The P:C ratio for human breastmilk is about 0.15. (That's based on "Nutrition Almanac" figures of 0.32g protein and 2.12g carbohydrate per ounce of breastmilk.) That's quite different from the Zone Diet's P:C ratio for adults. Obviously a baby's brain needs a whole bunch of lactose to function and grow!

So I'm wondering...How do kids "grow" from a P:C ratio of 0.15 to a P:C ratio of 0.75? How fast does this metabolic transition occur, and when, and is it a "smooth curve" or does it occur in steps?

On a practical level, I'm wondering...When you want to get a child into the Zone, does it really work to just put the kid on the adult P:C ratio? I'd think that at least for kids under 6, and maybe even for kids under 12, the ideal P:C ratio for reaching the Zone would be lower than for adults. After all, kids' brains are growing and changing in major ways at least through puberty, and maybe they still need more glucose (to do that growing and changing) than adult brains need?

The diet is ideal for children because they need to be in the Zone even more than adults do. The average child before puberty will need about 15 grams of protein per meal with the appropriate amounts of fat and carbohydrate. After puberty, they should eat the same amounts as a typical adult. This is to ensure more than adequate protein for growth spurts. The one protein source that virtually every child will eat is string cheese. Although a little high in saturated fat, string cheese is a good way to introduce more protein in your child's diet. That leaves just the hard part for parents: getting your kids to eat fruits and vegetables instead of pasta and bread

I understand what you're saying here, but your response doesn't really answer my question about how children transition from the 0.15 P:C ratio that Mother Nature provides in breastmilk to the adult P:C ratio of 0.75.

I know this is really "technical" -- and maybe nobody tries to put a kid under age 5 on the Zone Diet, and maybe experience has shown that kids 5 and up do well with the adult P:C ratio so nobody worries about the transition from 0.15 to 0.75? But if Barry Sears or somebody else has researched how and when this transition occurs, I'd like to hear what they figured out, or even just what their "best educated guess" is.

For every breast feed child that ratio changes when he starts eating baby foods(ie. cereal(carb), fruit(carb)etc. Usually that is a slow transition with cereal and breast milk until the breast milk is totally eliminated. The Zone is all about keeping insulin in a moderate zone. There are plenty of kids in the Zone. The P:C ratio keeps silent inflammation at bay and is a starting point and must be tweeked by some people. For some it may be a 1:1 ratio for others it may tip the other way. It depends on satiety and mental focus. I hope that helps.