Obesity is an increasing health problem with numerous metabolic complications. Vitamin D deficiency is common in obesity, and in epidemiological studies vitamin D deficiency has been linked to metabolic complications, such as type 2 diabetes, insulin resistance and cardiovascular disease, as well as myopathy, osteoporosis and depression. In obesity, a low grade inflammation is present in the fat tissue, thereby releasing inflammatory molecules to the blood stream. In cell line studies as well as small clinical studies vitamin D has been shown to have the ability to reduce inflammation and cell growth.

In the present study the investigators wish to investigate the effect of vitamin D on fat-, muscle and bone metabolism. 30 healthy obese subjects will be treated with cholecalciferol 175 micrograms daily for 6 months and will be compared with 30 healthy obese subjects treated with placebo.

The investigators hypothesize that restoring vitamin D levels in vitamin D deficient obese subject will reduce inflammation and thereby reduce obesity-related complications.

The effect will be evaluated as follows:

• Levels of circulating inflammatory markers will be examined in blood samples collected prior to and after treatment.
• Effects on fat- and muscle metabolism will be evaluated in fat- and muscle samples taken before and after treatment.
• Effects on fat distribution will be evaluated by MRI scan before and after treatment.
• Effects on insulin sensitivity will be evaluated by hyperinsulinaemic euglycaemic clamp performed on a subgroup of subjects with impaired fasting glucose.
• Effects on bone marrow density will be evaluated by DEXA scans before and after treatment.
• Effects on quality of life and depression score will be evaluated by questionnaires used before and after treatment.

Inclusion Criteria:

• Healthy obese (BMI 30 - 45 kg/m2)
• 25(OH) vitamin D < 50 nmol/l

Exclusion Criteria:

• Diabetes,
• Pregnancy or non-safe contraception,
• Vitamin D treatment within 3 months,
• Hypercalcaemia, renal failure,
• Liver failure, non eligibility for MRI-scan,
• Severe osteomalacia,
• Allergy towards study drug