Products like milk have been fortified with Vitamin D for decades because of its importance in uptake of calcium in the bones, along with other cellular and immune processes. The body creates vitamin D in the form of cholecalciferol within the skin itself, if there is a sufficient amount of sunlight. 

Saltwater fish contain larger quantities of natural vitamin D while smaller quantities can be found in eggs, meats, milk and butter but in the winter months and in some regions the sun is scarce so fortified food is common. Calcium up take is important for senior citiziens. Osteoporosis is one of the chief reasons why the elderly often suffer broken bones from relatively minor injuries. Postmenopausal women in particular experience a relatively rapid loss in bone mass due to a reduced concentration of estrogen, which is responsible for strong bone growth during youth. Maintaining bone mass requires physical exercise and vitamin D which is why during winter months many elderly women are prescribed a vitamin D supplement by their doctor to maintain bone mass.

What about correct dosage? Vitamin D has become the latest supplement craze and the source of books and articles, even linking it to autism. Correct levels are a debate. Some assert that it takes high doses of vitamin D supplements to prevent or slow down bone degeneration in elderly women.

How much is enough?

A group of researchers at ETH Zurich and the Universities of Zurich and Bern led by ETH Professor for Human Nutrition Michael B. Zimmermann took a closer look at this question. The team of scientists wanted to find out how much vitamin D there needs to be in the bloodstream to maintain bone strength.

They came out in favor of higher supplement doses, particularly in the winter. In the study, the researchers come to the conclusion that a vitamin D concentration of 40 micrograms per liter of serum in the bloodstream is ideal for slowing or preventing bone degeneration in postmenopausal women.

During the study, test participants were first given a single dose of calcium-41. This disperses like normal calcium throughout the body and into the bones and, given enough time, will mark the entire skeletal system evenly. "It's after about six months that things get interesting, because from that point on we can trace the absorption and depletion of calcium in the bones," says Zimmermann. However, highly sensitive measuring equipment is required to detect the minute quantities of calcium-41 present.

Researchers took urine samples from test participants at regular intervals and then used highly sensitive accelerator mass spectrometry equipment - which is found at ETH Zurich's Laboratory for Ion Beam Physics and just a handful of other facilities worldwide - to measure the quantities of calcium-41 and calcium-40 and determine the ratio between them. To put it simply, a very low ratio means more calcium is being added to the bones than released; a high one means the bones are releasing more calcium than they are taking up.

Increased calcium absorption

Over a period of nine months beginning half a year after the calcium-41 marking of their bones, the women were given daily vitamin D supplements. The first dose was administered in early spring, when the vitamin D concentration in the blood is expected to be at its lowest, and the dosage was increased in step increments every three months. In addition, the scientists led by Zimmermann modelled the paths the calcium took through the various segments of the body in order to calculate an ideal vitamin D quantity.

At the beginning of the experiment, participants showed a concentration of 16 micrograms per litre of serum, which is to say they already had a deficiency. By the end of study, the average vitamin D concentration in their serum had risen to over 46 micrograms per litre thanks to the vitamin D supplementation - and to the sunshine, which increased over the course of the study to promote the body's natural vitamin D production.

At the same time, the researchers noted that the ratio of calcium-41 to calcium-40 decreased abruptly following the start of the supplementation regimen - a sure sign that bone degeneration had been reduced.

Increasing vitamin D supplementation

For healthy postmenopausal women with sufficient calcium absorption and physical activity, a serum concentration of around 40 micrograms of vitamin D per litre of serum has the optimum effect on bone calcium absorption. "That the figure was so high was surprising," says Zimmermann, "as previously I had tended to believe that a low dose of vitamin D was sufficient."