The World Health Organization (WHO) estimate that the global prevalence of obesity has almost tripled since 1975. In 2016, more 650 million adults had obesity.
In the United States, the prevalence of obesity among adults increased from 30.5% to 42.4% of the population between 1999 and 2018, according to the Centers for Disease Control and Prevention (CDC).
Obesity is associated with an increased risk of heart disease, stroke, type 2 diabetes, and some types of cancer, making it a major cause of preventable premature death.
Estimates of the number of deaths that can be attributed to obesity vary widely, however, because of the complex interactions between body weight and factors such as age, smoking, diet, and physical exercise.
In addition, scientists are still unsure whether carrying excess weight in young adulthood causes harmful physiological changes that cannot be reversed by subsequent weight loss.
A team led by researchers at the Boston University School of Public Health, in Massachusetts, set out to discover whether losing weight after having obesity in early adulthood is associated with a reduction in mortality risk later in life.
The scientists recently published their findings in the journal JAMA Network Open.
Doctors often use a measure called body mass index (BMI) to determine whether a person has a healthy weight. To calculate this, the doctor divides the person’s weight in kilograms by their height in meters squared.
Experts define a healthy BMI as 18.5–24.9, one indicating overweight as 25.0–29.9, and one indicating obesity as 30 or higher.
The researchers behind the present analysis analyzed data from 24,205 people in the U.S. who were part of the National Health and Nutrition Examination Survey (NHANES).
As part of this survey, scientists recorded the weights of the participants when they were between the ages of 40 and 74. They also asked the participants how heavy they had been 10 years earlier, which the researchers described as midlife, and at the age of 25, which they defined as early adulthood.
The team behind the current study looked at BMI changes from early adulthood to midlife and their effects on mortality, measured during the NHANES follow-up period. Then, they took into account other factors that affect mortality risk, such as sex, past and current smoking, and education level.
Over a mean follow-up period of 10.7 years, there were 5,846 deaths among the participants.
The team found that going from a BMI indicating obesity in young adulthood to one indicating overweight by midlife was associated with a 54% reduction in mortality risk, compared with maintaining a BMI of obesity over the same period.
Also, people who shifted from having obesity to overweight between young adulthood and midlife had much the same mortality risk as people who had overweight only during this period.
The results taken together suggest that the harmful effects of obesity can be reversed.
The scientists estimate that 3.2% of all the early deaths in the study would have been prevented if people with BMIs indicating obesity had reduced this measure to fall within the overweight range by midlife.
In addition, they calculate that 12.4% of all the early deaths could have been avoided if everyone with a BMI indicating overweight or obesity had reduced it to fall within the healthy range by middle age.
“The results indicate an important opportunity to improve population health through primary and secondary prevention of obesity, particularly at younger ages,” says senior study author Prof. Andrew Stokes.
Another study author, Dr. JoAnn Manson, chief of preventive medicine at Brigham and Women’s Hospital, in Boston, refers to the link between obesity and a range of chronic illnesses:
“Although this study focused on preventing premature deaths, maintaining a healthy weight will also reduce the burden of many chronic diseases, such as hypertension, diabetes, heart disease, and even cancer,” she says.
Reflecting the public health challenge posed by overweight and obesity, however, the study found that weight loss was rare among the participants.
Only 1.3% of individuals with BMIs in the overweight range at 25 years of age had a healthy BMI 10 years before the NHANES interview, while 0.8% went from obesity to overweight by this measure, and 0.2% went from obesity to a healthy BMI.
And while losing weight in early adulthood was associated with a substantial reduction in mortality risk, the same was not true for those who lost weight later in life.
The authors believe that this is because weight loss later in life may result from worsening health, rather than adopting a more healthful lifestyle.
They write:
“The discrepancy likely reflects the different nature of weight loss at an earlier versus later life course. Weight loss at an older age is often unintentional, associated with underlying health conditions, and/or age-related loss of muscle mass, whereas weight loss earlier in life tends to capture changes in fat mass and is less likely to be affected by the onset of chronic diseases.”
Transitioning from a healthy or overweight BMI in young adulthood to a BMI in the obesity range in midlife was associated with increases in mortality risks of 32% and 47%, respectively, compared with staying in the healthy range.
However, the researchers report that going from a healthy BMI to an overweight BMI was not associated with a significant change in mortality risk, compared with maintaining a healthy weight.
This is in keeping with other studies, which suggest that being in the overweight range does not necessarily shorten lives, though this remains controversial.
Rounding off their report, the scientists explain that their study had several limitations.
To assess weight change between early adulthood and midlife, the scientists had relied on the participants’ recollections of their weights 10 and 25 years earlier. In addition, the team was unable to account for physical activity or diet in early adulthood.
Finally, the proportion of participants who lost weight was small, which the scientists say limits the precision of their estimates.
