Neurology

Why It Happens: The Biology of Nerve Damage

At a Glance

Diabetic neuropathy usually develops gradually and symmetrically in the feet because chronic high blood sugar injures nerve fibers and their blood supply. Sudden, one-sided, or weakness-dominant symptoms may signal another cause that needs testing.

When you are diagnosed with diabetic neuropathy, it is natural to assume that high blood sugar is the only culprit. However, the biology of nerve damage is complex, and many other conditions can produce nearly identical symptoms. Doctors must perform a “differential diagnosis”—a medical process of elimination—to ensure they aren’t missing a treatable condition that isn’t related to your diabetes.

The Biology of Nerve Damage in Plain Language

In a healthy body, nerves are like high-performance electrical cables that require a constant supply of energy and protection. Chronic high blood sugar (hyperglycemia) disrupts this system through several overlapping pathways:

Metabolic Stress: When blood sugar is high, your cells process excess glucose in ways that create harmful byproducts ^[1]. These act like biological rust, damaging the structures within your nerve fibers ^[2].
Energy Failure: Your nerves have long fibers that require massive amounts of energy. High sugar damages the cellular power plants, leading to energy failure and the eventual breakdown of the nerve fiber ^[3]^[4].
Vascular Issues: Nerves rely on tiny blood vessels for oxygen. Hyperglycemia damages these vessels, reducing blood flow and allowing inflammatory markers to leak into the nerve tissue, causing further injury ^[5]^[6]^[7].
Neuroinflammation: This stress triggers an immune response. Inflammatory cells can attack both the nerve fiber and the protective insulation around the nerve ^[8]^[9].

Atypical Features: When It Might Be Something Else

Standard diabetic peripheral neuropathy typically follows a very specific pattern: it is gradual, symmetric (affecting both sides), and starts in the longest nerves (the toes) before slowly moving up ^[10]^[11].

If your symptoms don’t fit this “glove-and-stocking” pattern, it requires evaluation for proximal diabetic neuropathy and other causes ^[12]^[13]. Atypical features that should prompt additional evaluation include:

Symptoms that started very suddenly (acute or subacute).
Weakness that is much more prominent than numbness.
Symptoms that are much worse on one side than the other.
Pain and numbness that starts in the thighs or hips before the feet (this may be Diabetic Amyotrophy, a proximal form of diabetic neuropathy, but requires assessment to confirm).

The “Mimics”: Other Causes of Nerve Pain

Just because you have diabetes does not mean your neuropathy is entirely caused by it. In fact, many patients have more than one cause for their nerve damage ^[14]. The most common mimics include:

Mimic Condition	Why it Matters
Vitamin B12 Deficiency	Very common in people taking metformin. It can cause numbness, balance issues, and spinal cord problems. If the B12 result is borderline, your doctor may run an additional test (methylmalonic acid) ^[15]^[16].
Hypothyroidism	An underactive thyroid can lead to fluid retention and metabolic changes that damage peripheral nerves ^[17]^[14].
Alcohol Exposure	Chronic alcohol use is directly toxic to nerve fibers and often goes hand-in-hand with nutritional deficiencies that worsen damage ^[18].
Abnormal Blood Proteins	Certain abnormal proteins in the blood can attack the nerve’s insulation. This is found in a small percentage of people with unexplained neuropathy, though it does not necessarily mean you have cancer ^[19]^[20].
Kidney Disease or Meds	Nerve damage can also result from severe kidney disease, chemotherapy, or certain medications.

To rule these out, your doctor will likely order a “neuropathy screen” of blood tests, selected according to your symptom pattern. Understanding the root cause is vital because some of these mimics, like B12 deficiency or thyroid issues, can be partially or fully reversed with the right treatment.

Common questions in this guide

What actually causes diabetic neuropathy?

Chronic high blood sugar can injure nerves through several processes, including harmful metabolic byproducts, reduced cellular energy, damage to the tiny blood vessels that supply nerves, and inflammation. These changes can gradually impair nerve fibers and their protective covering.

Is it normal for diabetic neuropathy to start suddenly or affect only one side?

Typical diabetic peripheral neuropathy develops gradually and affects both sides, often beginning in the toes and moving upward. Sudden symptoms, marked weakness, or symptoms that are much worse on one side are atypical and should be assessed for other causes or a proximal form of diabetic neuropathy.

Should I have my vitamin B12 checked if I take metformin?

Metformin use is associated with vitamin B12 deficiency, which can cause numbness, balance problems, and other nerve symptoms. Ask your healthcare professional whether a B12 test is appropriate; if the result is borderline, an additional methylmalonic acid test may help clarify the result.

What tests can look for causes of neuropathy besides diabetes?

A healthcare professional may order a blood-test evaluation for neuropathy, choosing tests based on your symptoms and medical history. This may include checks for vitamin B12 deficiency and thyroid problems, with additional testing when other causes are suspected.

Can something besides diabetes cause nerve pain or numbness?

Yes. Vitamin B12 deficiency, an underactive thyroid, chronic alcohol exposure, abnormal blood proteins, severe kidney disease, chemotherapy, and some medicines can cause similar nerve symptoms. Some of these causes, especially vitamin deficiency or thyroid disease, may improve with appropriate treatment.

Curated prompts to bring to your next appointment.

1.Since I am taking metformin, should we check my Vitamin B12 levels to make sure that isn't contributing to my nerve pain?
2.My symptoms started very suddenly—is that typical for diabetic neuropathy, or should we look for other causes?
3.Are my symptoms relatively similar on both sides, and if not, does that change what tests you want to run?
4.Do I have any signs of 'motor' involvement, like muscle weakness, that might point away from standard diabetic neuropathy?
5.Which specific blood tests are we running to rule out things like thyroid issues or vitamin deficiencies?

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References (20)

1
Diabetic Polyneuropathy in Type 2 Diabetes Mellitus: Inflammation, Oxidative Stress, and Mitochondrial Function.
Román-Pintos LM, Villegas-Rivera G, Rodríguez-Carrizalez AD, et al.
Journal of diabetes research 2016; (2016()):3425617 doi:10.1155/2016/3425617.
PMID: 28058263
2
The pivotal role of immunometabolism in diabetic neuropathy and its potential therapeutic strategies.
Lin R, Zhang X, Yang X, et al.
Frontiers in endocrinology 2026; (17()):1791057 doi:10.3389/fendo.2026.1791057.
PMID: 42369056
3
Bioenergetic failure in diabetic peripheral neuropathy: from glucotoxicity to multidimensional metabolic imbalance.
Tao W, Yu Y, Wu X, et al.
Journal of neurology 2026; (273(7)).
PMID: 42342866
4
Axonal transport deficits in the pathogenesis of diabetic peripheral neuropathy.
Yang C, Zhao X, An X, et al.
Frontiers in endocrinology 2023; (14()):1136796 doi:10.3389/fendo.2023.1136796.
PMID: 37056668
5
Redox-Driven Blood-Nerve Barrier Dysfunction in Diabetic Peripheral Neuropathy: Mechanisms and Therapeutic Opportunities.
Huang WH, Wong CS
Antioxidants (Basel, Switzerland) 2026; (15(6)) doi:10.3390/antiox15060670.
PMID: 42351976
6
Blood-Nerve Barrier (BNB) Pathology in Diabetic Peripheral Neuropathy and In Vitro Human BNB Model.
Takeshita Y, Sato R, Kanda T
International journal of molecular sciences 2020; (22(1)) doi:10.3390/ijms22010062.
PMID: 33374622
7
Targeting Oxidative Stress and Mitochondrial Dysfunction in Diabetic Neuropathy: Mechanisms and Therapeutic Opportunities.
Sztanek F, Tóth LI, Hernyák M, et al.
Antioxidants (Basel, Switzerland) 2026; (15(3)) doi:10.3390/antiox15030367.
PMID: 41897513
8
Cytokine Signaling in Diabetic Neuropathy: A Key Player in Peripheral Nerve Damage.
Nashtahosseini Z, Eslami M, Paraandavaji E, et al.
Biomedicines 2025; (13(3)) doi:10.3390/biomedicines13030589.
PMID: 40149566
9
Interleukin-10 Protects Schwann Cells against Advanced Glycation End Products-Induced Apoptosis via NF-κB Suppression.
Xu S, Bao W, Men X, et al.
Experimental and clinical endocrinology & diabetes : official journal, German Society of Endocrinology [and] German Diabetes Association 2020; (128(2)):89-96 doi:10.1055/a-0826-4374.
PMID: 30641609
10
Diabetic peripheral neuropathy essentials: a narrative review.
Chang MC, Yang S
Annals of palliative medicine 2023; (12(2)):390-398 doi:10.21037/apm-22-693.
PMID: 36786097
11
Diabetic peripheral neuropathy: pathogenetic mechanisms and treatment.
Zhu J, Hu Z, Luo Y, et al.
Frontiers in endocrinology 2023; (14()):1265372 doi:10.3389/fendo.2023.1265372.
PMID: 38264279
12
Distal Symmetric Polyneuropathy: A Review.
Callaghan BC, Price RS, Feldman EL
JAMA 2015; (314(20)):2172-81 doi:10.1001/jama.2015.13611.
PMID: 26599185
13
Understanding the role of hyperglycemia and the molecular mechanism associated with diabetic neuropathy and possible therapeutic strategies.
Kaur M, Misra S, Swarnkar P, et al.
Biochemical pharmacology 2023; (215()):115723 doi:10.1016/j.bcp.2023.115723.
PMID: 37536473
14
Diabetic distal symmetric polyneuropathy: More than just "tingling in the feet".
Bell DSH, Jerkins TW
Diabetes, obesity & metabolism 2026; (28(2)):817-825 doi:10.1111/dom.70235.
PMID: 41198580
15
Metformin-Associated Functional Vitamin B12 Deficiency Presenting as Subacute Combined Degeneration in a 57-Year-Old Man With Diabetes Mellitus.
Zhang R, Gou L, Zhou X
The American journal of case reports 2026; (27()):e952205 doi:10.12659/AJCR.952205.
PMID: 42035456
16
Correlation Between Nitrous Oxide and Functional Vitamin B12 Deficiency Resulting in Subacute Combined Degeneration of the Spinal Cord: A Case Report.
Kyaw KY, Lwin MT, Lwin ZT
Cureus 2024; (16(11)):e74383 doi:10.7759/cureus.74383.
PMID: 39723261
17
How Well do We Evaluate Small Fiber Neuropathy?: A Survey of American Academy of Neurology Members.
Thawani S, Chan M, Ostendorf T, et al.
Journal of clinical neuromuscular disease 2025; (26(4)):184-195 doi:10.1097/CND.0000000000000502.
PMID: 40512966
18
Expert opinion on screening, diagnosis and management of diabetic peripheral neuropathy: a multidisciplinary approach.
Atmaca A, Ketenci A, Sahin I, et al.
Frontiers in endocrinology 2024; (15()):1380929 doi:10.3389/fendo.2024.1380929.
PMID: 38952393
19
Monoclonal Gammopathy-Associated Peripheral Neuropathy: Diagnosis and Management.
Chaudhry HM, Mauermann ML, Rajkumar SV
Mayo Clinic proceedings 2017; (92(5)):838-850 doi:10.1016/j.mayocp.2017.02.003.
PMID: 28473042
20
Clinical Utility of Routine Monoclonal Gammopathy Testing in the Evaluation of Peripheral Neuropathy.
Maberry MD, Smith CJ, Go RS
Journal of general internal medicine 2026; (41(3)):753-758 doi:10.1007/s11606-025-09528-0.
PMID: 40379937

This page is for informational purposes only and does not constitute medical advice. A healthcare professional should evaluate sudden, one-sided, or weakness-predominant symptoms and interpret any blood-test results.

Get notified when new evidence is published on diabetic neuropathy.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.

Guide Contents