The timing and the contents of breakfast make it perhaps the most important meal of the day.

Whether it features soup in Vietnam, biscotti in Italy, or scrambled eggs in the United States, breakfast revs up the body after a night's sleep, giving us energy and nutrients to face the day. Studies suggest that eating breakfast regularly is associated with good health — and that the timing of the meal, as well as what's in it, matters.

Eating first thing

As we sleep, chemicals in our bodies are at work digesting food from the previous night. By morning, we are ready to "break the fast" after a stretch of not eating. The blood sugar (glucose) we need to power our muscles and brains is normally low when we wake up, and breakfast helps replenish it. But if we miss the day's first meal, notes Dr. David S. Ludwig, a nutrition expert at Harvard-affiliated Children's Hospital Boston, we may start tapping our energy reserves — including what's stored in our muscle. In addition to making us feel tired, missing breakfast is likely to increase the temptation to reach for an unhealthy pick-me-up snack later on and to overeat in general.

"The whole system gets stressed," Ludwig explained in an interview. "Skipping breakfast throws off the normal circadian rhythm of fasting and feeding. Breakfast is the worst time to skip a meal." For that reason, eating even a small amount within an hour or so of waking is a good idea. Fueling up in the morning can be especially important for children and adolescents, whose metabolic needs are relatively greater than adults. In that one limited respect, all those breakfast cereal commercials may be right.

Yet many American children and adolescents don't eat breakfast. Health surveys have shown that 20% of American children and 32% of adolescents usually miss the morning meal. The unhealthful pattern is not limited to the United States or to children and teens. A review of two dozen studies showed that between 1.7% and 30% of the population in several developed countries, including the United States, doesn't eat breakfast regularly.

Correlation or causation?

Studies have associated regular breakfast-eating with everything from enhanced memory and concentration to lower levels of "bad" LDL cholesterol to reduced risk of obesity, diabetes, and heart disease. It's hard to prove, though, that breakfast is directly responsible for all these favorable effects. A case can be made that breakfast eaters tend to have healthier lifestyles, so it may not be breakfast itself but rather healthier living overall that deserves the credit. Moreover, a fair number of breakfast studies rely on people reporting what they've eaten, which may not be entirely accurate. Plus, some studies have been sponsored in whole or part by companies that make breakfast food, so the pro-breakfast outcome seems preordained. But despite all these problems, the amount of research suggesting that breakfast has health benefits ends up being pretty persuasive.

A number of studies have focused on weight control, and researchers have found that breakfast eaters are, on average, thinner than breakfast skippers. Putting some protein and fiber into your stomach first thing may curb your appetite during the rest of the day. An interesting study published in 2010 in The American Journal of Clinical Nutrition examined the breakfast patterns of several thousand Australians in 1985, when they were children, and then about 20 years later, when they were adults. The study participants who reported skipping breakfast both as children and adults were heavier and had larger waists, higher LDL cholesterol levels, and less healthful diets than those who reported eating breakfast at both times in their lives.

But breakfast doesn't necessarily get us started on the weight-loss path. In 2011, German researchers reported that the size of the breakfast matters: the people in their study who ate big breakfasts took in more, not fewer, calories on a daily basis. Overweight people, they politely advised, should consider having a smaller breakfast if they want to trim their calorie counts.

A good cereal can be hard to find Breakfast cereal can be a nutritious and easy way to start the day, but choosing one can be dizzyingly difficult, even with nutrition labels at your fingertips. A typical suburban supermarket stocks about 130 varieties of cold cereal in its main cereal aisle, plus another 50 instant and to-be-cooked hot cereals. Many boxes and bags tout nutritional virtues and beckon with promises like "whole grain guaranteed," "good source of vitamin D," and "simply nutritious." Some depend on bright colors and cartoon-type characters. Breakfast cereal is the packaged food that's most heavily marketed to children, according to a study published in 2010 in the Journal of Public Health Policy. The researchers reported that in 2007, the average American child saw about 758 cereal advertisements on television. Why not reach for America's original breakfast cereal, Kellogg's Corn Flakes? Each serving has a reasonable 2 grams of sugars and only 100 calories. But wait: it's made from refined grain, so it delivers little dietary fiber (1 gram) and is considered high-glycemic. How about a cereal with whole grains, like one of the raisin brans? One serving of Post's famous version has 8 grams of dietary fiber, which is more than the 5-gram threshold for "high dietary fiber" used by Consumer Reports for rating cereals. But a serving also harbors 19 grams (about 4.5 teaspoons) of sugars. That's more than what you get in a Dunkin' Donuts glazed donut or some of the chocolate-flavored cereals marketed to kids. The search for a fiber-hearty cereal might draw a shopper to General Mills's Wheat Chex. It has 5 grams of dietary fiber per serving but also 5 grams of sugar and 300 milligrams of sodium, which is more than several other wholesome cereals out there. Oatmeal brims with nutritional virtue. The drawback there is that it takes between five and 30 minutes to cook the non-instant kind, which defeats one of the main purposes of breakfast cereal: getting the meal on the table fast before school or work. Many stores have a separate "natural" foods section that may have more cereals with whole grains as the main ingredient, but even there, you need to be on the look out for added sugars. Especially with children, it helps to be creative. To spruce up a bland low-sugar cereal, you might garnish it with a small amount of a sweeter cereal. You can also strike a balance by mixing a few brands together. Topping the mixture with fresh fruit adds dietary value to the meal. Another suggestion: do some homework online before you go to the store. Companies post nutritional information about their products on their Web sites. Nonindustry resources include the Glycemic Index Foundation (www.glycemicindex.com), the Cereal FACTS (Food Advertising to Children and Teens Score) project (www.cerealfacts.org), and the Harvard School of Public Health's Nutrition Source (www.hsph.harvard.edu/nutritionsource).

Going with the (whole) grain

Dr. Ludwig and other nutritionists say a good breakfast should include some carbohydrates with fiber (whole grains, fruits, or vegetables), some lean protein sources such as eggs or yogurt (Greek yogurt has more protein than regular), and some healthful fats such as those in nuts or salmon. A vegetable omelet with a slice of whole-grain toast qualifies as a good breakfast, as does a bowl of high-fiber cereal topped with fresh fruit and reduced-fat or soy milk, along with a handful of almonds or walnuts.

Carbohydrates often get a bad rap in diet books, but they belong in a good breakfast. They have been labeled simple or complex, based on the sugar molecules they contain. Another way to classify them is by their effect on blood sugar levels: foods with a high glycemic index get digested quickly and cause blood sugar levels to spike, triggering an extra-large release of insulin to bring them back down. Bagels, pastries, and sugary breakfast cereals generally have a medium-to-high glycemic index. Some research shows that high-glycemic foods wind up making people hungry sooner. And high-glycemic diets have been correlated with increased risk for diabetes, heart disease, weight gain, and other conditions that nobody wants.

Whole-grain foods enable you to have carbohydrates and keep the glycemic index of your breakfast down. In addition to moderating blood sugar spikes, whole grains supply vitamins, minerals, healthy fats, dietary fiber, plant enzymes, and other potentially valuable substances, many of which get stripped away during the refining processes that make grains easier to digest and store. You don't have to look very hard to find epidemiologic evidence for whole grain having protective effects against obesity, constipation, type 2 diabetes, and heart disease.

8 tips for breakfast

1. Read food labels carefully. Look for the serving size, calories, and nutrient information. For grain foods, choose products with whole wheat, oats, rye, or other whole grains listed first in the ingredients.

2. Know your coffee drink. For example, a 16-ounce White Chocolate Mocha at Starbucks contains 470 calories, 12 grams of unhealthful saturated fat, and 59 grams of sugars, versus just five calories and no fat or sugars (but more caffeine) in a cup of black coffee.

3. Make processed meats like bacon and sausage a very occasional treat. Processed meats have been associated with a higher risk of colorectal cancer, heart disease, and type 2 diabetes.

4. Get your carbohydrates from whole grains, fruit, and vegetables, not from food that has been highly processed.

5. Eggs in moderation (up to one a day) are okay for healthy people. Although yolks are high in cholesterol, eggs have proteins, vitamins, and other nutrients and don't appear to increase the risk for developing heart disease.

6. Go easy on fruit juice. Whole fruit is often a better choice because it tends to have more fiber.

7. Eat in, not out. You can enjoy a healthful breakfast out if you stick to oatmeal or yogurt (preferably no fat and nonsweetened). But much of the traditional fare (eggs and bacon, pancakes) will start your day with loads of calories and saturated fat. Like most processed food, the breakfast offerings from fast- food chains tend to be high-sodium, low-fiber disasters. McDonald's Egg McMuffin has 300 calories (not bad) but 820 mg of sodium (36% of the daily limit, according to new government guidelines) and just 2 grams of fiber.

8. Blend up a breakfast smoothie. A little home processing is okay. You can combine fruits, juice, yogurt, wheat germ, tofu, and other ingredients. Lots of recipes can be found online.

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