Vitamin D is a vital nutrient, helping the gut to absorb calcium while keeping calcium and phosphate at the right concentrations to support healthy bone growth and maintenance. Without adequate levels in the body, bones can become brittle and misshapen.

Low vitamin D levels have also been linked with a range of other conditions, including cardiovascular disease and cancer.

Vitamin D is not naturally present in many foods. Instead, the bulk of our requirement is synthesized in the skin after exposure to ultraviolet light from the sun.

Despite the importance of vitamin D, many people in the United States do not have sufficient levels in their bodies. For example, one study found that overall, more than 40 percent of the U.S. population were vitamin D deficient. So much so, that some authors have referred to vitamin D deficiency as a pandemic.

Furthermore, in one study published in 2009, only 3 percent of black people in their sample of thousands of U.S. individuals had the recommended vitamin D levels, representing a decrease of 9 percent over the previous 20 years.

For this reason, it is becoming increasingly important to understand how the vitamin works and to ensure that the right type of supplements are reaching individuals most at risk.

Not all vitamin D types are equal

There are two types of vitamin D, which are known as D-2 and D-3. The former is derived from plant sources, particularly fungi, while the latter comes from animal sources.

The two types of vitamin D are very similar, differing only in the structure of their side-chains, and it is generally accepted that both perform similarly well as a supplement. In fact, on the National Institutes of Health website, they write, "The two forms have traditionally been regarded as equivalent."

Researchers from the University of Surrey in the United Kingdom recently set out to test whether or not this widely held belief is correct. They wanted to understand which of the two nutrients raises levels of vitamin D in the body most effectively.

The researchers measured vitamin D levels in 335 South Asian and white European women over two winter periods. They chose winter because, due to a reduction in sunlight exposure, vitamin D levels tend to be lower at this time.

The women were split into five groups: those consuming vitamin D-2 in a biscuit; those consuming vitamin D-3 in a biscuit; those consuming vitamin D-2 in a juice drink; those consuming vitamin D-3 in a juice drink; and those receiving a placebo.

The study found that vitamin D-3 was twice as effective at raising vitamin D levels in the body as vitamin D-2.

Participants who received the D-3 in a biscuit raised their levels of vitamin D by 74 percent, while those receiving the vitamin in juice saw a 75 percent increase. Those receiving D-2 had a 33 and 34 percent increase, respectively. The placebo group experienced a drop of 25 percent across the same period.

Changes to guidelines and products

These findings have implications for the medical community, of course, but they also impact the retail sector; many companies add vitamin D-2 to beverages and foods. Some may therefore now want to think their choices.

As lead author Dr. Laura Tripkovic says, "The importance of vitamin D in our bodies is not to be underestimated, but living in the U.K., it is very difficult to get sufficient levels of it from its natural source, the sun, so we know it has to be supplemented through our diet." The same can be said for many parts of the U.S.

She continues, "[O]ur findings show that vitamin D-3 is twice as effective as D-2 in raising vitamin D levels in the body, which turns current thinking about the two types of vitamin D on its head."

"Those who consume D-3 through fish, eggs, or vitamin D-3-containing supplements are twice as likely to raise their vitamin D status than when consuming vitamin D-2-rich foods, such as mushrooms, vitamin D-2-fortified bread, or vitamin D-2-containing supplements, helping to improve their long term health."

Dr. Laura Tripkovic

Vitamin D deficiency appears to be widespread, and, as more research is conducted, it becomes increasingly clearer that this nutritional deficit is having a significant impact on the health of the country overall. Studies such as this may play a role in improving awareness, and, eventually, reversing the trend.