For those of you who take your vitamins and minerals everyday and lack a nutritional deficiency, you may be surprised by a review of several studies that show little benefit, and in certain cases harm, from regular supplement use.

A group of researchers published a review in the Annals of Internal Medicine of 24 randomized, controlled clinical trials and 2 cohort studies. Cohort studies include only participants that fall under a specific class or demographic group.The data reviewed did not provide consistent evidence that supplements decreased the risk of health problems, such as cardiovascular disease (CVD) and cancer, or prolonged the life of healthy and nutrient sufficient people. Claims about vitamin supplements being unnecessary unless there's clear evidence of a nutrient deficiency have been made in the past. Yet individuals don't appear to be giving up their supplements and industry data shows growth in sales.

Studies reviewed show a few limitations

As the researchers sought studies to include in their review, they defined "multivitamins" as single, paired, and combinations of three or more vitamins and minerals. It was also noted that the conclusion of the review was tempered by the fact that there are few good quality studies available for all supplements, except vitamin E and beta-carotene. Evidence provided by good quality studies did show that vitamin E did not decrease the risk of CVD, cancer, or general mortality. 

The review also confirmed a "probable increase" between beta-carotene supplements and incidence of lung cancer and death for individuals at high risk, such as smokers. Its also interesting to note that most of the studies reviewed looked at participants with a mean age over 50 and the majority of the trials provided less than 10 years of follow-up data. This is important because findings from two large studies reported a "borderline-significant" lower cancer incidence in men, but not women, who took a multivitamin for more than 10 years . Since the data is skewed toward those aged 50+, its unknown whether results would have varied if the mean age of participants had been much younger.

Vitamin and supplement sales show strength

As many doctors stress that multivitamins are unnecessary for nutrient sufficient individuals, it's business as usual for vitamin producers like Pfizer (NYSE:PFE), Bayer Co. (NASDAQOTH:BAYRY), and vitamin and supplement specialty retailer GNC Holdings (NYSE:GNC)

Pfizer is the maker of the popular Centrum vitamin line, a part of the company's Consumer Healthcare segment. Consumer Healthcare revenues rose 1% in the third quarter of fiscal 2013 to $788 million. The higher revenues were attributed mostly to the international growth of Centrum after the launch of new products and increased advertising in key markets. Pfizer has also found success with Emergen-C, a vitamin supplement drink mix that contains high amounts of vitamin C and other nutrients. The segment's revenues made up 6% of total quarterly revenues of $12.6 billion and was one of three of Pfizer's seven business units to report growth .

Another pharmaceutical company with a popular vitamin supplement is Bayer, makers of the One A Day vitamin line. The company's Consumer Health segment rose 2.9% in the third quarter of fiscal 2013, supported by strong growth in emerging markets and the Consumer Care division. The Consumer Care division includes Bayer's dietary supplement Supradyn and One A Day. Both products did well overseas, but One A Day did experience lower volumes in the U.S. market. Supradyn and One A Day were two of the best selling products within Consumer Health, whose sales accounted for 42% of total sales for the quarter .

Vitamin and supplement retailer GNC had strong results in the third quarter of fiscal 2013 from its three main operating segments – retail, franchise, and manufacturing/wholesale. Retail saw revenue grow 9.5% to $487.3 million, driven mostly by a 6.7% comp store sales increase and an additional 148 stores were opened since the third quarter of 2012.

GNC's franchise segment grew 9.3% to $118.9 million, as royalties increased from both domestic and international franchisees and from wholesale product sales. While, the manufacturing/wholesale segment increased 2.4% to $69.5 million. For the first nine months of the year, the company opened 296 new stores, which included 124 new international franchise locations.

My Foolish conclusion

While sufficient evidence from available studies may not exist to support the use of multivitamins in nutrient sufficient individuals, I believe a case can be made for additional research. The review's discussion section provides considerations for future research:

"Future studies of multivitamin supplements should recruit from a general population with representation of multiple minority groups and both sexes, use a multivitamin that is reasonably similar to the popular brands in the current market, continue for at least a decade, and include enough participants to provide adequate power to detect benefits and harms within important subgroups, including men and women. This is a tall order, and any such study would also face other difficulties..."

It may be nearly impossible to conduct this type of study, which could provide important information on the effect of multivitamins on different population subgroups. In the meantime, the global vitamin industry, which had revenues in 2012 of $23.4 billion, keeps selling and consumers keep buying. 

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Eileen Rojas has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.

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