Starvation Ketosis and the Kidney

<b><i>Background:</i></b> The remarkable ability of the body to adapt to long-term starvation has been critical for survival of primitive man. An appreciation of these processes can provide the clinician better insight into many clinical conditions characterized by ketoacidosis. <b><i>Summary:</i></b> The body adapts to long-term fasting by conserving nitrogen, as the brain increasingly utilizes keto acids, sparing the need for glucose. This shift in fuel utilization decreases the need for mobilization of amino acids from the muscle for purposes of gluconeogenesis. Loss of urinary nitrogen is initially in the form of urea when hepatic gluconeogenesis is dominant and later as ammonia reflecting increased glutamine uptake by the kidney. The carbon skeleton of glutamine is utilized for glucose production and regeneration of consumed HCO₃⁻. The replacement of urea with NH₄⁺ provides the osmoles needed for urine flow and waste product excretion. Over time, the urinary loss of nitrogen is minimized as kidney uptake of filtered ketone bodies becomes more complete. Adjustments in urine Na⁺ serve to minimize kidney K⁺ wasting and, along with changes in urine pH, minimize the likelihood of uric acid precipitation. There is a sexual dimorphism in response to starvation. <b><i>Key Message:</i></b> Ketoacidosis is a major feature of common clinical conditions to include diabetic ketoacidosis, alcoholic ketoacidosis, salicylate intoxication, SGLT2 inhibitor therapy, and calorie sufficient but carbohydrate-restricted diets. Familiarity with the pathophysiology and metabolic consequences of ketogenesis is critical, given the potential for the clinician to encounter one of these conditions.