Stacey C. Cahn, associate professor of clinical psychology at Philadelphia College of Osteopathic Medicine, wrote this for Philly.com's "Goal Getter" blog: philly.com/goalgetter

Depending on how you define it, only 3 percent to 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 were simply the result of sloth and gluttony, the rate of obesity would have been constant throughout human history. It hasn't been.

So, why do lost pounds boomerang back?

After weight loss of 10 percent or more, our bodies change in important ways to "defend" the higher weight. These biological adaptations serve to promote regaining weight and seem to persist for at least a year post-weight loss, even after weight regain begins. Some of the most significant biological changes are:

After major 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.

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

Once people lose significant weight, their metabolism slows, their hunger increases, 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.

To maintain significant weight loss, people need to eat less, despite feeling hungry. 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.

However, indefinite self-denial is unrealistic. All of us have limited willpower. Finite willpower combined with the obesogenic environment is a recipe for regaining weight, unless you can reengineer your personal environment. Steps to try:

Log what you eat, and you will make better food choices, and eat less, even without a specific diet.

Try programs with high-frequency, in-person contact, such as meetings.

Limit added sugars and processed foods, including artificial sweeteners. They may have "addictive" properties and may harm your microbiome, or "gut bacteria."

Regular, frequent exercise is a powerful method to maintain weight loss, likely because it helps counter metabolic adaptations.