If it seems like every other person you know, including yourself, has tried to diet, you’re not wrong. 42% of all people are actively trying to lose weight. Not “have tried in the past” or “are thinking about it in the future.” Rather, nearly half of all adults are trying to shed pounds, right this minute.
It’s staggering to find out, then, that the overwhelming majority of diets end in failure. Depending on the numbers and source, either one-third of dieters return to their old weight, or as many as 65% do.
Our other articles will focus on which diets are trending, and overall weight loss statistics in the US; this article will bear down on only the failure statistics.
How Many Diets Fail, and By How Much
One of the most common measuring sticks for a diet failing is weight regain after a certain period of time. The Journal of Clinical Nutrition  has compiled data from a registry of volunteers, and here are some of their findings.
Meanwhile, more nuanced data was taken from a different population by the Medical Clinics of North America.  These data are compiled from an analysis of dozens of diet groups, spanning thousands of participants.
Another set of studies quoted by UCLA found that:
Why Diets Fail
Some diets fail because they are inherently ineffective, some fail because of individual factors.
Some diets fail because of the misunderstanding of changes in the body:
This can explain why people do not keep their weight off; their appetite is increasing while their calorie needs are actually decreasing.
Repeated Diet Failures
One of the datum to track to conceptualize diet failure is the number of people who have tried to diet multiple times. Logically speaking, if a first or second diet succeeds it obviates the need for a subsequent diet attempt.
How People Diet
Ascertaining whether someone is dieting alone, or in conjunction with multiple disciplines, can give insight as to their success, as well as what types of dieting the engage in. 
Much of the research into diet failure is subsumed into larger data frameworks of weight loss, obesity, and fitness as a whole. Those data will be compiled in future articles.