski said. People who cooked more often at home were likely to have larger households with more children in them.
One of the limitations on the study was that people had to remember everything they ate in the past week, and perhaps some memories are not perfect. But he pointed out that almost all nutritional research is done using self-reported information.
The message of healthier-doesn't-cost-more is one he is sharing immediately with students in his Nutrition 303 class, where one of the assignments is to carefully pencil out how much your typical dinner costs.
"There's a lot of ramen," he joked, in regard to homework papers.