More than 60% of Americans are classified as overweight or obese. What’s going wrong?

For answers, Bottom Line/Health spoke with Marion Nestle, PhD, MPH, one of the country’s leading nutritionists and coauthor of Why Calories Count.

Instead of focusing on calories, the most popular diets these days focus on eating less of certain food groups such as fats and carbohydrates—or more of foods rich in, say, protein or fiber. Is this a good idea? Numerous studies have compared popular diets such as Atkins, Ornish, The Zone and South Beach—all of which promote eating one type of food over others or eliminating certain food types. And the studies have concluded that all low-calorie diets help people lose weight, regardless of how much protein, fat and carbohydrates are included. Some people do find higher-fat diets, such as Atkins, to be more satisfying, so they aren’t so hungry all the time.

What’s the best way for a person to determine his/her individual daily caloric needs? “Calorie” is such a widely known term, you might assume that most people would already know their daily caloric requirements. But research suggests there’s a widespread “calorie oblivion” in which consumers are clueless about the number of calories they should be eating and how many they actually are eating. A study conducted in 2011 found that only 9% of respondents even came close to estimating the number of calories they should consume each day.

The average number of calories required for healthy living, according to guidelines, is 2,000 to 2,500 calories per day for adult males and 1,800 to 2,300 for adult females.

The formula used to accurately determine an individual’s exact calorie requirements is complicated and requires a professional to help calculate. It involves your basal metabolic rate (BMR), the heat effects of food and the energy expended in physical activities.

Of course, energy expenditure levels decline with age—by as much as 20% by age 50 and by 30% for those over age 70. Also, men of all ages expend more energy than women, on average, and people with more lean muscle burn more calories doing the same activity than those with more fatty tissues.

If it’s as simple as counting calories, then why do so many people have such problems maintaining normal body weight? The truth is, nobody wants to think about calories. They are too abstract. You can’t see them, smell them or taste them. You can only see their effects on your weight and waistline. It’s impossible to know exactly how many calories are in many foods unless you identify and weigh every ingredient. Plus, Americans have no idea of proper serving sizes.

So it’s not very surprising that virtually everyone significantly underestimates the number of calories eaten each day—typically by about 30%, according to research. Even when calorie information is available (for example, on labels or posted at restaurants), people still are shocked to find out that a bagel, for instance, can contain up to 600 calories…and the supersized 64-ounce sodas sold at convenience stores have nearly 800 calories each.

What’s the best way to figure out how many calories one needs to cut daily to lose weight? It takes approximately 3,500 extra calories to put on a pound of body fat. So cutting back by 500 calories a day is reasonable and should result in losing about one pound a week—but it varies a lot with individuals. To lose weight, you need to balance your caloric intake with your physical activity…eat more fruits, vegetables and whole grains…eat less meat and dairy (and choose low-fat)…and stay out of the center aisles of supermarkets—that’s where junk food is found. To see how well you’re doing at maintaining a healthy body weight—or even dropping a few pounds—simply weigh yourself on a household scale. The most successful dieters do that routinely and compensate for small increases by eating less.

Why do some people lose weight more easily than others—even when they consume the same number of calories? Some of it has to do with metabolic rate and some with so-called “spontaneous non-exercise activity,” also known as fidgeting, which can account for as much as 100 to 800 calories a day. There are also factors that remain unknown. Studies on overeating demonstrate that all people gain weight if they overeat calories, but some gain a lot more than others.

Is that due to genetics? A genetic propensity to obesity can be a factor—especially when there is food around all the time and it’s served in big portions. People must learn to manage that environment to control their weight.

Does chewing food slowly help with weight loss? It might. The research is mixed, but when Chinese researchers recently counted the number of times that obese and lean people chewed their food, they found that those who chewed each bite 40 times before swallowing consumed 12% fewer calories than those who chewed only 15 times. The number of calories burned during the act of chewing is trivial, so it could be that slow eaters simply feel more satisfied than fast eaters or get bored with eating so slowly.

What about so-called “calorie-restricted” diets that limit people to, say, 25% fewer calories than the recommended daily intake. Do you consider these diets healthful? No. Research shows that sustained calorie restriction improves the lives of fruit flies, rats, mice and other animals. But it’s not at all clear that it extends human life. The lowest rates of mortality occur among people who have a “normal” body mass index (BMI) of 18.5 to 24.9. When a person’s BMI is considered “underweight” (less than 18.5), which often occurs in those who are practicing this form of calorie restriction, mortality rates increase. This isn’t a healthful approach to eating—and it can’t be much fun.

How can we ensure that we’re getting adequate amounts of the various nutrients that we need in our diets? Unless you restrict many different types of foods, you can take care of your nutrient needs by eating a variety of nonprocessed foods—that is, ones that don’t come in packages with “Nutrition Facts” labels. Whole foods contain many different nutrients in varying proportions—and all of them complement each other. Even people who want to, or must, restrict certain foods should vary the foods they do eat.

Source: Marion Nestle, PhD, MPH, Paulette Goddard Professor in the department of nutrition, food studies and public health and professor of sociology at New York University in New York City. She is the author of several books, including What to Eat (North Point), and coauthored, with Malden Nesheim, PhD, Why Calories Count (University of California).