Protective Effects of Metformin and Vitamin C on Diabetes-Induced Testicular Dysfunction

Combination metformin + vitamin C outperformed either drug alone on sperm viability and Johnsen scores in streptozotocin-diabetic mice (n=6/group)

Journal: Reproduction (Cambridge, England) | Published: 2026-06-02 | Type: Journal Article | PMID: 42172611 Authors: Özkoçer Süheyla Esra (Gazi University, Ankara); Guney Eskiler Gamze (Sakarya University); Konac Ece (Gazi University, Ankara) Funding/COI: Scientific and Technological Research Council of Türkiye. COI not declared.

Summary

Diabetic mice treated with both metformin and vitamin C showed better sperm viability, higher Johnsen spermatogenesis scores, and greater expression of testicular genes (Dnah7, Tomm20) than mice treated with either drug alone. The study tested this in streptozotocin-induced diabetic C57BL/6 mice — a common but artificial model that doesn't replicate the full metabolic profile of human type 2 diabetes. With six mice per group, the sample is too small to draw firm conclusions about effect sizes or generalizability.

Claims

• Diabetic mice (DM group) showed hyperglycemia and reduced superoxide dismutase (SOD) activity compared to controls
• Combination Met+VitC produced higher sperm viability than either Met alone or VitC alone (exact percentages not provided in abstract)
• Johnsen scores — a histological index of spermatogenesis completeness — were higher in the combination group than monotherapy groups
• Testicular Tomm20 (mitochondrial import marker) and Dnah7 (dynein axonemal heavy chain, linked to sperm motility) mRNA expression was greater in the combination group than monotherapy
• Caspase-3 (apoptosis marker) and PCNA (cell proliferation marker) levels were assessed by western blot and immunohistochemistry; abstract does not report the direction or magnitude of these findings

Study Quality

This is a six-group rodent intervention study — 36 mice total, six per group. That sample size is adequate for detecting large effects in highly controlled conditions but leaves no margin for biological variability or multiple comparisons. Diabetes was induced by streptozotocin injection, which selectively destroys pancreatic beta cells and produces insulin-deficient (type 1-like) hyperglycemia. This is a reasonable and widely-used model for studying hyperglycemic damage, but it doesn't capture the insulin resistance, dyslipidemia, or obesity component typical of human type 2 diabetes, where metformin is actually prescribed.

Metformin was dosed at 250 mg/kg/day and vitamin C at 100 mg/kg/day. Mouse-to-human dose translation is nonlinear and context-dependent; these doses don't map cleanly to standard human therapeutic doses. The study measured a commendable range of endpoints — sperm parameters, oxidative stress, spermatogenesis histology, gene expression, and protein levels — but the abstract reports no statistical values, effect sizes, or confidence intervals, making independent quality assessment impossible without the full paper.

Red Flags

• Six mice per group — extreme underpowering for multi-endpoint analysis; high risk of false positives across the battery of tests
• STZ model ≠ type 2 diabetes — the model produces insulin deficiency, not insulin resistance; results may not translate to the diabetic men most likely to be prescribed metformin
• No statistical correction detail — multiple comparisons across sperm viability, Johnsen score, PCR for two genes, and two protein assays without reported correction inflates false discovery risk
• COI not declared — the authors did not explicitly state no conflict of interest; absence of disclosure is not the same as no conflict
• Dose rationale unstated — why 250 mg/kg/day and 100 mg/kg/day specifically; no pharmacokinetic justification noted
• Abstract withholds key numbers — caspase-3 and PCNA findings are mentioned but not quantified, making this summary incomplete

Strengths

• Multi-method approach (flow cytometry, real-time PCR, western blot, immunohistochemistry, histological scoring) provides converging evidence rather than a single endpoint
• Combination vs. monotherapy comparison is the right experimental design to justify a combination claim
• Funded by a public research council, not pharmaceutical industry — no product to sell
• Dnah7 and Tomm20 are mechanistically plausible targets: Tomm20 links to mitochondrial function (relevant to oxidative stress), Dnah7 to sperm flagellar motility

Verdict

A small, tightly controlled mouse study that shows a combination drug effect in an artificial diabetes model. The finding — that metformin and vitamin C together outperform either alone on testicular endpoints — is biologically plausible and worth following up, but the sample size (n=6/group) means the effect size estimates are unreliable and the multiple-comparison exposure is uncontrolled. The STZ model's mismatch with human T2DM is the bigger conceptual gap: this tells you something about hyperglycemic oxidative damage in general, not specifically about what happens in a metformin-treated diabetic man's testes. File under "suggestive animal data that needs replication at meaningful scale before it means anything clinically."