Stroll down any pharmacy aisle these days and you’ll find that the low-carb craze has invaded the $20 billion dietary supplement industry.

From multivitamins to starch blocker pills, loosely regulated supplements are popping up in the burgeoning low-carb market dominated by food companies.

For vitamin makers, their biggest marketing tool is exploiting the pitfall of high-protein diets, namely that you lose some nutrients when you cut back on carbohydrates.

Low-carb followers may lack key B vitamins found in grain-based foods like bread and pasta. Skimping on fruits and vegetables may cause deficiencies in antioxidants like Vitamin C and beta-carotene.

About 44 million Americans are either on a low-carb diet or watching their carb intake. Of those, about 5 million are taking supplements, according to the Natural Marketing Institute.

While U.S. retail sales of brand-name, all-purpose multivitamins have declined, specialty vitamins targeted at dieters are growing dramatically.

Sales of One-A-Day WeightSmart, which contains a natural green tea extract claiming to increase metabolism, shot up 55 percent over the previous year, while sales for the popular Centrum multivitamin fell by 9 percent, according to Information Resources Inc., a market research firm.

Health experts generally agree that the best place to get the vitamins and minerals you need is from food. People on special diets may benefit from taking supplements, but nutritionists stress that a pill is not a replacement for healthy foods.

Carb-conscious diets from Atkins to the Zone recommend supplements. Atkins Nutritionals sells its own line of diet products and supplements.

Now mainstream pharmaceutical companies are betting that low-carb dieters will turn to tailor-made vitamins to make up for their nutritional deficiencies.

In April, drug maker Bayer launched its over-the-counter One-A-Day CarbSmart multivitamin containing higher doses of B vitamins and antioxidants. Wyeth, the Madison, N.J.-based pharmaceutical firm, followed suit a month later with Centrum Carb Assist.

“There’s good science behind this in terms of the needs of low-carb dieters and what they’re not getting in their nutritional intake through food,” said Andy Davis with Wyeth Consumer Healthcare.

But not everyone is convinced that low-carb dieters will get more out of a low-carb-focused vitamin, which can cost twice as much as the ordinary kind.

David Levitsky, a professor of nutritional sciences at Cornell University in Ithaca, N.Y., questions the need for increased amounts of B vitamins, and believes taking a regular multivitamin will suffice.

“There’s so much hype around the low-carb diet,” Levitsky said. “People who buy into it will buy anything that’s associated with low-carb.”

The low-carb vitamin makers are hoping to imitate the success of Bayer’s One-A-Day WeightSmart for dieters, which grossed $32 million in U.S. sales for the one-year period ending July 11. Since its debut about four months ago, One-A-Day CarbSmart has raked in $2.8 million in U.S. sales and Centrum Carb Assist, $1.7 million, according to IRI.

Bayer already makes multivitamins targeting men and women while Wyeth markets the popular Centrum Silver brand for the elderly. Both companies say it was only natural to develop a vitamin for low-carb dieters given the diet’s popularity.

But some dietitians are adamant that nothing beats getting nutrients from whole foods no matter what supplements people take.

“To take a pill to make up for a deficiency in your diet is not really a panacea,” said Elisa Zied, a spokeswoman for the American Dietetic Association and a registered dietitian who owns a private practice in New York City.