The investigators want to make sure that people get the right dose of Vitamin D treatment. They will therefore investigate how skin colour, body mass index, ethnicity, vitamin D binding protein and genetic variation affect the response to a standard course of vitamin D in young adults, as a prelude to further studies in younger children.

The Department of Health and the Chief Medical Officer have identified vitamin D deficiency as a key area of interest and concern for public health.

The main function of vitamin D is to enable dietary calcium to be absorbed from the intestine. Low levels of vitamin D can lead to diseases of bone such as rickets and osteomalacia and are linked to a higher risk of fracturing bones in older women with osteoporosis.

Vitamin D levels may be affected by the skin colour, body mass index (BMI), lifestyle or environment in which someone lives, and by their genetic makeup. Vitamin D levels tend to be lower in people with higher BMI and / or darker coloured skin or if the skin is covered by clothing because a lot of vitamin D is made from the action of sunlight on natural chemicals in the skin.

Vitamin D does occur naturally in the diet in foods like oily fish, and also vitamin D can be given as a supplement either on its own or as part of a multivitamin tablet.

There is natural variation from one person to another in how well the system controlling vitamin D blood levels works. Vitamin D circulates bound to a carrier protein, vitamin D binding protein (VDBP). When vitamin D levels are measured, both vitamin D bound to the protein and "free" vitamin D are measured.

A recent study in America showed that when "free" vitamin D levels (total vitamin D minus vitamin D bound to VDBP) are measured, they correlate very closely with other factors that help determine blood calcium levels.This variation is determined in part by a person's genetic makeup, and recent large studies have identified specific genetic variations that are linked to blood levels of vitamin D; some of these vary with the person's ethnic origin.

At present if someone has low vitamin D levels that put them at increased risk of bone problems, a course of vitamin D treatment is given.

When the investigators assessed their regular treatment given to children recently, they found some individuals developed very high blood vitamin D levels and others didn't.

They don't know how VDBP levels affect the response to treatment with vitamin D.

Further variation can occur because of the distribution of vitamin D into fat tissue. The investigators will measure height and weight, and waist and hip circumference and calculate Body Mass Index, body surface area (BSA) and waist: hip ratio as proxy measures of fat mass.

They will also evaluate whether blood or saliva tests give better information about vitamin D levels. The information about how these factors affect the response to vitamin D will help the clinicians choose the right dose of vitamin D for studies in younger children who are still growing.

Inclusion Criteria:

• Healthy young male adults aged 18 25 years
• Free from any condition affecting bone health, general nutrition, growth and glucose metabolism.

Exclusion Criteria:

• Subjects with any chronic illness involving the liver and kidney
• Use of steroids, anticonvulsants or any medication that might affect calcium and vitamin D metabolism.
• Potential participants who have made plans to travel abroad during the study period.