Vitamin D Supplementation and Muscle Characteristics in Trained Subjects
Condition: Healthy Subjects, Trained
Study type: Interventional

Summary: 
      This study evaluates the effect of vitamin D supplementation on outcomes muscle performance
      and characteristics in 70 trained healthy individuals (18-40 years of age). Subjects will be
      allocated to ingest either vitamin D (35) or placebo (35) in a double-blinded fashion
    

Detailed description: 
      Vitamin D insufficiency compromises muscle functionality, leading to changes in muscle mass
      and strength, manifested at the cellular level. While this is increasingly recognized as a
      problem for individuals with vitamin D deficiency (typically 25(OH)D < 25 nmol/l), there is
      no general consensus for the remainder of the vitamin D-spectra. This lack of knowledge is
      probably linked to the variation in protocols of studies that have investigated the
      physiological role of vitamin D. These variations vary from aspects such as interventional vs
      observational studies to aspects related to subject characteristics such as age, health and
      training status. In this project, we will evaluate the effects of 12 weeks of vitamin D
      supplementation (2000 IU/day) on muscle performance and characteristics in 70 trained healthy
      individuals (18-40 years of age). Subjects will be allocated to either a vitamin D (35) or a
      placebo group (35) in a double-blinded manner. They will be instructed to continue their
      ordinary modes and frequency of training.

      We anticipate baseline vitamin D levels in blood (and hence changes in vitamin D obtained
      through supplementation), measured as 25(OH)D, to be a major determinant of the efficacy of
      the intervention. This means that we expect individuals with low baseline levels of 25(OH)D
      to display more pronounced changes in functional and biological outcome measures than those
      with high baseline values. Such a scenario could be related to an inability of our protocol
      to increase blood levels of 25(OH)D in individuals with higher physiological levels of
      vitamin D (which essentially means that ingestion of vitamin D3 is leveled out or exceeded by
      elimination of vitamin D derivatives). Alternatively, it may be related to inabilities of
      tissues to respond to the resulting elevation in vitamin D levels. To study the individual
      variation in vitamin D responses, data on both functional and biological variables will be
      divided into quartiles based on baseline 25(OH)D-levels, whereupon comparisons will be made
      between the low-end and high-end quartile. Individual variation will also be assessed using a
      mixed model approach.
    

Intervention name(s): Vitamin D3; Placebo
Intervention type(s): Dietary Supplement; Dietary Supplement
Intervention description(s): Vitamin D3 dissolved in olive oil, encapsuled; Olive oil, encapsuled
Other intervention name(s): cholecalciferol
Intervention MeSH terms: Vitamin D; Cholecalciferol
Allocation: Randomized
Intervention model: Parallel Assignment
Masking: Triple (Participant, Investigator, Outcomes Assessor)
Primary purpose: Treatment
Keywords: Muscle; Vitamin D; Strength; Endurance; Muscle fiber