At the beginning of 2015, market data showed that the weight loss industry was worth roughly $60 billion. The industry targets hundreds of millions of dieters that are on a quest for a smaller waistline and a lower number on the scale.

Yet, despite the money spent on weight loss programs, diet foods and supplements, Americans aren't getting much slimmer.  The US Department of Health and Human Services reports that more than two-thirds of adults are overweight and that number has remained steady over the past 10 years.

In fact, a 2015 study in the Annals of Internal Medicine examined 32 national weight loss programs in an attempt to help physicians decipher which ones to recommend to patients. Of the programs studied, only two met the highest standards and even the two most successful ones were only moderately effective.

There are several explanations as to why diets and weight loss programs don't work. Here are my top three reasons:

Everyone is Different

Most diets are rigid, one-size-fits-all programs that don't take into account an individual's unique biochemical make up. There is no one diet that works for everyone. The theory, also known as bio-individuality, explains why some people can eat foods that are considered healthy but won't see the same results as another person eating those same foods. Instead of dieting, get to know your body and how it responds to certain foods and exercise.

A Quick-fix Focus

Many diets focus on eliminating or restricting certain foods. This process can be successful for a certain period of time, but after a while, the deprivation mentality can sabotage dieters results if they binge on the very foods they've been restricting. This explains why a large majority of people who lose weight, gain it back. In many cases, not only does the dieter return to their previous weight, but they often gain even more than the initial figure. Deprivation and restriction aren't sustainable long-term. The most successful programs focus on changing a few habits at a time that build on each other to trigger more healthy habits down the road.

Too Few Calories 

Finally, some of the most popular diets are in the business of cutting calories. Dropping your daily caloric intake too low can upset your metabolism and have the reverse effect of weight loss. Instead of dropping weight and burning fat for energy, dieters who get too few calories, will burn muscle and hold onto weight. Rather than cutting calories, those looking to lose weight should replace empty calorie foods (like packaged snacks, soda, sugar and refined white carbs) with nutrient-dense foods (i.e. vegetables, whole grains, fruit, lean proteins and healthy fats).

Fad diets are bound to fail due to the fact that they are a one-size-fits-all quick fix to a problem that is individualistic with goals that realistically take several months or years to successfully achieve. When trying any weight loss program, time is key.

Brian Maher is the owner of Philly Personal Training, a Philadelphia-based studio offering 1-on-1 personal training, physical therapy, and nutrition counseling. Philly Personal Training is the only personal training studio or gym in Philadelphia that requires its personal trainers to possess a college degree in an exercise-related field, as opposed to a basic certification.

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