So-called “functional food” has turned into a big business as more Americans buy into marketing claims that food products improve health. While food companies cite their right to free speech, government agencies try to oversee an industry fraught with exaggerated or even false claims.

Start in Aisle 2, third shelf from the bottom: Here is grape juice for your heart. Over to Aisle 4: There are frozen carrots for your eyes.

In Aisle 5: Vitamin-packed water for your immune system. In the dairy case: Probiotic yogurt for your insides and milk for your brain.

Push a cart through the D’Agostino store in Manhattan, or any supermarket anywhere in America, and you just might start believing in miracles — or at least in miracle foods.

In aisle after aisle, wonders beckon. Foods and drinks to help your heart, lower your cholesterol, trim your tummy, coddle your colon. Toss them into your cart and you might feel better. Heck, you might even live longer.

Or not. Because this, shoppers, is the question: Are all these products really healthful, or are some of them just hyped?

The answer to that question matters to millions of Americans who are wagering their money and their waistlines on hot new products in the grocery aisles called “functional foods.”

Food giants like Dannon, Kellogg and General Mills don’t claim these products actually prevent or cure diseases. Such declarations would run afoul of federal regulations. Nor do they sell them as medical foods, which are intended to be consumed under a doctor’s supervision.

Rather, food companies market functional foods with health-promoting or wellness-maintaining properties. Such claims are legal, provided that they are backed up by some credible science.

All those heart-healthy red hearts on your box of Quaker Oats cereal or that can of Planters peanuts? That happy-colon yellow arrow on the tub of Activia yogurt? It’s all part of the marketing of functional food.

Over the past decade, despite all those sales pitches for “natural,” “organic” and “whole” foods, functional food has turned into a big business for Big Food. And more Americans are buying into the functional story. Sales of these foods and beverages totaled $37.3 billion in the United States in 2009, up from $28.2 billion in 2005, according to estimates from the Nutrition Business Journal, a market research firm.

Dubious claims

But as sales soar, federal regulators worry that some packaged foods that scream healthy on their labels are in fact no healthier than many ordinary brands. Federal Trade Commission officials have been cracking down on products that, in their view, make dubious or exaggerated claims. Overwhelmed regulators concede that they are struggling to police this booming market, despite recent settlements with makers of brands like Kellogg’s Rice Krispies and Dannon’s Activia, which the authorities say oversold their health benefits.

Consumer advocates and some nutritionists are equally blunt. They say shoppers are being bamboozled by slick marketing. Many people grab products with healthy claims on the front of the package and overlook crucial nutritional information, like calorie counts, in the small print on the back.

“Functional foods, they are not about health,” says Marion Nestle, a professor of nutrition, food studies and public health at New York University. “They are about marketing.”

Walk through any grocery store, and you’ll see what she means.

Here in Aisle 2 is a box of Quaker Oatmeal Squares cereal, made by the Quaker Oats Co. The front of the box, in large white print, proclaims: “Oatmeal helps reduce cholesterol!” Scientists generally agree that fiber can be good for your heart. But read the adjacent smaller print, which the Food and Drug Administration requires, and you’ll find that one serving of Quaker Oatmeal Squares contains only a third of the amount of soluble fiber needed daily to help reduce the risk of heart disease. In other words, you may have to eat three bowls of cereal daily — 630 calories’ worth, without milk — to benefit.

Down the aisle is Welch’s 100% Grape Juice, with no fat and emblazoned with a red-heart certification from the American Heart Association. An 8-ounce glass has 36 grams of sugar; a regular-sized Snickers, by comparison, has 30.

No one is saying that these products are unsafe or unhealthy, or that there isn’t science behind them. But nutritionists like Nestle contend that the kaleidoscopic array of functional foods on offer, with all those different claims, has left many consumers confused about the products’ actual health value. And, in some cases, regulators say, manufacturers are bending, or even breaking, the rules about how they market these products.

“If people can’t rely on even the most trusted food brands to have good science backing up their claims, who can they rely on?” asks Mary Engle, the director of the advertising practices division at the Federal Trade Commission in Washington.

Today, companies promote myriad processed foods that have been loaded with vitamins and nutrients, or contain a potentially beneficial ingredient, as wellness aids. For many, these healthified foods have become the new health food. Many Americans are willing to pay a premium for ready-to-eat, ready-to-heat and on-the-go foods that seem to promise shortcuts to healthier living.

What’s more, most people can’t evaluate the claims for such products, like functional foods and medical treatments. So they rely on experts and regulators.

“The majority of American consumers really believe in the concept that certain foods provide benefits that go beyond basic nutrition or reduce the risk of disease,” says Wendy Reinhardt-Kapsak, senior director of health and wellness at the International Food Information Council. “Most of the big companies are making those claims within the letter of the law.”

Most, regulators say, but not all. Over the past two years, the FTC, which oversees food advertising, has filed complaints of deceptive marketing against Kellogg, Dannon and a subsidiary of Nestle.

None of the companies has admitted wrongdoing. But each has separately settled with the agency, agreeing to certain restrictions on health-related claims.

The agency’s concern, says David Vladeck, director of its bureau of consumer protection, is not only that people might be paying more for foods that are no more healthful than other brands. At a time when millions lack health insurance, he also worries that people who buy foods that, for instance, claim to bolster immunity or reduce the risk of prostate cancer might forgo a flu shot or a doctor’s visit.

“If people are going to spend their money for health benefits,” Vladeck says, “they ought to get them.”

Still, regulators at the Food and Drug Administration, which oversees food labeling, say it has been hard to curb every questionable claim.

Michael Taylor, the FDA’s deputy commissioner for foods, has acknowledged that the agency is acutely aware that as soon as it proves that one claim is misleading, savvy market types may dream up another. “Going after them one by one with the legal and resource restraints we work under is a little like playing Whac-A-Mole, with one hand tied behind your back,” Taylor wrote last year on TheAtlantic.com.

FDA-sanctioned claims

To protect consumers, the FDA, which oversees food labels, has a variety of rules on label claims. It maintains a short list of bona fide “health claims” that are backed by generally accepted science and have regulatory pre-approval. These include statements like “diets low in sodium may reduce the risk of high blood pressure.”

The FDA also permits more general package claims about how a nutrient can promote the normal functioning of the body, like “calcium supports strong bones.” The more general claims don’t require agency pre-approval.

Conventional foods, though, are prohibited from being marketed like drugs to prevent, mitigate or cure disease.

But regulators say they are concerned about incidents in which marketing crossed a line.

People want to believe

Pom Wonderful has long championed the power of pomegranate juice.

“Cheat death,” one of its billboard ads exhorted; “death defying,” said another. Pom Wonderful says it has spent about $34 million to sponsor pomegranate research. Sixty-five of those studies have been published in peer-reviewed journals, says Lynda Resnick, a California entrepreneur who founded Pom with her husband, Stewart Resnick.

But last year, the FDA accused the company of violating of the Food, Drug and Cosmetic Act by marketing its juices and supplements as treatments for disease.

The FTC, meanwhile, said the company employed misleading marketing, contending that Pom had exaggerated or overstated research results in advertising, on the Web and in press communications.

Pom denies that it is misleading anyone. The First Amendment protects commercial speech that is truthful and not misleading, Lynda Resnick says. Companies have a constitutional right to publicly discuss their research results. A second study, she says, also suggested that the juice holds promise for people with prostate cancer.

Jennifer Thomas, director of the division of enforcement at the FDA’s Center for Food Safety and Applied Nutrition, recommends that consumers take a close look at nutrition panels and ingredient lists, and not just read marketing claims. People may also want to peruse the FDA’s website.

Then again, many people just want to believe. Buying foods marketed as healthful may satisfy our yearning to feel we are doing something healthful for ourselves and our families. At least, that’s why many people thirst after Pom Wonderful, Lynda Resnick writes in her book on branding, “Rubies in the Orchard.”

As she put it: “Lots of people drink Pom because it makes them feel healthier — data or no data.”