Washington ? Go ahead, have a piece of bread. Have three. Make it whole-grain, and you’ll be following government advice for eating right.

Three servings of whole grains each day will reduce your risk of heart disease and type 2 diabetes. It doesn’t have to be bread. Brown or wild rice, oatmeal, cold cereal flakes, popcorn — without the salt and butter — and even trail mix will do.

Of all the new advice in the government’s new dietary guidelines, eating enough whole grains may prove the easiest.

But if eating whole grains is so easy, then why aren’t people doing it now? Most Americans are eating one serving or less each day, according to the Agriculture Department.

One reason may be that a little sleuthing is needed to figure out which foods have whole grains.

Just because a bread slice is dark, or a cracker looks grainy, doesn’t mean the whole grain is there. They could be darkened by molasses or other coloring.

You need to look on the ingredient list to make sure you’ve got whole grains. The words “whole” or “whole grain” should come before the grain ingredient. And that should be the very first thing listed.

Food companies are trying to make to make it easier. General Mills Inc. last year converted all its breakfast cereals to whole grain and now puts a big “Whole Grain” logo on the front of the box.

Some companies are using a black-and-gold label shaped like a postage stamp for identifying products that contain whole grains. Bruegger’s Bagels, Kashi, Gardenburger and Snyder’s of Hanover are among those using the stamp, which was developed by Oldways Preservation Trust, a Boston-based think tank that specializes in food issues.

Once purchased, eat three ounce-equivalents a day. It’s not hard to do. These measurements equal about an ounce:

• A half-cup of cooked cereal, rice or pasta.
• A slice of bread.
• 1 cup of cold cereal flakes.
• An ounce of dry pasta or rice.

“If you have a cereal high in whole grains, and you have a sandwich with whole grain bread for lunch, you can get your number of servings right there,” said Joanne Lupton, a Texas A&M University nutrition professor who helped write the guidelines.

Mark Andon, technical director for nutrition at Quaker, said a cup of oatmeal for breakfast counts as two servings of whole grains.

“When you think about a snack, think about trail mix. It’s not weird food,” said K. Dun Gifford, president of Oldways.

This effort may seem to contradict the thinking behind popular low-carb diets such as Atkins and South Beach. But while both of those diets insist that people eliminate most carbohydrates in the first two weeks, after that it’s all about choosing “good” carbs, such as whole grains.

Scientists don’t quite understand how and why whole grains are good for you.

“It would be nice if we knew the answer to that question; unfortunately, we don’t,” said Lupton. “It’s not just the fiber. It’s something in addition to that that has to do with having the whole grain.”

A whole grain is the entire seed or kernel — from grains like wheat, oats, corn or rye. They are packed with fiber, protein, carbohydrates, vitamins and minerals, and besides helping guard against heart disease and diabetes, they also contain cancer-fighting antioxidants and other nutrients.

They lose the good stuff when grains are refined or processed. Manufacturers generally fortify their food with nutrients, but it’s not the same.