For decades, doctors and nutritionists have advised weight-loss patients to follow a simple formula: one pound of fat equals 3,500 calories. Create a daily deficit of 500 calories through exercise or diet and one can expect to lose a pound per week.

Over the past decade, researchers have learned that weight loss is much more complicated. The “3,500 calorie rule” is a myth and fails to account for many factors, including how a person’s metabolism adapts over time.

Although scientists have debunked the “3,500 calorie rule,” it continues in textbooks, advice websites and in conversations between clinicians and their patients. Researchers are calling for an end to the myth so that expectations can be reset and patients who lose weight can be better prepared for success.

The making of a myth

The 3500 calorie rule derived from a Article from 1958 c Clinical Journal of Nutrition by a doctor who removed half a kilogram of fat from a patient and then calculated the energy content of the adipose tissue.

From there, the doctor found that by burning 3,500 calories, a person could lose a pound of fat. Over the next few decades, the 3,500 calorie rule became the norm standard advice. Weight loss patients were advised that if they cut or burned 500 calories per day, they would be rewarded with a weight loss of one pound per week.

And so it continued until the end of the century and beyond. But in 2010, scientists increasingly began to question the 3,500-calorie rule. Obesity had been on the rise for decades and diet advice was plentiful, but it seemed to fail people. Researchers took a closer look at the 3,500 calorie rule and found it greatly overestimated weight loss.

Read more: The myth of negative calorie foods.

Part of the problem was that the 3,500 calorie rule proved itself static equation. It is assumed that a person’s body will continue to respond in the same way to calorie restriction over time. The rule does not account for other biological factors during this period, including how an individual’s metabolism will develop I am adapting to these time constraints.

For most people, the equation can work in the beginning on a diet. One can initially see results from one’s efforts. But as their bodies adapted to the calorie restriction, their daily caloric needs changed without them realizing it. For some people, greater caloric deficit was necessary to continue weight loss.

The 3,500 calorie rule also failed to take into account how a person’s starting point might affect their long-term weight loss. A person with more adipose tissue, e.g. it may take longer to reach a stable weight from a thinner person.

Long term struggles

For people who have managed to lose weight, long-term maintenance has proven difficult. In analysis of 29 long-term weight loss studies, researchers found that dieters regained 50 percent of their lost weight within two years. After five years, they recovered an average of 80 percent.

2016 study in the diary obesity helped explain why weight loss can be difficult to maintain. The researchers worked with 14 people who were contestants on the reality show The biggest loser and just finished the race. The researchers measured the participants’ resting metabolic rate (RMR) and then followed up six years later.

Over time, only one participant kept the weight off. The rest gained an average of 88 percent of their weight back. The researchers found that the participants’ RMR was reduced during the race, meaning they needed fewer calories even though they built lean muscle mass. Over time, the participants needed an average of 500 fewer calories per day than when they finished the race, even as they gained weight.

Additionally, the study found that participants with the greatest weight loss during the race experienced the greatest deceleration in RMR. For some participants, there was evidence that their RMR was still slowing six years later.

Other diets have also undergone a change in RMR so that over time a diet that once worked no longer works. Scientists beware clinicians against dropping the 3,500 calorie limit and then blaming patients for a lack of will. A person, for example, who gave up two sodas a day would initially see weight loss from the new 300-calorie daily deficit. But over time, the body will adapt, weight loss will slow, and expectations will be shattered.

Instead of leaving a person to blame themselves, the researchers urged dieters to forget the 3,500-calorie myth and readjust their expectations. They reasoned that the body’s metabolism would adjust overtime, and a person looking for long-term weight loss would need to make changes.

Several online calculators have been created to help people follow more dynamic weight loss expectations. one gives a forecast weight loss over time. Another is a body weight planning program which allows the user to enter physical activity and select the phase of their weight loss goals. Both are designed with the idea that weight loss is not static and that as one’s body adapts, one’s diet must also be modified to see results, beyond what any 60+ year old nutrition document can offer .

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