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The authors’ affiliations are listed in the Appendix. Address reprint requests to Dr. Pittas at the Division of Endocrinolo‑ gy, Diabetes, and Metabolism, Tufts Medical Center, 800 Washington St., Box 268, Boston, MA 02111, or at apittas@ tuftsmedicalcenter . org.

*Deceased.

†A list of the members of the D2d Research Group is provided in the Supplementary Appendix, available at NEJM.org.

This article was published on June 7, 2019, and updated on July 18, 2019, at NEJM.org.

BACKGROUND Observational studies support an association between a low blood 25-hydroxyvitamin D level and the risk of type 2 diabetes. However, whether vitamin D supplementation lowers the risk of diabetes is unknown.

METHODS We randomly assigned adults who met at least two of three glycemic criteria for pre- diabetes (fasting plasma glucose level, 100 to 125 mg per deciliter; plasma glucose level 2 hours after a 75-g oral glucose load, 140 to 199 mg per deciliter; and glycated hemoglobin level, 5.7 to 6.4%) and no diagnostic criteria for diabetes to receive 4000 IU per day of vitamin D

3 or placebo, regardless of the baseline serum 25-hydroxyvitamin

D level. The primary outcome in this time-to-event analysis was new-onset diabetes, and the trial design was event-driven, with a target number of diabetes events of 508.

RESULTS A total of 2423 participants underwent randomization (1211 to the vitamin D group and 1212 to the placebo group). By month 24, the mean serum 25-hydroxyvitamin D level in the vitamin D group was 54.3 ng per milliliter (from 27.7 ng per milliliter at baseline), as compared with 28.8 ng per milliliter in the placebo group (from 28.2 ng per milliliter at baseline). After a median follow-up of 2.5 years, the primary outcome of diabetes oc- curred in 293 participants in the vitamin D group and 323 in the placebo group (9.39 and 10.66 events per 100 person-years, respectively). The hazard ratio for vitamin D as compared with placebo was 0.88 (95% confidence interval, 0.75 to 1.04; P = 0.12). The incidence of adverse events did not differ significantly between the two groups.

CONCLUSIONS Among persons at high risk for type 2 diabetes not selected for vitamin D insuffi- ciency, vitamin D

3 supplementation at a dose of 4000 IU per day did not result in a

significantly lower risk of diabetes than placebo. (Funded by the National Institute of Diabetes and Digestive and Kidney Diseases and others; D2d ClinicalTrials.gov number, NCT01942694.)

A B S T R A C T

Vitamin D Supplementation and Prevention of Type 2 Diabetes

Anastassios G. Pittas, M.D., Bess Dawson‑Hughes, M.D., Patricia Sheehan, R.N., M.P.H., M.S., James H. Ware, Ph.D.,*

William C. Knowler, M.D., Dr.P.H., Vanita R. Aroda, M.D., Irwin Brodsky, M.D., Lisa Ceglia, M.D., Chhavi Chadha, M.D., Ranee Chatterjee, M.D., M.P.H.,

Cyrus Desouza, M.B., B.S., Rowena Dolor, M.D., John Foreyt, Ph.D., Paul Fuss, B.A., Adline Ghazi, M.D., Daniel S. Hsia, M.D.,

Karen C. Johnson, M.D., M.P.H., Sangeeta R. Kashyap, M.D., Sun Kim, M.D., Erin S. LeBlanc, M.D., M.P.H., Michael R. Lewis, M.D., Emilia Liao, M.D.,

Lisa M. Neff, M.D., Jason Nelson, M.P.H., Patrick O’Neil, Ph.D., Jean Park, M.D., Anne Peters, M.D., Lawrence S. Phillips, M.D., Richard Pratley, M.D.,

Philip Raskin, M.D., Neda Rasouli, M.D., David Robbins, M.D., Clifford Rosen, M.D., Ellen M. Vickery, M.S., and Myrlene Staten, M.D.,

for the D2d Research Group†

Original Article

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V ita min D a nd Pr e v en tion of Di a be tes

More than 84 million adults in the United States have an increased risk of type 2 diabetes, based on a fasting glucose or glycated hemoglobin level above the normal range but below the threshold for diabe- tes.1 Persons at high risk for type 2 diabetes who are overweight or obese and who have elevated fasting glucose levels and glucose intolerance (according to a 75-g oral glucose-tolerance test) can slow progression to diabetes with lifestyle changes.2 However, achieving and maintaining sufficient lifestyle change is challenging, and the residual risk of diabetes remains elevated, even after successful weight loss.

Over the past decade, a low blood 25-hydroxy- vitamin D level has emerged as a possible risk factor for type 2 diabetes, and vitamin D supple- mentation has been proposed as a potential in- tervention to lower diabetes risk.3,4 The hypothe- sis that vitamin D status may influence the risk of type 2 diabetes is biologically plausible, be- cause both impaired pancreatic beta-cell func- tion and insulin resistance have been reported with low blood 25-hydroxyvitamin D levels.5 Ob- servational studies support an association be- tween a low blood 25-hydroxyvitamin D level and the risk of diabetes.6 In short-term mechanistic studies, vitamin D supplementation improved the disposition index, a measure of pancreatic beta-cell function, by 40%.7 However, whether vitamin D supplementation lowers the risk of diabetes is unclear.8-10 The Vitamin D and Type 2 Diabetes (D2d) trial was conducted to test whether vitamin D supplementation reduces the risk of type 2 diabetes among adults at high risk for the disorder.

M e t h o d s

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 plan- ning committee and the primary sponsor (Na- tional Institute of Diabetes and Digestive and Kidney Diseases) without input from manufac- turers11 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-cap- ture database. The statistical team at the coordi- nating center analyzed the data and vouches for its accuracy. All the authors vouch for the accu- racy 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 glu- cose 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 cri- teria 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 kilo- grams divided by the square of the height in meters) of 24 to 42 (22.5 to 42 for Asian Ameri- cans). A low serum 25-hydroxyvitamin D level was not an inclusion criterion.

Key exclusion criteria were any glycemic crite- rion 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 pri- marily on electronic-health-record identification of potentially eligible adults who were then

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screened in person and, if qualified, had a sec- ond 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 multivita- mins. 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 infor- mation 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 (tele- phone or email) took place. All visits and con- tacts were designed to promote retention, encour- age adherence to the trial regimen, and assess for diabetes, occurrence of adverse events, and use of high-dose vitamin D supplements, diabe- tes medications, and weight-loss medications.

Outcomes

The primary outcome in this time-to-event analy- sis was new-onset diabetes, based on annual glycemic testing of fasting plasma glucose, gly- cated hemoglobin, and 2-hour post-load plasma glucose and semiannual testing of fasting plas- ma 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 out- come. When only the measure for fasting plasma glucose or glycated hemoglobin met the thresh-

old, 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-toler- ance 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 out- come. A diagnosis of diabetes that was made outside the trial was validated by in-trial labora- tory 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 out- come. Safety was assessed by means of partici- pant report and annual fasting measurements of serum calcium, serum creatinine, and morning spot urine calcium:creatinine ratio (a rough esti- mate of urine calcium excretion).14

Laboratory Testing

Serum calcium and creatinine were analyzed lo- cally 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 chroma- tography method certified by the National Glyco- hemoglobin Standardization Program.16 Plasma glucose was measured with the use of a hexoki- nase 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 vali- dated 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 equal- ly 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 analy-

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V ita min D a nd Pr e v en tion of Di a be tes

sis taken into account), a power of 90%, a re- cruitment 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 in- cluded 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 occur- rence 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 medi- cation would be considered a “competing event” for the primary outcome, we prespecified a sen- sitivity analysis in which the primary outcome was the time to new-onset diabetes according to trial criteria or use of a diabetes-specific medica- tion. 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 medi- cation, or took out-of-trial vitamin D from sup- plements 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 pre- specified interim analysis for the data and safety monitoring board to examine harm or superior efficacy with the use of a Haybittle–Peto bound- ary20 was conducted when approximately 70% of the required events (364 of 508) had accrued, and the data and safety monitoring board rec- ommended 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 speci- fied 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 oc- curred 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 Wil- coxon rank-sum test, or the pooled-variance t-test.

Variability of response to vitamin D supple- mentation was assessed in prespecified subgroups defined by key baseline variables. Rates of ad- verse 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 mul- tiple 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.

R e s u l t s

Participants

From October 2013 through February 2017, a to- tal of 7133 persons were screened (Fig. 1), and 2423 were randomly assigned to receive vitamin D (1211 participants) or placebo (1212 participants); these participants were included in the intention- to-treat population (Table 1, and Table S1 in the Supplementary Appendix). A total of 44.8% of the participants were women, 33.3% were of non- white race, and 9.3% were of Hispanic ethnic background.24 The participants had a mean age of 60.0 years, a mean BMI of 32.1, and a mean glycated hemoglobin level of 5.9% (48 mmol per mole). A total of 84.2% of the participants met the glycemic criteria for both fasting plasma glucose and glycated hemoglobin; approximately one third met all three glycemic criteria.

The last trial encounter was in November 2018. In the two groups, the median follow-up was 2.5 years (interquartile range, 1.9 to 3.5 [vitamin D] and 1.7 to 3.5 [placebo]). Before reaching a pri-

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mary outcome event, 10 participants (5 in each group) died, and 62 (34 in the vitamin D group and 28 in the placebo group) withdrew consent (Fig. 1). In total, 99.1% of the cohort (1201 par- ticipants in the vitamin D group and 1199 in the placebo group) contributed follow-up data, through either a visit that included central-labo- ratory testing or a nonvisit encounter to capture a diagnosis of diabetes outside the trial.

The mean baseline level of serum 25-hydroxy- vitamin D was 28.0 ng per milliliter (69.9 nmol per liter), with no significant difference between the two groups; 78.3% of the participants had a level equal to or greater than 20 ng per milliliter (50 nmol per liter) (Table 1). The mean 25-hydroxy- vitamin D levels in the vitamin D group at month 12 (52.3 ng per milliliter [130.5 nmol per liter]) and month 24 (54.3 ng per milliliter [135.5 nmol per liter]) were higher than those in the placebo group (28.1 ng per milliliter [70.1 nmol per liter] and 28.8 ng per milliliter [71.9 nmol per liter], respectively) (Fig. S1 in the Supplementary Appendix).

Primary Outcome

By the end of the trial, diabetes had developed in 616 patients. New-onset diabetes (the primary outcome) occurred in 293 participants (273 cases diagnosed by trial-specific laboratory testing and 20 diagnosed by adjudication) in the vitamin D group and 323 patients (305 cases diagnosed by trial-specific laboratory testing and 18 diagnosed by adjudication) in the placebo group (9.39 events and 10.66 events per 100 person-years, respec- tively). The hazard ratio in the vitamin D group was 0.88 (95% confidence interval [CI], 0.75 to 1.04; P = 0.12) (Fig. 2). When the stratification variables were not included in the model, the hazard ratio in the vitamin D group was 0.87 (95% CI, 0.75 to 1.02). In a sensitivity analysis to account for missing data, the hazard ratio did not change substantially (see the Supplementary Appendix).

In the sensitivity analysis in which diabetes was defined as new-onset diabetes according to trial criteria or the use of a diabetes-specific medication, the hazard ratio in the vitamin D group was 0.88 (95% CI, 0.75 to 1.02). The re- sults of the subgroup analyses were consistent with the findings of the main analysis; there was no apparent heterogeneity of treatment ef- fect across the prespecified subgroups (Fig. 3).

Figure 1. Screening, Randomization, and Follow-up.

One participant in the vitamin D group was withdrawn administratively after a clinical site closed down early in the trial. Protocol‑specified adverse events that led to discontinuation of the trial pills were hypercalcemia, a fasting urine calcium:creatinine ratio of more than 0.375, a low estimated glomerular filtration rate, and nephrolithiasis. Of the 2423 participants who underwent randomization, 14 (9 in the vitamin D group and 5 in the placebo group) were subsequently found not to meet all eligibility criteria.

2423 Underwent randomization

7133 Persons were assessed for eligibility

4710 Did not meet eligibility criteria 3850 Did not meet glycemic

criteria 106 Used supplements contain-

ing vitamin D at dose of >1000 IU/day or calcium at dose of >600 mg/day

159 Had abnormal laboratory result

223 Withdrew consent or were not interested

372 Had other reason

1211 Were assigned to receive 4000 IU of vitamin D daily

1211 Received vitamin D

1212 Were assigned to receive placebo daily

1211 Received placebo

1201 Completed ≥1 follow-up encounter

1199 Completed ≥1 follow-up encounter

1131 Met primary outcome, died, or completed last follow-up encounter

1130 Met primary outcome, died, or completed last follow-up encounter

1211 Were included in the intention- to-treat analysis

1212 Were included in the intention- to-treat analysis

3 Had no contact after randomi- zation

5 Died 34 Withdrew 1 Was withdrawn administratively

137 Discontinued trial pills 27 Had protocol-specified adverse

event 20 Had other adverse event 86 Chose to discontinue 4 Had other reason

5 Had no contact after randomi- zation

5 Died 28 Withdrew

108 Discontinued trial pills 24 Had protocol-specified adverse

event 13 Had other adverse event 66 Chose to discontinue 5 Had other reason

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In a post hoc analysis of data from participants with a baseline 25-hydroxyvitamin D level of less than 12 ng per milliliter (30 nmol per liter) (103 participants), the hazard ratio in the vitamin D group was 0.38 (95% CI, 0.18 to 0.80). Among those with a baseline 25-hydroxyvitamin D level equal to or greater than 12 ng per milliliter (2319 participants), the hazard ratio in the vita- min D group was 0.92 (95% CI, 0.78 to 1.08).

Adherence

A total of 170 participants (14.0%) in the vita- min D group and 172 (14.2%) in the placebo group stopped trial pills, took diabetes or weight- loss medications, or took outside-of-trial vita-

min D supplements above the trial limit before the diagnosis of diabetes. During the trial, more participants in the placebo group than in the vitamin D group started diabetes or weight-loss medications (Fig. S2 in the Supplementary Ap- pendix). Although overall adherence to the trial regimen was high (85.8% of prescribed pills were taken), more participants in the vitamin D group (11.3%) than in the placebo group (8.9%) stopped trial pills (difference, 2.4 percentage points; 95% CI, 0.0 to 4.8) (Fig. S3 in the Sup- plementary Appendix). During follow-up, more participants in the placebo group (5.2%) than in the vitamin D group (2.6%) reported use of outside-of-trial vitamin D supplements above the

Characteristic Overall

(N = 2423) Vitamin D (N = 1211)

Placebo (N = 1212)

Demographic

Age — yr 60.0±9.9 59.6±9.9 60.4±10.0

Female sex — no. (%) 1086 (44.8) 541 (44.7) 545 (45.0)

Race — no. (%)†

Asian 130 (5.4) 66 (5.5) 64 (5.3)

Black 616 (25.4) 301 (24.9) 315 (26.0)

White 1616 (66.7) 810 (66.9) 806 (66.5)

Other 61 (2.5) 34 (2.8) 27 (2.2)

Hispanic or Latino ethnic group — no. (%)† 225 (9.3) 120 (9.9) 105 (8.7)

Body‑mass index 32.1±4.5 32.0±4.5 32.1±4.4

Laboratory assessments

Fasting plasma glucose — mg/dl 107.9±7.4 108.0±7.4 107.8±7.4

2‑Hr post‑load plasma glucose — mg/dl 137.2±34.3 136.9±34.3 137.6±34.3

Glycated hemoglobin — % 5.9±0.2 5.9±0.2 5.9±0.2

Serum 25‑hydroxyvitamin D

Mean — ng/ml 28.0±10.2 27.7±10.2 28.2±10.1

Distribution — no./total no. (%)‡

<12 ng/ml 103/2422 (4.3) 60/1211 (5.0) 43/1211 (3.6)

12–19 ng/ml 422/2422 (17.4) 216/1211 (17.8) 206/1211 (17.0)

20–29 ng/ml 876/2422 (36.2) 453/1211 (37.4) 423/1211 (34.9)

≥30 ng/ml 1021/2422 (42.2) 482/1211 (39.8) 539/1211 (44.5)

* Plus–minus values are means ±SD. Percentages may not total 100 because of rounding. To convert the values for glu‑ cose to millimoles per liter, multiply by 0.05551. To convert the values for 25‑hydroxyvitamin D to nanomoles per liter, multiply by 2.496.

† Race and ethnic group were reported by the participant. The category “other” includes American Indian or Alaska Native; Native Hawaiian or other Pacific Islander; and other race. Ethnic group includes any race.

‡ Categories of serum 25‑hydroxyvitamin D are based on the 2010 Dietary Reference Intakes for calcium and vitamin D recommended by the Food and Nutrition Board of the Institute of Medicine.23

Table 1. Baseline Characteristics of the Participants.*

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trial limit of 1000 IU per day (difference, 2.6 percentage points; 95% CI, 1.1 to 4.2) (Fig. S2 in the Supplementary Appendix). There was no sig- nificant difference between the two groups in the use of outside-of-trial calcium supplements above the trial limit.

In the exploratory per-protocol analysis that censored follow-up data when a participant start- ed a diabetes or weight-loss medication, stopped the trial pills, or took out-of-trial vitamin D from supplements above the trial limit of 1000 IU per day, the primary outcome occurred in 265 par- ticipants (21.9%) in the vitamin D group and 304 (25.1%) in the placebo group (hazard ratio, 0.84; 95% CI, 0.71 to 1.00).

Safety

There were no significant between-group dif- ferences in the protocol-specified adverse events of interest: hypercalcemia, a fasting urine calcium:creatinine ratio of more than 0.375, a low estimated glomerular filtration rate, and

nephrolithiasis (Table 2). Overall, 47 participants (3.9%) in the vitamin D group stopped the trial pills because of an adverse event, as compared with 37 (3.1%) in the placebo group (difference, 0.8 percentage points; 95% CI, −0.7 to 2.3).

D i s c u s s i o n

In this multicenter, randomized, placebo-con- trolled trial involving persons at high risk for type 2 diabetes not selected for vitamin D insuf- ficiency, vitamin D

3 supplementation at a dose of

4000 IU per day did not result in a significantly lower risk of diabetes than placebo after a me- dian follow-up of 2.5 years.

While our trial was being conducted, two other trials that were designed to test whether vitamin D supplementation lowers the risk of type 2 diabetes among persons at risk showed hazard ratios with vitamin D that were similar to those in our trial.25,26 In the Tromsø Vitamin D and T2DM Trial (Norway), which randomly as- signed 511 white adults with prediabetes to 20,000 IU per week (approximately 2900 IU per day) of vitamin D

3 or placebo, the risk of diabe-

tes was numerically lower in the vitamin D group than in the placebo group, but the difference was not significant (hazard ratio, 0.90; 95% CI, 0.69 to 1.18).25 In the Diabetes Prevention with Active Vitamin D study (Japan), which randomly assigned 1256 adults with prediabetes to an ac- tive form of vitamin D analogue (eldecalcitol) or placebo, the risk of diabetes was also lower in the vitamin D group than in the placebo group, but the difference was again not signifi- cant (hazard ratio, 0.87; 95% CI, 0.68 to 1.09).27 We powered our trial to detect a 25% lower risk of diabetes with vitamin D than with placebo. On the basis of the results from all three trials, vitamin D supplementation may decrease diabe- tes risk among persons at risk for diabetes not selected for vitamin D insufficiency by a smaller effect size (10 to 15%), but none of these trials were powered to test this effect size.

Our trial has several strengths. We used con- temporary glycemic criteria to assemble a diverse cohort at high risk for diabetes with a hypergly- cemic pattern closely matching how prediabetes is diagnosed in clinical practice, most commonly with fasting plasma glucose and glycated hemo- globin. The vitamin D dose of 4000 IU per day

Figure 2. Kaplan–Meier Curves for Survival Free from Diabetes among Adults at Risk for Type 2 Diabetes.

The hazard ratio for new‑onset diabetes between the vitamin D group and the placebo group is derived from Cox regression, with stratification accord‑ ing to trial site, body‑mass index, and race.

P ro

b ab

ili ty

o f

D ia

b et

es -f

re e

S u

rv iv

al 1.0

0.8

0.9

0.7

0.6

0.4

0.3

0.1

0.5

0.2

0.0 0 12 18 24 30 42 54

Months since Randomization

Hazard ratio for diabetes, 0.88 (95% CI, 0.75–1.04) P=0.12

No. at Risk Vitamin D Placebo

1211 1212

1089 1091

1171 1171

1001 975

812 779

625 577

283 258

466 419

21 13

141 121

Placebo

Vitamin D

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V ita min D a nd Pr e v en tion of Di a be tes

was selected to balance safety and efficacy and resulted in a large difference in the serum 25-hydroxyvitamin D level between the trial groups in the first 2 years of follow-up. In 94% of cases, the primary outcome was ascertained by trial-specific laboratory testing based on cur- rent ADA criteria and required two tests in the diabetes range for diagnosis. Our cohort was recruited at a constant rate throughout the cal-

endar year, which reduced the potential of con- founding by seasonal variability. Finally, the ob- served rate of new-onset diabetes in the placebo group (10.7 events per 100 person-years) was consistent with our estimate of 10 events per 100 person-years.

Overall adherence was high, and overall use of off-protocol concomitant therapies was low. However, among nonadherent participants, more

Figure 3. Prespecified Subgroup Analyses.

Participants met at least two of three glycemic criteria for prediabetes: 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) (impaired glucose tolerance); and glycated hemoglobin level, 5.7 to 6.4% (39 to 47 mmol per mole). To convert the values for 25‑hydroxy‑ vitamin D to nanomoles per liter, multiply by 2.496.

0.75 1.00 1.501.25

Placebo BetterVitamin D Better

Serum 25-hydroxyvitamin D

<20 ng/ml

≥20 ng/ml

Race

White

Black

Other

Glycemic criteria for prediabetes

Met all three criteria

Met two criteria

Body–mass index

<30

≥30

Impaired glucose tolerance

Yes

Ethnic group

Hispanic

Non–Hispanic

Sex

Female

Male

Waist circumference

<Median of 104.2 cm

≥Median of 104.2 cm

Age

<Median of 60.9 yr

≥Median of 60.9 yr

Geographic location

At or above 37° north latitude

Below 37° north latitude

Calcium intake from supplements

No intake

Any intake

Vitamin D Hazard Ratio for Diabetes (95% CI)

PlaceboSubgroup

0.50

73/276

220/935

207/810

64/301

22/100

143/427

150/784

82/435

211/776

191/604

102/607

36/120

257/1091

131/541

162/670

127/620

166/591

158/622

135/589

205/892

88/319

198/826

95/385

66/249

256/962

227/806

69/315

27/91

163/429

160/783

105/429

218/783

215/635

108/577

27/105

296/1107

127/545

196/667

135/585

188/627

153/587

170/625

235/898

88/314

216/793

107/419

0.87 (0.61–1.22)

0.89 (0.74–1.06)

0.90 (0.75–1.09)

0.83 (0.58–1.18)

0.86 (0.48–1.56)

0.86 (0.68–1.09)

0.90 (0.72–1.13)

0.71 (0.53–0.95)

0.97 (0.80–1.17)

0.92 (0.75–1.12)

0.86 (0.65–1.13)

1.14 (0.68–1.92)

0.86 (0.72–1.02)

0.98 (0.77–1.26)

0.82 (0.66–1.01)

0.82 (0.64–1.05)

0.95 (0.76–1.17)

0.97 (0.77–1.21)

0.80 (0.64–1.01)

0.85 (0.70–1.03)

0.97 (0.72–1.32)

0.81 (0.66–0.98)

1.05 (0.79–1.40)

no. of events/no. of participants

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n engl j med 381;6 nejm.org August 8, 2019528

T h e n e w e n g l a n d j o u r n a l o f m e d i c i n e

in the placebo group started diabetes or weight- loss medications and took outside-of-trial vita- min D supplements above the trial limit, whereas more in the vitamin D group stopped the trial pills for any reason. Whether these differences among nonadherent participants shifted the risk difference between the two groups toward or away from null in the intention-to-treat or per- protocol analysis is unknown.

Response to a nutritional intervention de- pends on nutritional status at baseline; thus, if vitamin D has an effect on diabetes prevention, persons with a higher baseline level of serum 25-hydroxy vitamin D would be expected to have less effect from supplementation than those with a lower baseline level.28 Owing to ethical and practical considerations, a lack of consen- sus on the preferred 25-hydroxyvitamin D level, and our desire to maximize the external valid- ity of the trial, we specifically did not include serum 25-hydroxyvitamin D as an eligibility criterion. Because vitamin D supplements are used increasingly in the U.S. adult popula- tion,29 approximately 8 of 10 participants had a baseline serum 25-hydroxy vitamin D level that was considered to be sufficient according to current recommendations (≥20 ng per milliliter) to reduce the risk of many outcomes,23,30 includ- ing diabetes.6 The high percentage of partici- pants with adequate levels of vitamin D may have limited the ability of the trial to detect a significant effect.

The vitamin D dose of 4000 IU per day is the recommended upper intake level to avert poten- tial toxicity,23 although data from large trials on the safety on this dose have been scant. There is concern that 25-hydroxyvitamin D levels above 50 ng per milliliter (125 nmol per liter) may be associated with adverse effects.23,30 In our trial, vitamin D

3 supplementation at a dose of 4000 IU

per day resulted in no significant differences be- tween the two groups in the protocol-specified adverse events of interest (hypercalcemia, a fasting urine calcium:creatinine ratio of >0.375, a low estimated glomerular filtration rate, and nephro- lithiasis). The 24-hour urine calcium level was not measured.31

In conclusion, among persons at high risk for type 2 diabetes not selected for vitamin D insuf- ficiency, vitamin D

3 supplementation at a dose of

4000 IU per day did not result in a significantly lower risk of diabetes than placebo.T

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n engl j med 381;6 nejm.org August 8, 2019 529

V ita min D a nd Pr e v en tion of Di a be tes

The views expressed in this article are those of the authors and do not necessarily represent the views of the National Insti- tutes of Health.

A data sharing statement provided by the authors is available with the full text of this article at NEJM.org.

The planning phase of the D2d trial was funded by the Na- tional Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) through a multicenter clinical study implementation planning grant to Tufts Medical Center in Boston (U34DK091958; principal investigator, Dr. Pittas). Planning was also supported, in part, by the Intramural Research Program of the NIDDK. The conduct of the trial was supported primarily by the NIDDK and the Office of Dietary Supplements of the National Institutes of Health through the multicenter clinical study cooperative agree- ment (U01DK098245; principal investigator, Dr. Pittas) to Tufts Medical Center, where the D2d Coordinating Center is based. The U01 grant mechanism establishes the NIDDK project scien- tist (Dr. Staten) as a member of the D2d Research Group. The trial also received secondary funding from the American Diabe- tes Association to Tufts Medical Center (1-14-D2d-01; principal investigator, Dr. Pittas). Educational materials were provided by the National Diabetes Education Program.

Dr. Dawson-Hughes reports receiving grant support, paid to Tufts University, from Pfizer and DSM and travel support from Abiogen Pharma; Dr. Aroda, receiving consulting fees, paid to her institution, from Adocia, grant support, paid to her institu- tion, and consulting fees from AstraZeneca/Bristol-Myers Squibb, Novo Nordisk, and Sanofi, consulting fees from BD and Zafgen, and grant support, paid to her institution, from Calibra Medical, Eisai, Janssen, and Theracos; Dr. Ceglia, receiving grant support from DSM; Dr. Desouza, receiving advisory board

fees from Novo Nordisk; Dr. Kim, receiving consulting fees from Sanofi; Dr. LeBlanc, receiving grant support, paid to her institution, from Merck; Dr. Neff, receiving grant support from GI Dynamics; Dr. O’Neil, receiving grant support from Weight Watchers International, advisory board fees from Janssen, advi- sory board fees and fees for presentations from Vindico Medical Education, fees for CME programs from WebMD, lecture fees from Robard, and grant support and lecture fees from Novo Nordisk; Dr. Phillips, receiving grant support and advisory board fees from Janssen Pharmaceuticals, receiving advisory board fees from Profil Institute for Clinical Research, receiving grant support from Merck, Amylin Pharmaceuticals, Eli Lilly, Novo Nordisk, Sanofi, PhaseBio, Roche, AbbVie, Vascular Phar- maceuticals, GlaxoSmithKline, Pfizer, and Kowa Research In- stitute, and serving as cofounder, officer, and board member of and holding stock in Diasyst; and Dr. Pratley, receiving lecture fees, paid to his institution, and consulting fees, paid to his in- stitution, from AstraZeneca, consulting fees, paid to his institu- tion, from Boehringer Ingelheim, Eisai, GlaxoSmithKline, Gly- tec, Janssen, Mundipharma, and Pfizer, grant support, paid to his institution, from Lexicon Pharmaceuticals, grant support, paid to his institution, and consulting fees, paid to his institu- tion, from Ligand Pharmaceuticals, Eli Lilly, Merck, and Sanofi, grant support, paid to his institution, and lecture fees, paid to his institution, from Novo Nordisk and Takeda, and consulting fees from Sanofi US Services. No other potential conflict of in- terest relevant to this article was reported.

Disclosure forms provided by the authors are available with the full text of this article at NEJM.org.

We thank the D2d investigators, staff, and trial participants for their dedication and commitment to the trial.

Appendix The authors’ affiliations are as follows: Tufts Medical Center (A.G.P., L.C., P.F., J.N., E.M.V.), the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University (B.D.-H.), Brigham and Women’s Hospital (V.R.A.), and Harvard School of Public Health (J.H.W.), Boston, and the Spaulding Rehabilitation Network, Charlestown (P.S.) — all in Massachusetts; National Institute of Diabetes and Digestive and Kidney Diseases, Phoenix, AZ (W.C.K.); the Maine Medical Center (I.B.) and the Maine Medical Center Research Institute (C.R.) — both in Scarborough; HealthPartners Institute, Minneapolis (C.C.); Duke University Medical Center, Durham, NC (R.C., R.D.); the University of Nebraska Medical Center and Omaha Veterans Affairs Medical Center, Omaha (C.D.); Baylor College of Medicine, Houston (J.F.), and the University of Texas Southwestern Medical Center, Dallas (P.R.) — both in Texas; MedStar Good Sa- maritan Hospital, Baltimore (A.G.), MedStar Health Research Institute, Hyattsville (J.P.), and the National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda (M.S.) — all in Maryland; Pennington Biomedical Research Center, Baton Rouge, LA (D.S.H.); the University of Tennessee Health Science Center, Memphis (K.C.J.); Cleveland Clinic, Cleveland (S.R.K.); Stanford University Medical Center, Stanford (S.K.), and the Keck School of Medicine of the University of Southern California, Los Angeles (A.P.) — both in Cali- fornia; Kaiser Permanente Center for Health Research–Northwest, Portland, OR (E.S.L.); the University of Vermont, Burlington (M.R.L.); Northwell Health Lenox Hill Hospital, New York (E.L.); Northwestern University, Chicago (L.M.N.); the Medical University of South Carolina, Charleston (P.O.); Emory University School of Medicine, Atlanta, and the Atlanta Veterans Affairs Medical Center, Decatur — both in Georgia (L.S.P.); AdventHealth Translational Research Institute for Metabolism and Diabetes, Orlando, FL (R.P.); the Uni- versity of Colorado Denver and the Veterans Affairs Eastern Colorado Health Care System, Denver (N.R.); and the University of Kansas Medical Center, Kansas City (D.R.).

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