Basically, the body needs fuel from food. All three: proteins, fats and carbohydrates provide fuel. Proteins and fats can provide fuel for all muscle and organ tissue, except the brain. The brain and nervous system needs carbohydrates. However, when no carbohydrates (i.e. glucose and glycogen) are available, the body uses its back-up system. It converts fats by partially burning a fat molecule into a ketone. The ketone acts as faux glucose and can be used to feed the brain and every body system.

There are contradictory opinions as to whether ketones fuel the brain as efficiently as glucose (or glucose stores i.e. glycogen).

Ketosis in diabetes is dangerous not because it is ketosis, but because a diabetic goes into ketosis while eating carbohydrates. This is because the glucose is not being converted by insulin into fuel for the body but rising higher and higher. In the diabetic, ketosis is a sign or signal to take insulin before coma from high sugar levels.

for non-technical supportive explanation of ketosis see:

http://lowcarbdiet4u.com/ketosis.html

http://www.lindora.com/ketosis.cfm

for a dissenting opinion:

http://www.nutrition.cornell.edu/nu...st/ketosis.html

For a very interesting argument by all the key, well-known diet doctors, see the Great Nutrition Debate at:

http://www.usda.gov/cnpp/Seminars/GND/Proceedings.txt