The implication, the researchers conclude, is that overweight individuals who lose weight may have to learn to live with feeling hungry.
They suggest that their recent findings, which have now been published in the American Journal of Physiology-Endocrinology and Metabolism, support the idea that obesity should be treated as a long-term illness.
Type 2 diabetes is treated in a similar way, and individuals with the condition are monitored closely to help them hold onto their gains.
"We have to stop treating [obesity] as a short-term illness," explains lead study author Catia Martins, an associate professor in the Department of Clinical and Molecular Medicine at the Norwegian University of Science and Technology in Trondheim, "by giving patients some support and help, and then just letting them fend for themselves."
'Gold standard in obesity treatment'
In the United States, obesity is common and affects 36.5 percent of the adult population. It costs more than $147 billion per year to treat.
Prof. Martins and colleagues studied morbidly obese adults who took part in a 2-year weight loss program during which they attended five 3-week residential sessions.
Morbidly obese is defined as having a body mass index (BMI) that is greater than 40.
At each residential session, the participants received advice and therapy and learned about weight loss and how to achieve it through diet and exercise.
"We gave 34 patients with morbid obesity the gold standard in obesity treatment over a period of 2 years," Prof. Martins notes.
Between the residential sessions, the participants were all urged to continue with what they had learned about maintaining a healthful diet and getting some exercise every day.
Hunger 'seems to override' satiety
All of the participants gave blood samples and completed questionnaires about their feelings of hunger and fullness 4 weeks, 1 year, and 2 years after the start of the program. From the blood samples, the team was able to assess levels of hormones that control hunger and fullness, or satiety.
The participants did not report a change in their feelings of fullness at the 4-week assessment but did report a reduction after 1 and 2 years of sustained weight loss. In contrast, they reported a significant increase in hunger at the 1- and 2-year assessments.
The blood tests showed higher levels of both satiety and hunger hormones after 1 and 2 years of sustained weight loss.
The study's authors suggest that the increase in the hunger hormone ghrelin "seems to have overridden" the rise in satiety hormones.
"This means," they write, "that patients with severe obesity who have lost significant amounts of weight with lifestyle interventions, combining diet and exercise, will have to deal with increased hunger in the long-term."
Overall, the participants lost an average of 11 kilograms (around 24 pounds) over the 2 years, with around half of that being shed in the first 3 weeks.
After the program, only 20 percent of the participants sustained their weight loss. Prof. Martins says that this is roughly in line with established research: most people with obesity can achieve weight loss — even by themselves — but 80 percent of them put it back on again later.
"Obesity is a daily struggle for the rest of one's life."
Prof. Catia Martins