With the same fat-rich diet, the mouse remained slender (right) with a lack of smell, while the normal animal (left) became obese.

With the same fat-rich diet, the mouse remained slender (right) with a lack of smell, while the normal animal (left) became obese.

Berkeley (USA) - How good our smell is, also depends on whether we are just hungry or full. An empty stomach improves the olfactory performance, after a meal it sinks. At least in mice, there is also a connection in the reverse direction, as a German-American researcher has now found out. Thus, smell signals regulate body weight by affecting how much body fat is stored or degraded. Mice without olfactory cells also remained slender in fat-rich diet. These animals built off deposits of fat and released more energy in the form of heat, the scientists report in the "Cell Metabolism". On the other hand, animals with artificially improved smelling ability were more obese than normal mice. Whether similar relationships exist between the sense of smell and body weight also exist in humans has not yet been investigated.

"I never expected the elimination of the smell to cause such a dramatic decrease in body weight," says Andrew Dillin of the University of California, Berkeley, who led the working group with Jens Brüning of the Max Planck Institute for Metabolic Research in Cologne , The researchers investigated genetically modified mice whose olfactory cells in the nasal mucosa had been almost completely eliminated by a special treatment. These animals had a 16 per cent lower body weight than normal mice when the diet was fat. The animals did not differ in appetite and caloric intake. Even mice that were already obese lost a third of their body weight after their odor was turned off.

The unexpected weight loss was due solely to a reduction in fat mass. As further investigations were made, the remaining odor signals activated parts of the sympathetic nervous system, which in turn reduced the mass of white adipose tissue. On the other hand, fat burning was associated with heat production in brown adipose tissue. The researchers checked their results with the help of another type of genetically modified mice. Briining had succeeded in producing animals whose smell performance exceeded the natural degree. In fact, these mice behaved exactly the same way as Dillin's experimental animals in dietary experiments: they became faster obese than normal mice.

The mice lacking olfactory power had an elevated blood level of adrenaline. It is known that this hormone stimulates the burning of fat in brown adipose tissue in stressful situations, says Dillin. The researchers suggest that - probably mediated by the hypothalamus - there is a connection between olfactory cells and sympathetic nervous system, which together with several hormones ensures a balanced energy balance and the control of body weight. It is quite possible that a temporarily blocked smell perception could trigger metabolic reactions that would be suitable for new obesity therapies. So far, however, it is still clear whether the results of animal experiments can be transferred to humans at all.