Participants will be randomly assigned in one of two groups. Group A: oral supplementation with 4,000 IU of vitamin D (cholecalciferol). Group B: oral supplementation with 400 IU of vitamin D (cholecalciferol). Both treatments will be consumed daily for 6 months.

Outcomes will be evaluated at baseline, three and six months. Variables related to leptin resistance will be evaluated.

The main hypothesis is that vitamin D will diminish leptin resistance in overweight and obese women. If the hypothesis is confirmed, women will show a reduction in the Resting energy expenditure: serum Leptin ratio (REE: Leptin ratio), as well as a reduction of hunger, body weight, body and abdominal fat and an increase in resting energy expenditure.

Study subjects will get dietary advice and physical activity counseling. At the end of the study, participants with suboptimal vitamin D levels (<80 nmol/L 25-OH-D) will receive treatment to normalize their vitamin D status.

Variables related to leptin resistance like resting metabolic expenditure, hunger, body weight and human body fat will be assessed. Outcomes will include, glucose, insulin, C-reactive protein, inflammatory interleukins (IL-6 & TNF-α) and non-inflammatory interleukins (IL-10 & TGF-β-1).

Inclusion Criteria:

• BMI: 25-29.9
• Serum vitamin D levels: 20-80 nmol/L (25-OH-D)

Exclusion Criteria:

• Liver disease
• Kidney disease
• Diabetes mellitus
• Malignity
• Any kind of hormonal disorder
• Medication that modify hunger/satiety answer and those medications that alter -the glucose/insulin metabolism
• Subjects with diet treatment to lose weight