Last month the American Society for Nutrition (ASN) reaffirmed in a report that fiber-rich whole grains lower the risks of diabetes and heart disease. Media outlets such as Reuters duly reported the news, but many failed to point out a crucial detail: some whole grains may do nothing to reduce disease risks. In fact, many foods legally marketed as whole grains could actually harm health.

The term “whole grain” might evoke an image of a whole, intact grain—that is, a fiber-rich coating of bran surrounding a starchy endosperm and a small reproductive kernel known as the germ. But in a definition created in 1999 by the American Association of Cereal Chemists (AACC) International, an organization of food industry professionals and scientists, and adopted by the U.S. Food and Drug Administration in 2006, “whole grain” refers to any mixture of bran, endosperm and germ in the proportions one would expect to see in an intact grain—yet the grains can be, and usually are, processed so that the three parts are separated and ground before being incorporated into foods. (Refined grains, on the other hand, are grains that have been stripped of their bran and germ.) For a food product to be considered whole grain, the FDA says it must contain at least 51 percent of whole grains by weight. Compared with intact grains, though, processed whole grains often have lower fiber and nutrient levels.

In what it calls a scientific statement the ASN reviewed studies published about whole grains between 1965 and 2010. Descriptions in many studies did not conform to the current definition in that they considered foods such as wheat germ and bran cereals to be whole grain. (Both are now considered partsof a whole grain.) Ultimately, the researchers found that the only whole-grain-rich diets that reduced the risk of heart disease and type 2 diabetes were those that included bran as a whole grain or those that contained high amounts of fiber. Studies of “whole grains using the currently accepted definition don’t have enough data to support them for preventing these different chronic diseases,” says co-author David Klurfeld, the national program leader for human nutrition in the Agricultural Research Service at the U.S. Department of Agriculture.

Missing fiber

One problem with the current definition of “whole grain” is that it doesn’t account for fiber—and many whole grain products on supermarket shelves contain very little. An individual would have to eat 10 bowls of Multi-grain Cheerios, 16 slices of whole-wheat bread, or nine cups of brown rice to get the fiber recommended for an American adult for one day. “There’s nothing wrong with eating brown rice, but you can't expect health benefits if you're going to be eating brown rice as your source of whole grains,” Klurfeld explains.

The common processing of whole grains—which can involve grinding, puffing and flaking them—can also impact their healthfulness. Processing can make whole grains tastier; it can give them a longer shelf life, too, by removing fats from the outer layer of the grain that can turn rancid. But some processing techniques have been shown to degrade natural antioxidants and reduce fiber content. In fact, the AACC International recently proposed modifying its definition of “whole grain” to allow for some nutrient losses during processing.

Individuals also absorb the sugars from some processed whole grains more quickly than they do those from intact whole grains, triggering blood sugar spikes that can “possibly increase hunger, lead to overeating and increase the risk for diseases related to insulin resistance, like diabetes and heart disease,” says David Ludwig, a professor of nutrition at the Harvard School of Public Health. For instance, bread made from 80 percent–whole-wheat kernels is absorbed much more slowly than bread made from ground whole wheat. When a person eats intact grains, the body has to break down the outer bran before digesting the inner endosperm and germ. Ground grains often don’t provide these metabolic brakes.

Fast-cooking penalty

But even when whole grains aren’t ground, they can be processed in ways that can cause metabolic problems. In a 1999 study Ludwig and his colleagues split 12 obese teenage boys into groups. Some were given a breakfast of instant oatmeal, in which whole oats have been rolled and steamed so that they cook quickly. Other boys were given steel-cut oatmeal, comprising whole oats that have been sliced but not steamed. Although both meals had the same calorie and fiber content, the instant oats triggered much larger blood sugar spikes (a reflection of how quickly they were absorbed) and caused the boys to consume 53 percent more calories at lunch than the boys who ate the steel-cut oats. (A third group of boys were given identically caloric breakfasts of vegetable omelets and fruit and ate 81 percent fewer calories at lunchtime than did the instant oat eaters.) Food companies lump ground whole grains, partially processed grains and intact unprocessed grains together under the same broad category of “whole grains,” so it’s difficult for consumers to know which they’re getting.

Whole-grain foods can also contain unhealthy additives. In January Ludwig and other Harvard researchers compared the nutrient composition of 545 grain products and found that those labeled with the “Whole Grain” stamp, an industry-sponsored label for foods containing at least eight grams of whole grains per serving, contained more calories and sugar than those without the stamp. They were also more expensive. When a food is marketed as containing whole grains, “it takes on a ‘health halo,’ and companies can slip in all sorts of junk without people recognizing what they're getting,” Ludwig says. Joanne Slavin, professor of food science and nutrition at the University of Minnesota, who served on the committee for the 2010 Dietary Guidelines for Americans, agrees that many foods with whole-grain labels are “really not very nutritious food products.”

More isn’t good

So how did “whole grain” become synonymous with “healthy”? Part of the confusion stems from the U.S. dietary guidelines, which recommend that individuals triple their consumption of whole-grain foods from an average of one ounce per day to three ounces. But the goal is for Americans to replace half of their refined grains with whole grains, not to eat whole grains in addition to the refined ones they have been eating, Slavin explains. “We don’t want people to think that because a food has whole grains, they should eat more of it,” she says. “Grains in general are overconsumed in the U.S.”

Another reason whole grains may seem more healthful than they really are: people who eat lots of them tend to make smarter lifestyle choices in general. A 2006 study (pdf) reported that the quartile of people who eat the most whole grains are less than half as likely to smoke and 25 percent more likely to regularly exercise as the quartile of people who eat the least whole grains.

If one clear piece of advice is emerging about whole grains, it is that individuals should buy whole grains that are high in fiber: All of the diets that reduced disease risk in the ASN’s review were high in fiber or fiber-rich bran, and Ludwig and his colleagues found in their 2013 study that whole-grain foods with a ratio higher than one to 10 of fiber to total carbohydrate also contained less sugar, sodium and trans fats than other whole-grain products. People should consider cooking with intact whole grains, too, such as whole-wheat berries or spelt. And when in doubt, consumers should always trust their nutritional instincts. “If it’s a whole-grain cookie, you probably don’t need it,” Slavin says. Deep down, most people already know that.