Effect of vitamin D replacement on indexes of insulin resistance in overweight elderly individuals: a randomized controlled trial

Abstract

Background: It is unclear whether and at what dose vitamin D supplementation affects insulin resistance (IR).

Objective: We sought to investigate whether vitamin D at doses higher than currently recommended decreases indexes of IR in an ambulatory population of overweight elderly subjects.

Design: This double-blind, randomized, controlled multicenter trial enrolled 257 elderly overweight individuals aged ≥65 y with baseline 25-hydroxyvitamin D [25(OH)D] concentrations between 10 and 30 ng/mL. All subjects received 1000 mg calcium citrate/d, with vitamin D administered weekly at an equivalent dose of 600 or 3750 IU/d. The homeostasis model assessment (HOMA) of IR index at 1 y was the primary outcome. We also assessed the McAuley index.

Results: In total, 222 subjects (55% women) with a mean ± SD age and body mass index (BMI; in kg/m(2)) of 71 ± 4 y and 30 ± 4, respectively, completed the study. Subjects' baseline characteristics, including IR indexes, were similar across groups: 69% had prediabetes, 54% had hypertension (47% were taking antihypertensive medications), and 60% had hyperlipidemia, nearly half of whom were receiving lipid-lowering drugs. At 1 y, mean ± SD serum 25(OH)D increased from 20 ± 7 to 26 ± 7 ng/mL in the low-dose arm (P < 0.0001) and from 21 ± 8 to 36 ± 10 ng/mL in the high-dose arm (P < 0.001). Median HOMA-IR indexes did not change compared with baseline concentrations and were similar in the high- [2.2 (IQR: 1.5, 2.9)] and low-dose [2.3 (IQR: 1.6, 3.3] treatment groups. Adjusted analyses showed that HOMA-IR was predicted by the baseline HOMA index and BMI but not by vitamin D dose, baseline serum 25(OH)D, or change in 25(OH)D.

Conclusion: Vitamin D3 at 3750 IU/d did not improve HOMA-IR compared with the Institute of Medicine Recommended Dietary Allowance of 600 IU/d in elderly overweight individuals. This trial was registered at clinicaltrials.gov as NCT01315366.

Keywords: HOMA; IOM RDA; high-dose vitamin D; insulin resistance; prediabetes.

© 2016 American Society for Nutrition.

Figures

FIGURE 1

CONSORT flow diagram detailing participant recruitment and retention status from prescreening to study completion. CONSORT, Consolidated Standards of Reporting Trials; DSMB, Data and Safety Monitoring Board; TSH, thyrotropin; vit D, vitamin D.

FIGURE 2

Subgroup analyses on the ΔlnHOMA by glucose tolerance status at entry (means ± SEMs). Impaired glucose tolerance: fasting blood sugar, >100 mg/dL; prediabetes: fasting blood sugar, 100–125 mg/dL or HbA1c 5.7–6.4%; and normal glucose tolerance: fasting blood sugar, P = independent t test for ΔlnHOMA (low compared with high dose). HbA1c, glycated hemoglobin.