People lose weight every day, on just about any kind of diet. What's far less common is the long-term maintenance of that weight loss. Depending on how you define it, only 3-28 percent of people achieve "long-term" weight loss. A year or two later, the vast majority have gained the weight back, a phenomenon I call "the Boomerang Effect."

One popular belief is that people regain weight for the same reasons they gained it in the first place: they shirk "personal responsibility;" they're lazy, incompetent, and disinterested in health and appearance. This notion is based in weight stigma, not fact.

The dramatic rise in U.S. obesity began in the late 1970s/early 1980s — absent any corresponding change in human personality. If obesity was simply the result of sloth and gluttony, the rate of obesity would be constant throughout human history. It's not.

So, why do lost pounds boomerang back?

  • Hormones help regulate our experience of hunger and fullness. After significant weight loss hormones associated with the sensation of hunger (e.g., ghrelin) increase, and those associated with the sensation of satiety, or fullness (e.g., leptin), decrease.

  • Metabolism becomes more "efficient"; people burn fewer calories, both at rest, and during exercise, than they did before the weight loss. Importantly, the decrease in body mass alone cannot fully explain the magnitude of this change.

Real-life example: Let's say Amy and Bea are the same height and weight (150 pounds). Amy has weighed 150 pounds all her adult life. Bea, however, used to weigh 210 pounds, but has now dieted her way down to Amy's weight.

Now that Amy and Bea are at the same height and weight (and activity level), they should require the same number of calories, right?

Wrong. To maintain the weight of 150 pounds, not only does Bea have to eat 28 percent less than she did at her heavier weight, but 15-24 percent less than Amy does at the exact same weight

The "formerly-obese" are biologically different than those who have never been obese. Once people with obesity lose significant weight, their metabolism slows, their hunger increases, and their feeling of fullness decreases…and the weight loss becomes that much harder to maintain.

It is, and may forever be, so much harder for Bea to maintain her weight than it is for Amy.

We're built for Survivor…but live in Willy Wonka's Chocolate Factory.

As a species, we've evolved to survive scarcity and famine -- the antithesis of today's obesogenic food environment, where calories have never been cheaper or more accessible.

To maintain significant weight loss, people need to eat less, despite hunger. Yet, the environment profoundly influences when, what, and how much we eat, and we're surrounded by constant cues to consume unhealthy food and beverages, even in health care centers, schools, and colleges.

However, indefinite self-denial is unrealistic. All of us, from the most pious to the most hedonistic, have limited willpower. Finite willpower combined with the obesogenic environment are a recipe for weight regain (unless you engineer your personal environment otherwise).

So how is anyone with obesity able to lose weight and keep it off (naturally)?

How to beat the odds for successful, long-term weight loss:

  1. Log what you eat, and you will make better food choices, and eat less, even without a specific diet. Consider using a smartphone app, or simply taking a picture on your smartphone of everything you eat; accountability is key.

  2. To maximize probability for weight-loss success, try programs with high-frequency, in-person, "contact" (e.g., programs that involve regular, in-person, meetings).

  3. To the extent possible, limit added sugars and processed foods, including artificial sweeteners. They may have "addictive" properties and may harm your microbiome or "gut bacteria." (What's your "microbiome"? Start here.)

  4. Forget our controversial, new dietary guidelines; Brazil's are better.

  5. Regular, frequent exercise is a powerful method to maintain weight loss, likely because it helps counter the metabolic adaptations described above. Ignore the "calories burned" counter on cardio machines, and ignore the 3500 calories=a pound rule: they're both inaccurate. We have much more accurate mathematical models now. Here's a new, user-friendly, free calculator, based on Dr. Kevin Hall's work at the National Institutes of Health (NIH): Body Weight Planner.

Stacey C. Cahn, Ph.D. is associate professor of clinical psychology at the Philadelphia College of Osteopathic Medicine (PCOM). Dr. Cahn specializes in obesity, eating disorders, and cognitive-behavioral therapy.

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