Table 1

Characteristics of randomized controlled trial studies included in the meta-analysis

Author (year)Study locationN subjects (Tx/Px)SexEthnicityBMI (kg/m2)Health statusVitamin D levels at baseline (nmol/L)Vitamin D levels after intervention (nmol/L)Type of vitamin D; mode of delivery; dose (IU/day)Vitamin D analytical measuresDuration (months)Outcome measured
Mitri et al. (2011)U.S.23/24M/FMultiethnic≥25 (≥23 if Asian)Prediabetes (IGT, IFG, HbA1c ≥5.8%)T 59.9 ± 2.7,§ P 60.4 ± 2.7§T 15.7 ± 3.7§ (change from baseline), P −20.5 ± 3.5§Vitamin D3; oral; 2,000LC-MS4Insulin sensitivity index (FIVGTT), 2-h glucose (OGTT)
Harris et al. (2012)U.S.43/46M/FAfrican American25–39.9Prediabetes or early T2D (HbA1c 6.7–7%), baseline 25(OH)D <50 nmol/LT 39.6 ± 12.9, P 38.2 ± 15.5T 41.6 ± 3.1§ (change from baseline), P 0.9 ± 2.9§Vitamin D3; oral; 4,000RIA3Matsuda index, 2-h glucose (OGTT)
Iraj et al. (2012)Iran20/20M/FMiddle Eastern≥25First-degree relatives with T2D, 25(OH)D <75 nmol/L, prediabetesT 27.5 ± 15.0, P 28.2 ± 15.0T 87.4 ± 49.9, P 38.7 ± 54.9Vitamin D3; injection; 10,000CLIA2Matsuda index, AUCglucose (OGTT)
Davidson et al. (2013)U.S.56/53M/FMultiethnic (Hispanic and African American)≥25Prediabetes, 25(OH)D <75 nmol/LT 54.9 ± 12.5, P 54.9 ± 12.5T 169.6 ± 37.4, P 54.9 ± 17.4Vitamin D3; oral; 12,695LC-MS3Matsuda index, 2-h glucose
Oosterwerff et al. (2014)Netherlands53/57M/FMultiethnic (Morocco, Suriname, Turkey)≥25IFG or IGT, 25(OH)D <50 nmol/LT 25.0 ± 10.8, P 22.0 ± 11.0T 60.0 ± 16.0, P 23.0 ± 15.0Vitamin D3; oral; 1,200LC-MS4Insulin sensitivity index, 2-h glucose
Gagnon et al. (2014)Australia35/45M/FMultiethnic25–4025(OH)D <50 nmol/L, prediabetesT 47.0 ± 13.0, P 43.2 ± 13.0T 89.1 ± 16.5, P 41.4 ± 16.3Vitamin D3; oral; 2,000CLIA6Matsuda index
Kampmann et al. (2014)Denmark7/8M/FNR≥25T2D, 25(OH)D <50 nmol/LT 31.0 ± 4.9,§ P 34.8 ± 3.8§T 104.9 ± 19.0,§ P 32.1 ± 3.8§Vitamin D3; oral; 6,400ELISA3M value derived from clamp
Mitchell et al. (2015)U.S.40/50M/FMultiethnicMedian 2525(OH)D <50 nmol/LT 44.9 ± 17.5, P 44.9 ± 17.5T 107.3 ± 29.9, P 49.9 ± 24.9Vitamin D2; oral; 7,142LC-MS3Insulin sensitivity index (FIVGTT)
Tuomainen et al. (2015)FinlandT1/T2/P (24/21/21)M/FNR≥25Prediabetes, 25(OH)D <75 nmol/LMean baseline T + P 57.0 ± 11.0T1 27.7 ± 17.2 (change from baseline)(T1) Vitamin D3; oral; 1,600HPLC-CEAD5Matsuda index
T2 45.0 ± 23.4 (change from baseline), P 4.1 ± 17.3(T2) Vitamin D3; oral; 3,200
Wagner et al. (2016)Sweden21/22M/FNR≤32Prediabetes or drug-naive diabetes, HbA1c ≤7.9%, FPG <9 mmol/L, 25(OH)D <75 nmol/LT median 43.0 (IQR 36.0–50.0), P 43.0 (37.0–54.0)Median change T 42.0 (IQR 32.0–50.0), P 0.0 (−7.0 to 11.0)Vitamin D3; oral; 4,285CLIA2GIR derived from clamp
Yeow et al. (2015)Malaysia13/13FAsian23–31History of GDM during last pregnancy 6–48 months postpartum; hypovitaminosis (25(OH)D 15–50 nmol/L)T median 35.6 (IQR 25.6–43.9), P 35.1 (21.6–40.7)Median change T 51.1 (IQR 39.9–76.1), P −0.2 (−10.18 to 11.8)Vitamin D3; oral; 4,000ECLIA6Insulin sensitivity index, 2-h glucose
Barengolts et al. (2015)U.S.87/86MAfrican American28–39Fasting glucose 5.3–6.9 mmol/L, HbA1c 5.7–6.4%, 25(OH)D 12.5–75 nmol/LT 36.7 ± 11.7, P 34.9 ± 12.0T 120.1 ± 45.9, P 49.7 ± 18.2Vitamin D2; oral; 7,142CLIA12Matsuda index
Garg et al. (2015)India15/17FAsian≥23PCOS, 25(OH)D <50 nmol/LP 16.9 ± 6.1T 78.6 ± 34.6, P 16.7 ± 5.8Vitamin D3; oral; 4,000CLIA6Matsuda index, AUCglucose
Mousa et al. (2017)Australia28/26M/FMultiethnic≥2525(OH)D <50 nmol/LT 31.4 ± 12.6, P 34.2 ± 10.0T 88.4 ± 21.0, P 36.1 ± 15.3Vitamin D3; oral; 4,000CLIA4M value derived from clamp
Gulseth et al. (2017)Norway33/29M/FMultiethnic (Nordic and South Asian)<45T2D, 25(OH)D <50 nmol/LT 38.0 ± 11.9, P 36.8 ± 12.6T 53.7 ± 9.2, P 38.2 ± 12.9Vitamin D3; injection and oral; 3,333RIA6GIR derived from clamp
Lerchbaum et al. (2017)Austria49/49MNRMedian 2525(OH)D <75 nmol/LT median 52.0 (IQR 42.0–65.0), P 51.0 (43.0–68.0)Median T 107.0 (IQR 89.0–119.0), P 69.0 (46.0–79.0)Vitamin D3; oral; 2,857LC-MS3Matsuda index, AUCglucose
Moreira-Lucas et al. (2017)Canada36/35M/FMultiethnic<4025(OH)D ≤65 nmol/L, HbA1c 5.4%–6.4%T 48.1 ± 14.3, P 47.6 ± 14.3Mean change T 50.6 (95% CI 36.7, 64.6), P −2.11 (−6.11, 1.89)Vitamin D3; oral; 4,000LC-MS6Matsuda index, AUCglucose
Cefalo et al. (2018)Italy9/9M/FNR>25BMI ≥30 kg/m2, 25(OH)D <75 nmol/LT 36.7 ± 13.2, P 34.7 ± 21.1T 74.8 ± 18.7, P 41.7 ± 7.7Vitamin D3; oral; 3,571NR3Insulin-mediated glucose uptake, AUCglucose (from OGTT)
  • CLIA, chemiluminescence immunoassay; ECLIA, electrochemiluminescence immunoassay; FPG, fasting plasma glucose; FIVGTT, frequently sampled intravenous glucose tolerance test; HPLC-CEAD, high-performance liquid chromatography-coulometric electrode array detector; IFG, impaired fasting glucose; IGT, impaired glucose tolerance; IQR, interquartile range; LC-MS, liquid chromatography–mass spectrophotometry; M, male; F, female; NR, not reported; P, placebo; RIA, radioimmunoassay; Tx/Px, treatment/placebo group; T, treatment; T2D, type 2 diabetes.

  • § Data are means ± SE.