The concentration of ketone bodies in the blood at any time represents a balance between the rate of hepatic ketogenesis and the rate of utilization by peripheral tissues. It is generally assumed that an increase in ketogenesis leads to a rise in blood ketone bodies, which in turn results in their increased utilization. In rare situations, such as congenital absence of key enzymes involved in ketone body utilization (e.g., 3-oxoacid-CoA transferase) or inhibition of these enzymes by pharmacological agents, blood ketone bodies may increase without any concomitant increase in ketogenesis.

The concentration of ketone bodies in the blood is exquisitely sensitive to changes in pathophysiologi-cal state. It is therefore useful to define normoketo-nemia in mammals as a concentration of total ketone bodies in blood below 0.2mmoll-1, hyper-ketonemia as above this level, and ketoacidosis (ketosis; by analogy to the definition of lactic acido-sis) as above 7mmoll-1. In adult mammals there are small but characteristic diurnal variations in ketone body concentrations. Larger increases in concentration occur in man in response to change in patho-physiological state (Table 1). The concentrations span a 200-fold range and it is this which underlines the important role of ketone bodies as substrates and signals.

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