This is it. This time, you’re really going to lose weight. Sure, maybe you’ve tried to lose weight before. But almost inevitably, the pounds come back. Why?

The reasons, as you’ve probably realized, are complicated. And that’s why weight loss is never a simple matter.

From a medical perspective, being over­weight means that you carry more pounds than recommended for your height and build by experts at places like the Centers for Disease Control and Prevention (CDC). Being obese means that you’re over your recommended weight by 20 percent or more, with body fat accounting for 30 percent of your weight if you’re a woman, 25 percent if you’re a man. (Women naturally carry more fat than men.)

But the experts still don’t agree on what the recommended weight ought to be. And weight standards have varied among cultures throughout history. Until the early twentieth century, Americans viewed being heavy-”portly” or “corpulent” in the lexicon of that bygone eraas a sign of good health. Then actuaries with the life insurance industry realized that compared with people of average weight, the fattest people died younger.

The CDC estimates that 35 percent of American adults, 12 percent of adolescents, and 14 percent of children are overweight.

How heavy do you have to be to experience weight-related health problems? Harvard researcher Joann Manson, M.D., investigated that issue using data from the ongoing Nurses’ Health Study, which has tracked more than 100,000 women nurses for almost 20 years. Her conclusion: Risk begins to increase when you weigh 22 pounds more than you did at age 18.

Actually, most of us don’t have to lose many pounds to reduce our risk of weight ­related health problems. “Convincing studies have shown that the diseases most commonly associated with obesity-type 2 diabetes, high blood pressure, high cholesterol, and heart disease-can be significantly improved with a loss of just 10 percent of your weight;’ says Robert Kushner, M.D., medical director of the nutrition and weight­management program at Northwestern Memorial Hospital in Chicago.

The fundamentals of weight loss are simple: You have to burn more calories than you consume. To lose 1 pound a week, you have to burn 500 more calories a day than you consume.

Frankly, that’s not easy. Eating 500 fewer calories leaves most people feeling deprived, and burning that many calories in exercise means engaging in strenuous aerobics for 45 minutes or walking briskly for over an hour.

A goal of reducing by 1 pound a week is simply beyond what most people are prepared to maintain for more than a month or two. That’s why psychologist Ronnette Kolotkin, Ph.D., of the Diet and Fitness Center at Duke University in Durham, North Carolina, suggests a goal of 1 pound every 2 to 4 weeks, meaning that you lose 12 to 24 pounds in a year.

But even at that pace, weight loss can be difficult. Part of the problem relates to genetics and upbringing. A child with one obese parent has a 40 percent chance of becoming obese. A child with two obese parents has an 80 percent chance. But genetics and upbringing are not destiny.

Most of us Americans are overweight because of our poor eating and exercise habits. On the nutrition side, our rising weight reflects our increased intake of fat. As a nation, we’ve become so fat-conscious that food manufacturers now make low-fat and nonfat versions of just about everything. But at the same time, our consumption of high­fat fast foods has soared. In 1984, fewer than one-third of Americans ate pizza twice a month. Now half of us do.

“Americans have unprecedented access to a poor diet-to highcalorie foods that are widely available, low in cost, heavily promoted, and good-tasting,” says weight-control expert Kelly Brownell, Ph.D., professor of psychology, epidemiology, and public health at Yale University. “We’ve created a toxic food environment.”

As for exercise, the average American gets nowhere near enough. A few generations ago, people had more physical jobs, and they walked a lot more. Today, we drive, take elevators, and use dozens of labor­saving devices. What’s more, most of us work in physically undemanding professions, spending most of our workdays sitting at a desk.

A big reason so many Americans are over­weight is that many of us equate weight loss with dieting-that is, food restrictions. Dieting American-style is noteworthy for two things: fads and failure. Every few months, it seems, some new fad diet appears, and legions of people try it.

But studies of very low calorie diets by researchers at the University of Pennsylvania School of Medicine in Philadelphia show that after a few months and weight loss of around 30 pounds, the weight comes back with a vengeance. Within a year, most people regain two-thirds of their lost weight, and within 5 years, they’re back to their prediet weight-or they’re heavier.

But if you strip away the hype from fad diets, diet books, and the various commercial and medically supervised weight-loss
programs that are generated by the diet industry, the experts generally agree that there are four interrelated keys to permanent weight loss.

• A low-fat, moderate-calorie diet
• Regular exercise
• Emotional readiness to commit to weight loss
• A lifelong commitment to weight control

This strategy has been embraced by the small but significant number of Americans who have successfully, permanently lost a great deal of weight. Very few of those who win at losing rely on fad diets, drugs, or commercially prepackaged diet meals. Instead, they change their lives, and slowly but surely, they shed unwanted pounds. You can do the same just by embracing the strategies that follow.