Trial Design

This randomized, double-blind, placebo-controlled clinical trial evaluated the safety and efficacy of oral administration of vitamin D 3 (cholecalciferol; 4000 IU per day) for diabetes prevention in adults at high risk for type 2 diabetes.11 The trial protocol (available with the full text of this article at NEJM.org) was designed by the planning committee and the primary sponsor (National Institute of Diabetes and Digestive and Kidney Diseases) without input from manufacturers11 and involved collaboration among 22 academic medical centers in the United States (https://d2dstudy.org/sites). A sponsor-appointed data and safety monitoring board approved the protocol and provided independent monitoring of the trial. The institutional review board at each clinical site also approved the protocol, and all the participants provided written informed consent. The data were collected by trial-site personnel and stored in an electronic data-capture database. The statistical team at the coordinating center analyzed the data and vouches for its accuracy. All the authors vouch for the accuracy and completeness of the data and for the fidelity of the trial to the protocol. They also contributed to the interpretation of the results and the preparation, review, and approval of the manuscript and made the decision to submit the manuscript for publication.

No pharmaceutical manufacturers contributed to the planning, design, or conduct of the trial. Trial pills were purchased from an independent nutritional-supplement manufacturing company that has no association with any members of the D2d Research Group.

Participants

Participants met at least two of three glycemic criteria for prediabetes as defined by the 2010 American Diabetes Association (ADA) guidelines: fasting plasma glucose level, 100 to 125 mg per deciliter (5.6 to 6.9 mmol per liter); plasma glucose level 2 hours after a 75-g oral glucose load, 140 to 199 mg per deciliter (7.8 to 11.0 mmol per liter); and glycated hemoglobin level, 5.7 to 6.4% (39 to 47 mmol per mole).12 Other inclusion criteria were an age of 30 years or older (25 years or older for American Indians, Alaska Natives, or Native Hawaiians or other Pacific Islanders) and a body-mass index (BMI, the weight in kilograms divided by the square of the height in meters) of 24 to 42 (22.5 to 42 for Asian Americans). A low serum 25-hydroxyvitamin D level was not an inclusion criterion.

Key exclusion criteria were any glycemic criterion in the diabetes range,12 factors (other than hyperglycemia and race) affecting the glycated hemoglobin level, use of diabetes or weight-loss medications, or use of supplements containing vitamin D at a dose of more than 1000 IU per day or calcium at a dose of more than 600 mg per day. For a complete list of eligibility criteria, see the Supplementary Appendix (available at NEJM.org). The recruitment process relied primarily on electronic-health-record identification of potentially eligible adults who were then screened in person and, if qualified, had a second screening visit to determine final eligibility according to measured fasting plasma glucose, 2-hour post-load plasma glucose, and glycated hemoglobin at the central laboratory of the trial.13

Intervention and Procedures

Participants were randomly assigned to take a single, once-daily soft-gel pill containing either 4000 IU of vitamin D 3 or matching placebo. Randomization was block-stratified according to trial site, BMI (<30 or ≥30), and race (white or nonwhite). Participants received a bottle of trial pills at the time of randomization and every 6 months thereafter. Bottles with unused pills were returned at each visit to estimate adherence.

To maximize the ability of the trial to observe a treatment effect, participants were asked to refrain from using diabetes-specific or weight-loss medications during the trial and to limit the use of outside-of-trial vitamin D to 1000 IU per day from all supplements, including multivitamins. Because of concern that high intake of calcium from supplements may be associated with adverse outcomes, participants were asked to limit calcium supplements to 600 mg per day. During the trial, participants were provided with information on diabetes prevention through information sheets and twice-yearly group meetings.

Follow-up visits occurred at month 3, month 6, and twice per year thereafter. Midway between the in-person visits, an interim contact (telephone or email) took place. All visits and contacts were designed to promote retention, encourage adherence to the trial regimen, and assess for diabetes, occurrence of adverse events, and use of high-dose vitamin D supplements, diabetes medications, and weight-loss medications.

Outcomes

The primary outcome in this time-to-event analysis was new-onset diabetes, based on annual glycemic testing of fasting plasma glucose, glycated hemoglobin, and 2-hour post-load plasma glucose and semiannual testing of fasting plasma glucose and glycated hemoglobin. If two or three of the glycemic measures met the 2010 ADA thresholds for diabetes,12 the participant was considered to have met the diabetes outcome. When only the measure for fasting plasma glucose or glycated hemoglobin met the threshold, confirmatory testing was performed for the positive measure within 8 weeks. If only the measure for 2-hour post-load plasma glucose met the threshold, then a 75-g oral glucose-tolerance test to reassess all three glycemic measures was repeated. If the repeat measure was positive or both fasting plasma glucose and glycated hemoglobin were positive (in the case of a repeat oral glucose-tolerance test), then the participant was considered to have met the diabetes outcome. A diagnosis of diabetes that was made outside the trial was validated by in-trial laboratory testing or adjudicated by an independent clinical-outcomes committee.

During the trial, research staff, caregivers, and participants were unaware of glycemic test results until a participant met the diabetes outcome. Safety was assessed by means of participant report and annual fasting measurements of serum calcium, serum creatinine, and morning spot urine calcium:creatinine ratio (a rough estimate of urine calcium excretion).14

Laboratory Testing

Serum calcium and creatinine were analyzed locally at each site, and the estimated glomerular filtration rate was calculated.15 Other blood and urine specimens were processed locally and shipped to the central laboratory. Glycated hemoglobin was measured with the use of an ion-exchange high-performance liquid chromatography method certified by the National Glycohemoglobin Standardization Program.16 Plasma glucose was measured with the use of a hexokinase method. Stored serum samples from the baseline, month 12, and month 24 visits were used to measure 25-hydroxyvitamin D by liquid chromatography–tandem mass spectrometry validated by a quarterly proficiency-testing program administered by the Vitamin D External Quality Assessment Scheme.17,18

Statistical Analysis

The trial was designed as an event-driven trial with a target of 508 diabetes events and a total sample size of 2382 participants assigned equally to the vitamin D group and placebo group, on the basis of a hypothesized hazard ratio of 0.75 in the vitamin D group, an incidence of diabetes of 10% per year in the placebo group, a type I error rate of 0.0501 (with a single interim analysis taken into account), a power of 90%, a recruitment period of 2 years, a trial duration of 4 years, and a withdrawal rate of 5% per year of follow-up.19

Intention-to-treat analyses compared groups defined by the randomization procedure and included all participants irrespective of adherence to the assigned intervention or to the protocol (e.g., use of diabetes or weight-loss medications). Follow-up time for all analyses was calculated as the time from randomization until the occurrence of the primary outcome, death, withdrawal, or last follow-up encounter free from diabetes. No imputation was performed for missing data, but we conducted a sensitivity analysis to assess for noninformative censoring of incomplete data (see the Supplementary Appendix).

Because the use of a diabetes-specific medication would be considered a “competing event” for the primary outcome, we prespecified a sensitivity analysis in which the primary outcome was the time to new-onset diabetes according to trial criteria or use of a diabetes-specific medication. As planned in the protocol, we conducted an exploratory per-protocol analysis that censored follow-up data when a participant stopped the trial pills, started a diabetes or weight-loss medication, or took out-of-trial vitamin D from supplements above the trial limit of 1000 IU per day.

The protocol specified that this event-driven trial would continue until the required number of diabetes events (508) was reached. A prespecified interim analysis for the data and safety monitoring board to examine harm or superior efficacy with the use of a Haybittle–Peto boundary20 was conducted when approximately 70% of the required events (364 of 508) had accrued, and the data and safety monitoring board recommended that the trial proceed to its planned conclusion. Because the efficiency of event-driven trials is increased by stopping when the required number of events is achieved,21 we conducted blinded monitoring of event count and specified that when the trial was within approximately 2 months of reaching 508 events, the subsequent scheduled follow-up visit for each participant would be considered the last visit. All events that occurred during the trial, including those that occurred after the target of 508 events was reached, were used to generate the primary results.

Kaplan–Meier estimates were plotted for each group. Cox proportional-hazards models were used to calculate the hazard ratio for new-onset diabetes between the two groups.22 The model included group assignment as its main predictor variable and the stratification variables (trial site, BMI, and race). We also show a model without the stratification variables. Comparisons between the two groups at baseline and with respect to the rate of withdrawal, discontinuation of trial pills, use of diabetes or weight-loss medications, and supplemental intake above the trial limit used Fisher’s exact test, the chi-square test, the Wilcoxon rank-sum test, or the pooled-variance t-test.

Variability of response to vitamin D supplementation was assessed in prespecified subgroups defined by key baseline variables. Rates of adverse events were compared between the two groups. When evaluating the significance of the prespecified subgroup analyses, we used the Hochberg sequential procedure to adjust for multiple comparisons, if necessary. No adjustments were made for the safety analyses or the planned exploratory or post hoc analyses for the primary outcome; therefore, only point estimates and 95% confidence intervals are presented without P values.