4. Discussion
The systematic review and meta-analysis of 11 RCTs with more than 900 subjects lasting between 1 to 12 months after cholecalciferol supplementation (from 25,000 to 600,000 IU monthly), revealed that cholecalciferol has a desirable effect on weight loss by reducing BMI and waist circumference in overweight and obese individuals, while no data was found on waist hip ratio and the other body compositions, such as fat mass and fat percentage.
From a theoretical point of view, there are many mechanisms reported in the literature that claim to explain the process through which cholecalciferol supplementation supports the weight loss process [17].
A recent study has reported that cholecalciferol has physiological and biochemical effect in a way that reduces metabolic abnormalities and tissue damage that can result from adiposity [7].
Cholecalciferol has a direct role in suppressing the PTH hormone, which promotes and triggers fat accumulation in the adipose tissue via increasing intracellular calcium [5].
A recent study reported that cholecalciferol supports intestinal calcium absorption which assists in weight loss [6]. Another suggested mechanism states that cholecalciferol stimulates insulin receptors and is responsible for maintaining the calcium homeostasis, which is an important factor for intracellular mediated processes. Increasing body size is the accumulative consequence of the claimed association between cholecalciferol and insulin resistance [10].
Some studies showed that taking Ca supplements in addition to cholecalciferol increased the inverse relationship and decreased the fat mass that was attributed to calcium metabolism. It is believed that a calcium-rich diet increases fat oxidation, promotes fat cell apoptosis and decreases lipid absorption through the process of insoluble calcium-fatty acid soap formation in the intestine. In addition, the presences of calcium in a diet leads to suppressing 25(OH)VitD levels, which in turn decreases the calcium influx into the cell and eventually triggers the lipolysis process and suppresses lipogenesis in the adipocyte [18,19].
It is noteworthy to highlight and emphasize the results of this meta-analysis and apply them in clinical practice. 25(OH)VitD levels must be taken into consideration as a strong factor that contributes to weight gain among overweight and obese people. Prescribing cholecalciferol and following an effective strategy for cholecalciferol supplementation should be an imperative practice, especially for overweight and obese individuals
One of the main strengths in this meta-analysis is the design of the studies that were analyzed, as all used randomized, placebo-controlled designs that led to causative conclusions.
In addition, this meta-analysis investigated the effect of vitamin D on weight loss using a comprehensive approach with holistic anthropometric measurements of percentage and fat mass to measure the differences before and after intervention. This approach did not only depend on just a single variable, which enhances and promotes the study characteristics. Moreover, one article discussed the effect of cholecalciferol combined with another intervention, such as dietary control, exercise, or calcium intake, which makes it difficult to make conclusive remarks to distinguish an independent vitamin D supplementation clinical effect for weight loss purposes [19].
Moreover, few clinical trials of cholecalciferol supplementation on body composition were done using a combination of calcium and cholecalciferol supplementation. Previous studies have demonstrated that cholecalciferol supplementation positively influences BMI, weight and waist circumference [15,16,20].
However, the adequate dosages and duration of cholecalciferol supplementation are still unclear. Therefore, further extensive clinical trials involving larger sample sizes are required to evaluate whether cholecalciferol supplementation has an effect on another anthropometric outcomes, such as waist to hip ratio and changes in body composition.
Several limitations of the present study should be noted. First, the number of eligible studies was small. We evaluated the efficacy of cholecalciferol supplementation on weight loss and anthropometric outcomes independent of researcher-imposed energy restriction or weight-loss counseling. Finally, other potential additional factors such as diet, follow up, cholecalciferol dosage and physical activity were different among the RCTs.