Diabetes Risk Linked to Low Vitamin D Levels

Miriam E. Tucker

December 03, 2012

Low serum vitamin D concentration was associated with an increased risk of developing "insulin-requiring" diabetes in a nested case-control study of active-duty military service members.

The study, led by Edward D. Gorham, PhD, MPH, a research epidemiologist from the Naval Health Research Center, San Diego, and assistant adjunct professor in the Department of Family and Preventive Medicine at the University of California, San Diego, was published online September 7 and in the December issue of Diabetologia.

Dr. Gorham and colleagues used prediagnostic serum taken as part of a US Department of Defense serological surveillance program between 2002 and 2008. They compared levels of 25(OH)D between 1000 consecutive patients who developed insulin-requiring diabetes and 1000 healthy matched control participants. Case and control participants had all been on active duty at the time of the initial blood draw. Individuals from the 2 groups were matched based on "date that the blood sample was drawn (±2 days), age (±3 months), length of military service (±30 days), sex, and whether the control was on active duty when the case was diagnosed," the authors write.

A median of 1 year (range, 1 month - 10 years) elapsed between the blood sample collection and diabetes diagnosis. More than two thirds of all the study participants were younger than 35 years.

Those in the lowest quintile of serum 25(OH)D levels (<43 nmol/L) had a 3.5-fold greater likelihood of developing insulin-requiring diabetes than those in the highest quintile of serum 25(OH)D levels (≥100 nmol/L). Those in the second-lowest quintile of serum 25(OH)D levels (43 - 59 nmol/l) had a 2.5-fold greater risk. Odds ratios for the next 3 serum 25(OH)D levels quintiles (60 - 77, 78 - 99, and ≥100 nmol/L) were 0.8, 1.1, and 1.0 (reference), respectively (P trend < .001).

"Based on the present study, it may be that no further reduction in risk would be expected once a serum 25(OH)D concentration of >60 nmol/l has been attained," the authors note.

Those who developed diabetes had significantly lower mean 25(OH)D levels than the healthy control patients (62.2 nmol/L vs 72.5 nmol/L; P ≤ .0001).

Among racial groups, blacks were more likely to develop diabetes than other races, but the association between 25(OH)D levels and diabetes was seen in all racial groups.

After adjustment for race, those with the lowest quintile of 25(OH)D were at nearly 2-fold greater risk of developing diabetes compared with those in the highest quintile (odds ratio, 1.9; 95% confidence interval, 1.4 - 2.7; P < .0001). Compared with whites, the odds ratio for blacks was 1.6 (95% confidence interval, 1.2 - 2.0; P < .001).

As the researchers explain, worldwide rates of type 1 diabetes vary by latitude, with annual age-standardized incidences ranging from a low in the tropics (0.5 per 100,000 in Venezuela) to a high near the Arctic Circle (60/100,000 in Finland).

It is possible, the researchers state, that vitamin D affects immune function. "Vitamin D deficiency is associated with major effects on the innate immune system. This could potentially influence the risk of diabetes by reducing risk of infection of islet cells," the authors write.

A potential study limitation includes the possibility that a small number of patients may have had complicated type 2 diabetes. "Cases in the present study were included solely because they were dependent on insulin," the authors note.

This research was supported by a Congressional allocation to the University of Miami Diabetes Research Institute through the Department of the Navy, Bureau of Medicine and Surgery. The authors have disclosed no relevant financial relationships.

Diabetologia. 2012;55:3224-3227. Full text