Energy Stores in Humans

A stable blood glucose level is absolutely essential for normal brain function. The brain can only use glucose or ketone bodies as its fuel. Ketone bodies (acetyl acetate or ?-hydroxybutyrate) cannot replace glucose as the brain s energy source on short notice. About 10-14 days are required to increase plasma ketone body levels such that they can provide energy for neural tissues. At most they can provide about 50% of the brain s energy, the rest must come from glucose.



Blood sugar levels are usually between 4.5 to 5.5 mmoles/l and swing about 10-15% around these values. We do not normally experience low blood sugar levels. Hypoglycemia does ram some few people including persons with diabetes who have not eaten after taking insulin, others with insulin-producing tumors, newborn with untreated galactosemia, some alcohol-poisoned people, athletes who exceed their capacity in a competition and others with a variety of liver diseases. For most of us (relatively healthy persons), low blood sugar just does not happen.
Most nutritionists recommend a diet in which between 50 and 60 % of the caloric content is contributed by carbohydrates. However, we can exist quite well on diets containing with little or no carbohydrate. Low starch and sugar intake does not reduce blood sugar levels: we maintain normal blood glucose levels in spite of large variations in sugar and starch consumption. The key to this is the ability of both the liver and kidneys to synthesize glucose from amino acids (derived from proteins in the diet or from the body s muscle mass). Loss of control of hepatic glucose production is a major factor in development of the high blood sugar levels seen in type 2 diabetes mellitus.