Despite the increasing number of people in the United States who say that they are trying to lose weight, the average body mass index (BMI) of US adults is on the rise.

This is according to new research appearing in the journal JAMA Network Open.

Dr. Lu Qi, director of the Tulane University Obesity Research Center in New Orleans, LA, is one of the corresponding authors of the study.

As per the study paper's title, the research examined "[t]rends in self-perceived weight status, weight loss attempts, and weight loss strategies among adults in the United States" in 1999–2006.

Dr. Qi and team used nationally representative data from nine applications of the National Health and Nutrition Examination Survey (NHANES) between 1999 and 2016.

In total, they had access to data from 48,026 participants. The researchers examined trends in measured BMI and weight, self-reported weight during the past year, and the difference between measured weight at present and self-reported weight in the prior year.

Two contradicting trends

The study demonstrated that the proportion of participants who tried to lose weight increased over the study period, despite the fact that current weight and weight in prior years had increased.

Specifically, the number of adults who said that they had tried to lose weight rose from 34% to 42% in 1999–2016. During this time, the most common weight loss strategies the participants applied were eating less, exercising more, and drinking more water.

US adults also reported changing their eating habits, such as "consuming less sugar, candy, and sweets" and eating less junk food or fast food.

However, despite these efforts, the "trends for actually measured weight and self-reported weight history increased during the same time period," the study authors write.

In fact, if 33.7% adults had obesity in 2007–2008, this number jumped to 39.6% in 2015–2016.

This suggests, the study authors explain, that the weight loss strategies "may not have translated into effective weight loss," even though studies show that these strategies should be effective.

"These findings," says Dr. Qi, "suggest that although 34–42% of US adults in our study reported weight loss efforts, many of them might either not actually implement weight loss strategies or apply a minimal level of effort, which yielded unsatisfactory results."

Sticking to weight loss strategies is key

In other words, US adults hoping to lose weight would need to apply more effort, particularly in terms of adherence. "Reduced food consumption is one common strategy to lose weight, but modified diets are difficult to maintain," says Dr. Qi.

"These findings suggest a need to increase the promotion of effective strategies for weight loss, including caloric reduction and increased physical activity, among all adults attempting to lose weight," she adds.

"Notably, adherence is the primary factor predictive of a successful response to a weight loss attempt. Therefore, weight loss strategies that consider a participant's preferences and abilities may help them stick with it long-term."

—Dr. Lu Qi

The researchers also mention the possibility that the contradiction in trends is down to the fact that the participants who tried to lose weight were not those who may have needed to.

Conversely, those who might have needed to lose weight for health reasons may not have tried very hard to do so because they considered their weight to be "normal."

In fact, the number of people who thought they were "approximately the right weight" also increased over the 15 year period, report the researchers.

Potential study limitations

The researchers acknowledge some other potential limitations to their study. Firstly, participants self-reported the data on weight self-perceptions and weight loss efforts, which makes the information subject to potential bias.

However, the study authors say that the difference between how the participants reported their current weight and their measured weight was minimal, indicating that their approximations may have been correct.

Secondly, the NHANES surveys did not collect information on the "frequency, duration, or number of weight loss attempts or strategies."

Finally, and importantly, the study could not establish causality or temporal relationships in the observed trends.