By Nick Wasson, M.D.:

Keeps bones strong. Improves heart health. May prevent cancer.

We've all heard the reasons why you need more vitamin D. However, the amount of vitamin D you're taking in those over-the-counter supplements may not be what you think it is.

In a new study by Kaiser Permanente, researchers performed an independent analysis of several over-the-counter vitamin D supplements, and the results were startling: vitamin content varied from as low as 9 percent to as high as 146 percent of the amount of vitamin D listed on the bottle.

"I was just surprised that there was so much variability between pills, and how low some of the pills were." said Dr. Erin S. LeBlanc, a researcher with the Portland, Ore.-based Kaiser Permanente Center for Health Research and lead author of the study published today in the Journal of the American Medical Association: Internal Medicine. "Since supplements aren't regulated, consumers can't be sure what listed on the label is what's in the bottle."

LeBlanc and colleagues tested 55 bottles of vitamin D from 12 different companies bought in five Portland-area stores and one compounding pharmacy. Differences in vitamin D content were noted not only among different manufacturers, but between pills within the same bottle.

"Some variance is given, but the larger variances here are the surprise," said Dr. David Katz, a board-certified specialist in Preventive Medicine and Public Health at Yale School of Medicine, who was not involved in the study. "In fact, most of us have heard that with regard to vitamin D in particular, choice of brand is not all that important. That appears to be untrue."

The study was not the first time such supplements have been scrutinized. Dr. Tod Cooperman of ConsumerLab.com, an independent tester of vitamins and supplements, remarked, "We did an independent survey involving 10,000 people who use supplements, and reportedly 55.5 percent of people were taking vitamin D daily, making it the third most popular supplement." Fish oil and multivitamins are the most common supplements.

Several nutrition specialists contacted by ABC News said that vitamin D has a wide range of doses and it is unlikely to have any dangerous health impact, unless you're taking doses that exceed the upper tolerable limit of 4,000 International Units or IU.

"For adults, the daily requirement is 600 to 800 IU, and most products provide about 1,000 IU," said Cooperman. If people are taking a supplement that has 2,000 IU and really has 4,000 IU, they may be doing harm and not know it."

Still, many consumers might be left wondering if there is any way they can ensure they get what they paid for.

Researchers noted that manufacturers whose product was voluntarily tested annually by the U.S. Pharmacopeial Convention or USP - an independent, nonprofit organization that sets standards for quality and safety of vitamin supplements - was more precise in listing correct amount of vitamin D than those that didn't go through USP tests. Once tested and approved, the USP verification mark is placed on the bottle.

And approval for one doesn't mean approval for all.

"We don't grant a blanket verification for all of a manufacturer's products; we verify each one individually," said Francine Pierson, a spokesperson for USP. "Before earning the mark, USP tests samples of products but also does a manufacturing facility audit and manufacturing and quality control/product document review - so the program is very thorough and involves more than just testing to help ensure quality."

In Kaiser's review, barely half of the products tested, and only one third of the compounding pharmacy's product, met USP standards.

The findings beg the question as to whether more should be done to police the multi-billion dollar a year dietary supplements industry. In a statement by Kaiser, quoting a recent editorial in the New England Journal of Medicine, "The U.S. Food and Drug Administration is considering new safety guidelines for some supplements but, for the most part, the industry remains unregulated."

Katz, for one, believes this should change.

"I favor oversight of supplements like drugs," he said. "Minimally, random batch sampling is necessary to ensure what's being promised is actually in the bottle. In the meantime, clinicians will need to know which brands have the highest quality standards, and recommend them preferentially."

A list of USP verified supplements can be found on the USP website.