Metabolic paradox: «Starvation amidst plenty»
At the biochemical level, diabetes mellitus largely mimics fasting, as in both cases the insulin/glucagon ratio decreases critically. The key difference lies in the sugar level: diabetes presents with severe hyperglycemia, yet the cells of insulin-dependent tissues (primarily muscle and adipose tissue) cannot utilize it.
Due to the insulin signaling deficit, GLUT-4 receptors fail to translocate into cell membranes, and glucose remains in the bloodstream. Intracellular energy deficiency ensues. Attempting to escape «starvation», the body initiates massive catabolism of its own structures:
- Protein breakdown (proteolysis) is enhanced, leading to muscle mass loss and azotemia.
- Glycogen breakdown is activated.
- The rate of fat breakdown (lipolysis) increases sharply.
The liver uses the released amino acids to synthesize new glucose (gluconeogenesis), which only exacerbates the already high hyperglycemia. This excess sugar begins to be excreted in the urine, dragging along massive volumes of water and causing osmotic diuresis (polyuria).
Pathogenesis of diabetic ketoacidosis
The removal of the inhibitory effect of insulin on hormone-sensitive lipase leads to a massive release of free fatty acids into the blood. They are transported to the liver, where they undergo intensive $\beta$-oxidation. This process results in an excess of acetyl-CoA.
Normally, acetyl-CoA is oxidized in the tricarboxylic acid cycle (Krebs cycle) by combining with oxaloacetate. However, in diabetes, oxaloacetate is massively consumed for gluconeogenesis. A metabolic block occurs: unutilized acetyl-CoA accumulates and is redirected toward the synthesis of ketone bodies (acetone, acetoacetate, and $\beta$-hydroxybutyrate).
When the rate of ketone body production exceeds the utilization capacity of tissues, they accumulate in the blood. Since these substances are acids, their excess shifts blood pH toward the acidic side, causing severe metabolic acidosis and toxic damage to the nervous system.
Mechanisms of late vascular complications
Chronically high blood sugar triggers non-enzymatic glycation reactions. Unlike normal enzymatic carbohydrate attachment to proteins, this process occurs chaotically: glucose molecules «stick» to free amino groups of proteins, irreversibly altering their structure.
The accumulation rate of damage directly depends on the lifespan of a specific protein. Slowly renewing protein structures are affected first:
- Macroangiopathies. Glycation of collagen and elastin in the walls of large arteries leads to loss of elasticity and contributes to the development of atherosclerosis, coronary artery disease, and lower extremity circulatory disorders.
- Microangiopathies. Renal glomerular capillaries (nephropathy) and retinal capillaries (retinopathy) are affected. The main pathological marker is the thickening of basement membranes in small vessels, which impairs microcirculation and filtration.
Glycated erythrocyte hemoglobin serves as an early indicator of glycation. A glucose molecule binds to the $\beta$-chains of hemoglobin, forming the $HbA_{1c}$ fraction, the level of which can increase 2–3 fold in diabetes.
Polyol pathway and nerve tissue damage
Another pathway of hyperglycemic toxicity is the activation of glucose conversion into the polyhydric alcohol sorbitol. This process occurs actively in target tissues: Schwann cells of nerve fibers, the eye lens, and vascular endothelium.
The problem is that sorbitol is practically not used in other biochemical reactions, and its rate of cellular efflux is extremely low. Accumulating intracellularly, sorbitol drastically increases internal osmotic pressure. The cell draws in water, swells, and becomes damaged.
In Schwann cells, osmotic edema leads to demyelination of nerve fibers and the development of severe diabetic neuropathy. In the eye lens, this same process, complemented by the glycation of structural proteins (crystallins), causes opacification and the formation of diabetic cataract.
Briefly: answers to topical questions
Which enzymes are involved in the conversion of glucose to sorbitol via the polyol pathway?
Sources indicate that the enzyme aldose reductase is involved in the conversion of excess glucose to sorbitol via the polyol pathway.
Hyperglycemia activates aldose reductase and enhances the synthesis of sorbitol from glucose. The consequences of the polyol pathway indicated in sources include: depletion of NADP/NADPH₂ reserves, generation of reactive oxygen species (ROS), and intracellular accumulation of sorbitol due to its low diffusion rate. In neurons, this leads to increased osmotic pressure, cell swelling, and tissue edema.
The paradox of diabetes can be remembered by the phrase «Starvation amidst plenty»：“tons” of sugar float around cells (hyperglycemia), but without the insulin key, the cellular doors remain closed. The body thinks it is starving and begins to burn its own muscles and fat.
Class and exam questions
Why does severe thirst and weight loss occur during diabetes decompensation?
High sugar causes osmotic diuresis (polyuria)—a colossal amount of water and ions is lost in the urine, which stimulates the thirst center in the brain. Weight loss against the backdrop of a good appetite (polyphagia) is explained by the fact that cells do not receive glucose and switch to self-consumption, burning fat reserves and breaking down muscle proteins.
What is the difference between ketoacidotic and hyperosmolar coma?
Ketoacidotic coma is characteristic of absolute insulin deficiency (type 1 diabetes mellitus), accompanied by massive ketogenesis and profound acidosis. Hyperosmolar coma occurs more frequently against the background of concurrent kidney disease; it is characterized by extremely high glucose figures and severe dehydration, whereas ketone bodies in the blood are usually absent.
Why do endocrinologists measure C-peptide?
C-peptide is released into the blood in equimolar amounts with endogenous insulin. Its high level in the context of hypoglycemia helps diagnose a hormonally active tumor (insulinoma) and differentiate types of diabetes (in type 1 it is critically reduced, in type 2 it is normal or elevated).
What else to study on this topic
- Mechanisms of lactic acidosis coma and tissue hypoxia
- Differential diagnosis of fasting, steroid-induced diabetes, and diabetes mellitus
- The effect of counter-regulatory hormones (glucagon, cortisol, adrenaline) on metabolism
- Hormonal regulation of insulin secretion by incretins (GLP-1, GIP)
- Water-electrolyte shifts in diabetes: the mechanism of intracellular potassium loss
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