You've heard for years that the French and Japanese are much thinner than Americans because their diets are so much better than ours. A new mathematical model assesses why that is and how much thinner Americans could be if they changed their eating habits.

According to the study, conducted by agricultural economists at the University of Nebraska-Lincoln, Americans could cut 2.57 points off their average Body Mass Index score by adopting a Greek (Mediterranean) diet, 2.13 points by eating like Finns, 1.96 by adopting a French diet and 1.48 by eating like the Japanese.

That's a lot of weight. The study listed the average BMI in the United States at 28.45 in 2009. Finland was at 26.25, Greece at 25.75 and France at 25.35. All of those qualify as overweight (25 to 29.9), though not obese, which is a BMI of 30 or more. Only the Japanese, at 22.7, had an average BMI in the healthy range.

The scenarios assume a daily diet of 3,688 calories, which the researchers acknowledge is probably a lot more than the average person consumes, even in the United States. It's the average number of calories per capita produced by the United States each day, according to the Food and Agriculture Organization of the United Nations.

Lighten up with plant-based diets

So if you ate the same number of calories as you do now, but did it the way the French, Japanese, Finns or Greeks do, why would you lose weight? It's largely because their diets are more plant-based, said Azzeddine Azzam, a professor of agricultural economics who conducted the research with Sarah Rehkamp, a graduate student who now works for the U.S. Department of Agriculture.

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"What we're finding is that the composition of plant-based vs. animal-based [diets] is correlated with BMI," Azzam said in an interview. Switching to the Japanese diet, for example, would mean eating 82 fewer calories from beef and 273 fewer calories from dairy, but 22 more calories from eggs and 115 more calories from seafood per day, he said.

Rehkamp said the mathematical model she constructed also "may be capturing other lifestyle factors associated with each diet, such as physical activity or eating habits in the country."

Azzam and Rehkamp next looked at what would happen if Americans adopted those alternative diets and reduced their calorie intake to the average levels of those countries. Not surprisingly, the gain would be even greater. Under those conditions, the Japanese diet would slice 3.05 points off the average American BMI. The Finnish diet would cut 2.78, the Greek 2.6 and the French 2.19.

Those big drops in BMI would save Americans health care dollars, the researchers figured. For example, adopting the composition and calorie totals of the Japanese diet would save the average American $617.36 per year.

A diet's environmental impact

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Next, they looked at each diet's impact on the environment, converting the carbon dioxide output of each one to various equivalents. Here there was something of a surprise: The U.S. diet was actually better than the French and Finnish diets. Using the U.S. Interagency Working Group's 2010 estimate that the social cost of carbon dioxide emissions was $21 per ton, they calculated the greenhouse gas cost of each diet.

If each culture consumed those 3,688 calories, the U.S. diet would cost $8.62 per person each year. The Greeks are best at $8.14 and the Finns worst at $10.88. Under the scenario that takes both calories and diet composition into account, the Japanese are by far the easiest on the environment, at $6.10 per person per year, and the French the worst, at $9.73.

Azzam speculated that the French and Finns rely more heavily on beef when they do eat meat products, while Americans are bigger consumers of poultry. Chickens and other poultry are much more energy efficient to raise than cattle, he said.