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updated 3/14/2013 10:49:40 AM ET 2013-03-14T14:49:40

An enduring mantra among nutritionists, from both a vegetarian and carnivorous perspective, has been to eat more whole grains to reduce the risk of heart disease, type 2 diabetes and colon cancer.

And with March touted as both National Colorectal Cancer Awareness Month and National Nutrition Month, this mantra can be deafening. The U.S. dietary guideline is three to five servings of whole grains daily.

The only problem is that true whole grains are an acquired taste. Cardboard comes to mind.

The food industry knows this and has long offered products merely sounding like they contain whole grain but usually have little or none, including foods with labels sporting the words multigrain, 12-grain, stoneground, high fiber, enriched, wheat flour and (unbelievably) whole grain.

Also, whole grain might be the first ingredient in food products tainted with less desirables, such as Lucky Charms cereal — "magically delicious," with magic perhaps derived from copious marshmallows (second ingredient) containing sugar, corn starch and corn syrup, followed by more sugar (third ingredient), more corn syrup (fifth ingredient) and more corn starch (sixth ingredient).

A study published in January in the journal Public Health Nutrition by researchers at Harvard School of Public Health found that most "whole grain" labeling is confusing, with the industry-supported Whole Grain stamp pointing to foods higher in sugar and calories than those without the stamp. [ 8 Reasons Our Waistlines Are Expanding ]

(Yes, Lucky Charms has a Whole Grain stamp. Yes, General Mills, the maker of Lucky Charms, is a sponsor of the Whole Grains Council, issuer of the stamp.)

And while you'd think that some whole grain is better than none, this might not be the case.

A 1999 study led by David Ludwig, now director of the New Balance Foundation Obesity Prevention Center at Boston Children's Hospital, found that obese teenagers who ate a breakfast of instant oatmeal — a highly processed "whole grain" that cooks in seconds — were much hungrier later in the day compared with those who ate steel-cut (long-cooking) oats or an egg omelet; and the instant-oatmeal eaters had blood sugar spikes similar to what's seen after eating simple, non-whole-grain white bread.

All in the processing

Ludwig's more recent work has demonstrated that how a grain — whole or refined — is processed truly determines its healthfulness. You guess it: In general, more processed means less healthful.

Part of the problem lies with the FDA definition of whole grain. Most grain contains inedible chaff (fed to animals), bran (the oil-rich outer layer), germ (the grain seed's nutrient-rich embryo), and the endosperm (the starchy center). Refined, white flour contains only endosperm.

The FDA definition of whole grain allows for grain to be milled and separated of its edible constituents, as long as the three are later mixed in proportions similar to the intact grain. This enables the food industry to make whole-grain products that taste — and act — nothing like whole grain.

So, a serving of steel-cut oats with raisins may be more healthful than a serving the oat-based Lucky Charms, but no amount of FDA-required labeling explains this to consumers lured into eating three servings of whole grains daily. [ 10 New Ways to Eat Well ]

Also, grain type matters. "Corn is arguably the lowest-quality 'whole grain,'" Ludwig told LiveScience. "I'd put oats, rye and barley at the top of the list, and corn at the bottom."

Slower is better

At the heart of Ludwig's research is the concept of the glycemic index (GI), a measure of how quickly blood sugar levels rise after eating a particular type of food.

Carbohydrates with a high GI on a scale from 0 to 100 tend to spike blood sugar levels and ultimately lead to problems such as obesity and diabetes. Refined grains such as packaged white bread and most breakfast cereals have a high GI, 70 or above; intact whole grains such as oat, spelt, millet and barley have a low GI, below 50. Non-intact whole grains such as whole-grain breads are in the middle.

You won't garner this information from a food label. But the general rule of thumb is that heavily processed foods (crackers, cookies or instant breakfast cereals compared with whole grains you need to cook for 30 minutes or more) will have little or none of the benefits said to come from eating whole grains.

One exception is pasta, Ludwig said. The process of extruding and drying the durum wheat (which itself is very different from wheat species used for bread) leads to a surprisingly low GI food with a slower digestion rate.

Popcorn is technically a whole grain and may provide some of the benefits that come from consuming other whole grains, said Joanne Slavin, a professor at University of Minnesota. Provided the popcorn isn't laden with sugar, salt, oil or butter, the advantage also might be what it replaces — snacks such as potato chips or crackers.

Along these lines, Slavin's approach is not merely to add more whole grains to your diet but rather to substitute them for less-healthful foods: to add a higher ratio of whole grain oat or wheat flour to white in recipes, for example.

As healthy as whole grains are, they aren't essential to human health, unlike fat and protein. Proponents of the paleolithic diet argue that grains entered the human diet only a few thousand years ago. Yet grains do provide many essential nutrients and a feeling of fullness … and the more whole the grain, the more nutrients it will have and the greater a sense of satiety it will provide.

Christopher Wanjek is the author of a new novel, " Hey, Einstein! ", a comical nature-versus-nurture tale about raising clones of Albert Einstein in less-than-ideal settings. His column, Bad Medicine, appears regularly on LiveScience.

© 2012 LiveScience.com. All rights reserved.

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