Anybody who has grappled with weight loss knows that even once the kilos come off, it can be hard to keep them off in the long term.

A study on the hormones that govern feelings of hunger and fullness has shown why it's such a struggle for some individuals, revealing that changes in body chemistry two years after shedding weight can have an impact on our desire to go back for second helpings.

Researchers from Norway and Denmark made the discovery after tracking the fitness, body mass index (BMI), hormone concentrations, and reported hunger levels of 35 adults over two years.

Each volunteer entered the program as severely obese before undergoing a rigorous weight loss program involving a calorie-restricted diet, exercise, and therapy.

Following a three week residential session, the participants went home and continued to exercise and eat healthier.

Two years later all of the subjects successfully lost significant amounts of weight and had better cardiovascular fitness. But their hormones and reported feelings of fullness and hunger told an interesting story.

A month into the weight loss program, the volunteers reported feeling fuller after meals, and experienced no significant change in their hunger levels while they fasted.

Coinciding with this sensation were higher levels of a hormone called peptide YY, which when released in the gut shuts down appetite and impedes our gastric functions.

All good news.

But one year later, those feelings of fullness went south as hunger levels and a desire to eat steadily climbed. Worse still, they remained high after another twelve months.

These hunger sensations were reflected in their levels of a hormone called ghrelin, which slowly crept up from day one.

Meanwhile those appetite-satisfying peptide YY levels stayed steady after the four week mark, not shrinking but not climbing any higher either.

In other words, their initial efforts in improving activity and changing diet were rewarded with a depressed appetite, but pangs of hunger quickly took over any sensations of a satisfied belly.

The study can't tell us much about the exact mechanisms behind these hormone changes, and the researchers admit in their report that they don't have enough information to comment on the energy balance of the participants at the time measurements were taken.

Knowing details about the metabolisms of those losing weight might help us better understand what's going on below the surface.

But the research does show how losing weight isn't always a simple case of setting up good habits and watching the fat fade.

For many in the overweight and obese category who want to lose weight, it's not enough to just muster the willpower to get up more often and eat smaller, healthier portions.

We are only just beginning to understand the effects weight gain has on how we store fat, and the difficulties involved in reversing the impact of being obese in the first place.

Research like this shows how complex weight loss can be, while also providing hope that with such knowledge there could be solutions that might just make it easier in the future.

This research was published in American Journal of Physiology-Endocrinology and Metabolism.