Apparently, if you're French or Japanese, chocolate croissants and vegetable tempura magically disappear from your waistline.

Less than a year after the publication of the best-selling book "French Women Don't Get Fat: The Secret of Eating for Pleasure," by Mireille Guiliano, comes a similarly titled book, "Japanese Women Don't Get Old or Fat: Secrets of My Mother's Tokyo Kitchen," by Naomi Moriyama — just in time for post-holiday diets.

Both books claim that if you eat a variety of fresh food, cook at home at least some of the time, pay attention to portions and eat only until you are almost full, you will lose weight.

But is this really the case?

Meg Corcoran, a registered dietitian and nutritionist in New York City, said both books have excellent advice.

"The French and Japanese diets include fish, veggies, fruit, whole grains, salads and high-fat foods in very low quantities," Corcoran said. "They eat very healthy oils and they walk a lot more than Americans. They use meat, cheese and butter in small increments like condiments — never for a meal — while Americans would have a steak. They eat in moderate proportions and small portions."

But Dr. Ruth Kava, director of the American Council on Science and Health Nutrition, is skeptical.

"You have to look at, is it true and why?" she said. "If they're not fat, what's their smoking rate? Exercise rate? It may have little to do with eating. It might be some genetic or environmental factor. It may be that their lifestyle is different."

The facts, however, back up these books' assertions: Japanese women have the lowest obesity rate in the industrialized world (3 percent for men and women) and the highest life expectancy (85 years). Eleven percent of French women and men are obese — as compared with 28 percent of American men and 34 percent of American women, Moriyama cites in her book.

But Kava says Americans won't get skinny just by doing as the French or Japanese do.

"Most of us won't take up either of their lifestyles," she said. "I think we have to adapt the American lifestyle to make it healthier. I don’t know if you can import just a piece of somebody else's lifestyle and expect it to cure your ails. You can say, 'What is it about this diet that can help me improve mine?'"

That said, both authors claim their diets are easily doable in America.

"It is very different than Western cooking — but once you get used to it, it takes the same amount of time and energy," Moriyama told FOXNews.com. "You already have a Tokyo kitchen — most ingredients you already have. And some essential ingredients, like benito flakes, are becoming more and more available in American supermarkets."

Moriyama also disputes the notion that the Japanese are genetically small.

"When I moved to America, I gained 25 pounds in two months — it's not in our genes. If I ate the wrong kind of food, I would gain again. When I went back to Japan, all I did was go back to my mother's Tokyo kitchen. I didn’t try to lose weight."

But Moriyama did lose weight — and she kept it off by importing her Japanese habits to New York City, where she now lives.

"It's about eating modest portions of different kinds of food on small, beautiful dishes," she said. "Eat slowly. Use canola oil — which has virtually no flavor — instead of other oil or animal fats, and enjoy the natural flavor of the ingredients. Eat brown rice, a good carb, rather than white bread. Drink Japanese tea instead of soda with no cream or sugar or any additive. Eat lots of fish and lots of veggies."

Guiliano — who also gained weight when she moved to America and lost it by reverting back to her French diet — says the advice in her book is much easier to follow here than the advice in Moriyama's book, as similar as the titles may be (Moriyama credits Guiliano with the inspiration for her book's title but says she was already working on her book when "French Women" hit stores).

"Americans are not going to eat rice and miso soup for breakfast, even in cities," Guiliano said. "That's a lot of cooking — let's be realistic. For most people here, French, Italian, Mediterranean food is OK. After that, it's too foreign. Americans like Chinese food, bread, french fries and pizza."

That said, Guiliano's advice is similar to Moriyama's.

"Eat slowly, eat more fresh food, lots of fruit and veggies. Sit down, try not to multi-task. Drink water, walk. Of course, nobody cooks everyday, but try to do it once or twice a week and on the weekend," she said.

But perhaps the biggest — and easiest to follow — lesson in Guiliano's book is balance, or learning to compensate for bigger meals with smaller, healthier ones.

"Last night I had a big dinner. I had champagne, red wine, a piece of bread and I shared some cheese and a dessert. This morning I'm not going to have a croissant — I had yogurt, half a grapefruit, half of piece of whole wheat toast. Lunch will be soup and salad."

But Guiliano says her book is not about a diet — it’s about a lifestyle.

"It's humor, exercise, breathing, yoga, sex, love, sitting at the table socializing," she said. "It's a much more sensual approach to eating, trying new things, more variety.

"Eat what's in season," she continued. "Wine and chocolate — the real thing — are OK sometimes, they're good for you. We do use cheese and butter and cream — in slivers, not a quarter pound. Americans say, 'We don’t have three hours for lunch!' French don’t have three hours, either, nowadays. Just don’t gulp it down."

Washington, D.C., resident Corey Brett, 31, lived in Japan for 5 1/2 years — and while she missed pizza, bagels and American Chinese food, she says she did lose weight while she was there.

"I thought the food was really good. It took a while to get used to it and I definitely did lose weight naturally. They eat rice there rather than bread and pasta, which is less fattening and better for you."

She didn't, however, take the cooking style home with her.

"I am not a really good cook so, no, I didn’t. Even there I ate the pre-prepared food, which happens to be healthier."

Brett also had kind words for the French philosophy.

“Everything in moderation is a great rule to live by.”

But Kava has her doubts.

“Gimmicks sell books. What's next, 'Swedish Women Live Forever'?"