Background Achieving desirable 25OHvitamin D serum levels with vitamin D supplementation may be influenced by many confounders including lifestyle patterns and vitamin D receptor genetic polymorphisms.

Objectives To test the effect of dietary intake confounding variables such total as calories, dietary intake of vitamin D, calcium, proteins, carbohydrates and fats, on the variance in serum 25-hydroxy vitamin D (25OHD) level after vitamin D supplementation.

Methods Study population: All subjects from one out of three centers who participated in a randomized clinical trial comparing high (3750 IU/day) versus low (600 IU/day) dose of vitamin D supplementation in ambulatory overweight (BMI exceeding 25) elderly subjects (age ≥65 years), with serum 25OHD level between 10 and 30 ng/ml at screening.

Outcome measure: 25OH D serum levels at 12 months of follow-up.

Confounders included gender, body mass index, baseline 25OHD serum level, genetic polymorphism of vitamin D receptor (Fok, BsmI and Taq) and dietary intake of vitamin D, calcium, proteins, carbohydrates, fats and total calories assessed by food frequency and 24h recall questionnaires, administered a year after the termination of the trial.

Statistical analysis: ANOVA for repeated measures and regression using STATA version 13. Numbers expressed as mean (SD).

Results We studied 117 Participants, 62 females and 55 males, age was 71 (5) years, of who 61 received 3,750 IU of vitamin D, and 55 received 600 IU/day. Vitamin D receptor genetic polymorphisms distribution was comparable in both arms. Daily vitamin D intake excluding provided supplements was 7.59 (1.21) mcg and 8.53 (1.36) mcg from the 24 h recall, and food frequency questionnaires, respectively (p was NS between the two arms). Calories, calcium, protein, carbohydrate and fat mean intake was also similar across both arms. The high-dose group achieved significantly higher 25OH-vitamin D serum levels at twelve months 34.9 (9.3) ng/ml compared to low-dose, 21.9 (6.1) ng/ml (p<0.0001). Difference between arms remained significant adjusting for gender, baseline BMI, Fok, Bsml and Taq genetic polymorphism, baseline serum vitamin D and dietary intake of vitamin D, calcium, calories, proteins, carbohydrates and fats. None of these dietary intake measures was found to have significant contribution to the model variance. Supplementation dose was the only significant predictor of 25OHvitamin D serum levels achieved at 12 months.

Conclusions Diet composition was not a confounder for the effect of high versus low dose vitamin D 12-month supplementation, on achieved 25OHD vitamin D serum levels.

Acknowledgements St Joseph University, Hotel-Dieu de France hospital

American University of Beirut Medical Centre

Lebanese National Council for Scientific Research

Lebanese Ministry of Social Affairs

Disclosure of Interest None declared