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# Diabetes: how hyperglycaemia damages tissue

- Mechanism

Years of high sugar damage small and large blood vessels and nerves, through several chemical routes.

Updated Nov 12, 2026

## Key points

- AGEs: glucose binds proteins non-enzymatically; AGEs cross-link collagen and basement membranes, trap LDL and activate RAGE on macrophages and endothelium.
- RAGE signalling: cytokines, growth factors (VEGF, TGF-beta), procoagulant endothelium, smooth muscle proliferation.
- PKC activation (via diacylglycerol): VEGF (retinal neovascularisation), TGF-beta (matrix), PAI-1 (impaired fibrinolysis).
- Polyol pathway: aldose reductase turns glucose into sorbitol, using NADPH; less glutathione, osmotic swelling of lens and nerves (cataract, neuropathy).
- Hexosamine pathway: fructose-6-phosphate to glucosamine-6-phosphate, glycosylation of transcription factors and more TGF-beta.
- HbA1c is itself a glycated protein: it measures the same chemistry.

## What it is

The long-term complications of diabetes, rather than the acute ones, cause most of its disability and deaths. Persistent hyperglycaemia injures tissues that take up glucose without insulin, such as endothelium, kidney, retina, nerves and lens. Four pathways explain much of the damage: formation of advanced glycation end products, activation of protein kinase C, the polyol (sorbitol) pathway, and the hexosamine pathway, all converging on oxidative stress.

## Step by step

- Glucose enters endothelium, mesangium, nerve and lens cells in proportion to its blood level.
- Inside, excess glucose feeds the polyol, hexosamine and diacylglycerol-PKC pathways.
- Outside, glucose glycates long-lived proteins such as collagen and basement membrane: AGEs.
- AGEs and RAGE activation, PKC and oxidative stress raise growth factors and matrix production.
- Result: thick basement membranes, leaky small vessels, accelerated atherosclerosis, damaged nerves and lens.

## The four pathways and their targets

| Pathway | Key step | Main consequence |
| --- | --- | --- |
| [Advanced glycation end products](https://durgical.com/en/conditions/diabetic-kidney-disease/) | Non-enzymatic glycation, RAGE binding | Basement membrane thickening, atherosclerosis |
| [Protein kinase C](https://durgical.com/en/conditions/diabetic-retinopathy/) | Diacylglycerol from glycolytic intermediates | VEGF, TGF-beta, PAI-1; neovascularisation, fibrosis |
| [Polyol](https://durgical.com/en/conditions/cataract/) | Aldose reductase: glucose to sorbitol | Oxidative stress, osmotic injury: cataract, neuropathy |
| Hexosamine | Glucosamine-6-phosphate formation | Altered gene expression, TGF-beta, matrix |

## Clinical notes

- Tight glucose control reduces microvascular complications in both types.
- Macrovascular disease also depends on blood pressure, lipids and smoking.

## Memory aid

Sugar sticks (AGEs), sugar signals (PKC), sugar swells (sorbitol) and sugar re-wires genes (hexosamine).

## Test yourself

Think of the answer, then tap the question to see it.

## How does the polyol pathway injure nerves and the lens?

Aldose reductase converts glucose to sorbitol using NADPH, reducing glutathione regeneration (oxidative stress) and causing osmotic swelling.

## What are advanced glycation end products?

Products of non-enzymatic binding of glucose to proteins that cross-link matrix and activate the receptor RAGE.

## Which tissues are most exposed to hyperglycaemic damage?

Tissues that take up glucose independently of insulin: endothelium, kidney, retina, peripheral nerve and lens.

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## Elsewhere in DurgiPedia

- [Glycated haemoglobin (HbA1c)](https://durgical.com/en/biochemistry/topics/glycated-haemoglobin/)
- [Oxidative stress and free-radical injury](https://durgical.com/en/general-pathology/oxidative-stress-and-free-radical-injury/)

## Connections

## Related 4

- [Diabetes: chronic lesions](https://durgical.com/en/general-pathology/diabetes-chronic-lesions/)
- [Diabetic nephropathy](https://durgical.com/en/general-pathology/diabetic-nephropathy/)
- [Atherosclerosis: risk factors](https://durgical.com/en/general-pathology/atherosclerosis-risk-factors/)
- [Oxidative stress and free-radical injury](https://durgical.com/en/general-pathology/oxidative-stress-and-free-radical-injury/)

## Related conditions 7

- [Type 1 diabetes mellitus](https://durgical.com/en/conditions/type-1-diabetes-mellitus/)
- [Type 2 diabetes mellitus](https://durgical.com/en/conditions/type-2-diabetes-mellitus/)
- [Diabetic kidney disease](https://durgical.com/en/conditions/diabetic-kidney-disease/)
- [Diabetic retinopathy](https://durgical.com/en/conditions/diabetic-retinopathy/)
- [Peripheral neuropathy (diabetic and other)](https://durgical.com/en/conditions/peripheral-neuropathy/)
- [Cataract](https://durgical.com/en/conditions/cataract/)
- [Diabetic foot](https://durgical.com/en/conditions/diabetic-foot/)

[← All of general pathology](https://durgical.com/en/general-pathology/)

Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Diabetes Complications](https://medlineplus.gov/diabetescomplications.html) (MedlinePlus, US National Library of Medicine, 2025); [Diabetic Retinopathy](https://www.ncbi.nlm.nih.gov/books/NBK560805/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/diabetes-how-hyperglycaemia-damages-tissue/