A new study analysis of various weight loss trials highlights the biological and environmental explanations for why keeping weight off can be difficult.
This is a fascinating question with potentially important social implications for overweight or obese unsuccessful dieters, looking to lower their risk for complications like type 2 diabetes, fatty liver disease or heart disease.
This study review, published in the Diabetes Spectrum journal, was conducted by researchers at the University of Washington Medical Diabetes Care Centre, in Seattle, US.
The authors explain that while about half of weight variance may be due to genetics, the other half is determined by environmental influences.
With regards to our environment, certain lifestyle factors play into how and why things somethings evolve as they do during weight loss.
These factors include eating hypercaloric foods and overeating, having a reduced physical activity or declining mental health leading to feeling demotivated to exercise, an impaired sleep or unhealthy eating patterns.
The body tightly regulates weight through metabolic, hormonal and neural regulation of feeding. Adaptations such as decreases in leptin mirrored by increases in ghrelin and dopamine in the brain reward system can cause constant unsatisfied hunger, for example.
When one or several of these systems become out of balance, it can also affect the expression of our genes, over which we can be rulers through putting in action healthy behavioural changes.
Genomic wide association studies (GWAS) have found that nearly 150 genetic variants are significantly associated with measures of Body Mass Index (BMI), waist circumference and obesity risk amongst different populations.
The researchers discuss the cycle of weight loss and subsequent weight regain, triggered by a slowing of resting metabolic rate (RMR) people are often confronted to, resulting in a decrease in energy expenditure and calories expended over time.
One study included in the review found that formely obese individuals had a 3 to 5 per cent lower RMR than did lean individuals. Nearly all participants in the reality show, The Biggest Loser, who experienced weight regain had a 500 calories lower RMR at the end.
The review points out that people with type 2 diabetes, compared to those without type 2 diabetes, tend to lose less weight from similar weight loss interventions.
Researchers concluded that relapse among obese people who have lost weight has a strong basis based on long-term hormonal changes and does not only reflect a lack of willpower.

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