One reason not to skimp on carbohydrate:
http://www.pleasanton.k12.ca.us/amador/Creek/AP98/JoyGobs/intro.html
A big question is: Does the food you eat really have a significant
effect on your cognitive processing and behavioral patterns?  According to
Katherine Milton, an anthropologist at the University of California at
Berkeley, " the behaviors and physiology that define us are the
consequences

of dietary-driven evolution. . . and everything comes back to diet." Milton
has spent the last twenty years on the island of Barro Colorado in Panama,
studying howler and spider monkeys. Her extensive research has resulted in
an increased knowledge of the effects of diet in relation to the
productivity
of the brain. After many years of observation and analyzation of the
monkeys,
their habitat, diet, and physiology she made some astounding discoveries.
Even though the two monkeys shared the same environment and were
approximately
the same size, they had dramatic differences. The spider monkeys, whose
diet
consisted ninety percent from fruits, had more energy, were brighter, and
had larger brains. Milton also found that their digestive tracts were much
smaller allowing for quicker removal of wastes. By eating high energy, hard
to find fruits, the monkeys were forced to memorize more locations of
plants,
the time of year and day the fruit would ripen, thus forcing their
brains to

expand over time. In contrast, their counterpart, the howler monkey, fed
themselves on protein rich leaves leading to a lack of energy that was
needed
to fuel the brain. Because of the excess in protein and lack of
carbohydrates
the howler monkeys were dull and unobservant. Milton was able to eat
bananas
and peanut butter sandwiches in front of the howler's making loud slurping
noises, and the howler's were seemingly oblivious to her action's. The
spider
monkeys, however, had a much different reaction. They would attack
Milton to
get her food, and she quickly learned that eating could become a hazardous
situation because of their attentiveness.

In conclusion, the focal point of our experiment was based on the idea that
diet has a direct impact on the function's of the brain.

-- Thanks to DeVries