Question explored with the scientific record
Are fungal toenails a sign of diabetes?
Fungal toenails are associated with diabetes, but they are not a reliable early sign of it.
The evidence shows that people with diabetes have a higher rate of fungal toenail infections than the general population. A systematic review found the prevalence of culture-confirmed toenail fungus in diabetic patients was 8.75%, compared to 3.22% in the general population [2]. An Italian study of over 8,000 patients found diabetes was present in 11.2% of those with onychomycosis [3]. A Greek study reported diabetes as a predisposing factor in 7.7% of cases [1, 5].
However, the most recent and direct evidence on this question comes from a 2026 register-based study of over 78,000 people [4]. It found that having a PCR-confirmed dermatophyte infection of the foot or nail did not predict a higher risk of developing type 2 diabetes later on. The adjusted incidence rate ratio was 1.0, meaning no increased risk [4]. This suggests that while diabetes makes you more susceptible to fungal infections, the fungus itself is not a signal that diabetes is coming.
| Population | Prevalence of Toenail Fungus |
|---|---|
| General population | 3.22% [2] |
| Diabetic patients | 8.75% [2] |
| Elderly | 10.28% [2] |
| HIV positive | 10.40% [2] |
The mechanism is straightforward. High blood sugar feeds fungi and impairs circulation and immune function in the extremities, making it easier for an infection to take hold and harder to clear. The fungus does not cause diabetes.
My call: Fungal toenails are a common complication of existing diabetes, not a useful early warning sign. If you have them, it is worth checking your blood sugar, but the absence of fungus does not rule out diabetes, and its presence does not predict it. Confidence: high for the association with existing diabetes; moderate for the lack of predictive value, based on the single 2026 study.
Sources used 5
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Epidemiology of onychomycosis in Crete, Greece: a 12‐year study
Over 12 years in Crete, fingernail onychomycosis was more common in women and largely caused by yeasts (Candida spp.), while toenail disease was more often dermatophyte-driven (Trichophyton rubrum), with an overall culture positivity around 16% among suspected cases.
DOI: meta/10.1111/myc.12533 -
The prevalence of culture‐confirmed toenail onychomycosis in at‐risk patient populations
This study systematically reviews the prevalence of culture-confirmed toenail onychomycosis across various at-risk patient populations, revealing significant differences in prevalence rates among groups such as the elderly, diabetics, and dialysis patients.
DOI: 10.1111/jdv.12873 -
Epidemiology of onychomycosis in Italy: prevalence data and risk factor identification
This study evaluates the prevalence of onychomycosis in Italy, finding a 14.2% diagnosis rate among 8331 patients, with significant associations to age, previous infections, and comorbidities such as diabetes and hallux valgus.
DOI: 10.1111/myc.12396 -
Can Foot Dermatophyte Infections Signal Future Diabetes Risk? Findings from a Register-based Study
PCR-confirmed dermatophyte infections of the feet or nails show no overall increase in subsequent type 2 diabetes risk, though exposure misclassification in sensitivity analyses hints at a modest risk in select subgroups.
DOI: 10.2340/actadv.v106.adv-2025-0125 -
Epidemiology of onychomycosis in Crete, Greece: a 12‐year study
A 12-year prospective epidemiological study in Crete, Greece (2004–2015) characterizing the frequency, sites, and etiological agents of onychomycosis, revealing predominance of yeasts in fingernail infections and dermatophytes in toenail infections, with rising non-albicans Cand…
DOI: 10.1111/myc.12533