Patients with type 2 diabetes and painful diabetic peripheral neuropathy have lower vitamin D (25-hydroxyvitamin D) levels compared to patients with type 2 diabetes who do not have neuropathy or have painless neuropathy and healthy controls as shown in a study published in Diabetic Medicine.

Painful diabetic peripheral neuropathy is a distressing and disabling condition. These findings therefore assume clinical significance because many patients with type 2 diabetes have painful diabetic neuropathy and what’s more, many patients are undiagnosed. And, for the first time, the researchers corrected for confounding factors of seasonal sunlight and physical activity.

People with type 2 diabetes were grouped into three in the study: Those with painful diabetic peripheral neuropathy, those with painless diabetic peripheral neuropathy and those who did not have diabetic peripheral neuropathy. Seasonal sunlight exposure and daily activity were assessed in all study participants. Levels of 25‐hydroxyvitamin D were measured during the summer months.

25-hydroxyvitamin D levels were found to be significantly lower in those with painful diabetic peripheral neuropathy (34.9 nmol/L) than healthy volunteers (62.1 nmol/L), those with no diabetic peripheral neuropathy (49.6 nmol/L), and those with painless diabetic peripheral neuropathy (53.1 nmol/L). With each unit decline in vitamin D, the likelihood of developing painful diabetic neuropathy increased by a factor of 1.11.

Lower 25-hydroxyvitamin D levels were also associated with lower cold detection thresholds and subepidermal nerve fiber densities. Patients with painful DPN had less sensitive small nerve fibers and only detected cold at 11°C.

The study suggests a role for vitamin D in the pathogenesis of painful diabetic neuropathy, though long-term prospective cohort or interventional studies to examine a cause and effect association between the two. The authors say: If causality is confirmed, this will have a significant impact on clinical practice as there would be a clear rationale for early screening and treatment for low vitamin D in people with painful diabetic neuropathy.”