The Japanese consume a diet high in carbohydrates, but I don't see them, or their children, growing up sick and obese.
There have been cultures that traditionally ate high-carb diets, without becoming obese. These traditionally ate grains and beans, rather than flour and sugar, and usually fermented them.
It's flour and sugar that are the problem, and I'll challenge you to find a single example of a culture that survives predominantly on refined carbs that isn't sick and obese - and often malnourished at the same time.
As for Japan, their obesity rates have been skyrocketing, as they've adopted western foods:
Obesity on the rise as Japanese eat more Western-style food
Of course, their kids are getting a better start, because their school lunches actually serve food, and insist that they eat it. (As opposed to ours, which consider sugar-and-chocolate flavored milk to be healthy, so long as it is low-fat.):
How Japan’s revolutionary school lunches helped slow the rise of child obesity
A few generations ago, we were a country with little obesity. That's because we employed a lot of people in agriculture, but mostly because people worked like hell and didn't sit on their butts. The diet consumed on the average farm was high in carbohydrates, many of them highly refined (flour, sugar, corn syrup, etc.), yet obesity was almost nonexistent, and the people could hardly be considered sick working 12+ hours a day.
Japan has always had a high carb diet, and some of those foods are most certainly considered to be high glycemic. The GI of rice is all over the board, with the average being around 65. This is higher than heavily processed high fructose corn syrup which has a GI in the 55~60 range.
People who want to sell diet books and the latest diet fad have seen the benefits of demonizing one macronutrient over another. But that's good for selling books and such, but does nothing for those who are genuinely interested in the cause(s) of obesity. Like it has been since the beginning, obesity, at least for the vast majority of people, is caused by people consuming more energy than they burn.
Lowfat milk in our schools is just plain stupid, but there it is. The fact that they include some flavoring to make it palatable shouldn't bother anyone who is truly interested in proper child nutrition. Milk casein, next to egg albumin, is probably the highest quality protein we consume. And growing children need a lot of high quality protein. Without that flavoring, the lowfat milk, along with all the nutrition, ends up in the garbage.
I spent a lot of time in Japan in the 80's and 90's, and it is pretty obvious that, like us, many of them have adopted a sedentary lifestyle and are now reaping the consequences of a slothful existence. But it wasn't always that way, and the Japanese can hardly be considered an obese or unhealthy society today. That just isn't true. This issue is, and always has been, about total calories consumed vs. calories burned. Same as it ever was.