New research has identified a mechanism in the body that appears to act as a kind of internal bathroom scale, registering body weight and telling the brain to reduce or increase food intake as necessary.
For now the system has only been observed in mice, but the scientists are hopeful of finding the same internal mechanism in humans too, and that could offer up a host of new ways to understand and treat obesity in people.
The last time we discovered an entirely new body fat regulatory process was back in 1994, when the leptin hormone was found. So far that hasn't led to any new obesity treatments, but this time could be different, according to the international team behind the new research.
"Quite simply, we have found support for the existence of internal bathroom scales," says one of the researchers, John-Olov Jansson from the University of Gothenburg in Sweden.
"The weight of the body is registered in the lower extremities. If the body weight tends to increase, a signal is sent to the brain to decrease food intake and keep the body weight constant."
Part of the motivation behind the new study was to work out why spending a lot of time sitting increases the risk of obesity – we know that getting up is good for us but we don't really understand the underlying reasons, at least as far as obesity goes.
To see if they could find an internal trigger, the scientists fitted extra weights to already obese rodents and observed what happened.
When the extra weights were added to the mice, body fat decreased and blood glucose levels improved. In fact, the animals lost the same amount of weight as had been added.
That suggests there's something inside the body, detecting extra bulk and working independently of any internal diet monitoring that's going on.
It's still early days, but the researchers say that this could potentially explain what we see happening in humans - sitting down could fool our bodies into thinking we're not that heavy, so the warning not to eat too much wouldn't get activated.
If this same mechanism is confirmed in humans, researchers suggest obesity treatments could be developed that can combine the leptin findings with this new bathroom scales idea.
"The mechanism that we have now identified regulates body fat mass independently of leptin," says one of the team, Claes Ohlsson from the University of Gothenburg.
"It is possible that leptin combined with activation of the internal body scales can become an effective treatment for obesity."
The precise workings of these internal scales – which have been dubbed the gravitostat – aren't yet clear, but based on follow-up tests the scientists think some of the sensing might be done by the bones lower down in the body.
What appears to be happening is that cells in the mice's bones - known as osteocytes - are somehow detecting the amount of weight and strain on bones, and then feed that message back to the brain.
The researchers tested this by creating mouse models without any osteocytes and repeating the experiment by adding weights to them. This time, the mice didn't decrease their food intake to make up for their additional body weight.
"We propose that much sitting time results in decreased loading of osteocytes in the weight-bearing long bones and, thereby, the homeostatic regulation of body weight does not activate its afferent signal to the brain, resulting in obesity," the team write in PNAS.
More research is needed to get a closer look at the process, and of course to find evidence that the same mechanism is happening in humans, but it's potentially a big, big step forward in our understanding of obesity.
"We have discovered a completely new system that regulates fat mass," says Jansson. "We hope this discovery will lead to a new direction in obesity research."
The research has been published in PNAS.