Is hormone the key to desire for food?

The hormone leptin, that is known to be involved in letting us know when we have had enough to eat, has also been identified as being involved in regulating the desire for food in the brain, reported The Daily Telegraph . People who are naturally deficient in leptin “find less-appetising food such as broccoli as mouthwatering as chocolate cake. The result is that these people overeat even when they are not hungry”, the newspaper explained.

The new research has demonstrated that when people who are leptin-deficient looked at pictures of food, whether or not they were hungry, the areas of the brain involved in pleasure response became active. For “healthy people”, this area of the brain was only active when they were hungry. The Daily Telegraph suggests that this means “it should be possible to design anti-obesity drugs which interfere with the brain’s “pleasure centres” and this “could offer a treatment for obesity”.

The study presents findings on two leptin-deficient patients compared with people without the hormone deficiency. The study findings cannot be interpreted to explain how leptin may contribute to obesity in the general population, or if this finding may lead to the development of new anti-obesity drugs.

Where did the story come from?

The research was conducted by Sadaf Farooqi and colleagues of the Departments of Medicine and Clinical Biochemistry, Psychiatry, Radiology and Brain Mapping Unit, of Addenbrooke’s Hospital, University of Cambridge. Funding for the study was provided by Wellcome Trust, Medical Research Council and Woco foundation and it was a short article published in the journal, Science – Sciencexpress .

What kind of scientific study was this?

This was an experimental study carried out on two individuals with a rare condition known as congenital leptin deficiency, to see how activity levels in different parts of the brain changed in response to food, before and after treatment with leptin.

The two subjects with leptin deficiency were a 14 year old boy and 19 year old girl. Initially, each patient had two brain scans, using magnetic resonance imaging (MRI), one after they had fasted and were hungry, and the other 30 minutes after eating a meal. As the scans were taken the patients were shown images, either of food or non-food items to see monitor their responses in the brain.

Immediately before each scanning session, the subjects rated how hungry or satisfied they felt on a score from 1 to 10. They also scored their liking for each of the food images during the scanning. The experiment was then completed with a further two scans after the two subjects had received a seven-day course of leptin treatment.

The researchers report that they carried out the same experiment on control subjects (people without leptin deficiency), but give no indication of how many controls there were or how they were selected for the study.

What were the results of the study?

The researchers found that, before treatment, there seemed to be a positive link between activity in a particular region of the brain and the amount the patients liked the food for which the image was provided whether they were hungry or not.

After treatment with leptin, the link seen between brain activity and the amount the patient liked the food image that they saw, was demonstrated only when the subjects were hungry. The researchers say that after leptin, the patients’ results were the same as those seen in the “healthy” controls.

The researchers also found that, after leptin treatment, the patients’ scores on the hunger rating scale when they were hungry and immediately after they ate were less than they had been before treatment, indicating that they felt more satisfied.

What interpretations did the researchers draw from these results?

The researchers conclude that leptin affects how the brain responds to images of food, and that people with the hormone leptin find it easier to be able to “discriminate between the rewarding properties of food”, that is, to decide whether they really need to eat or not.

When the level of leptin hormone is low, “liking” the food seems to be linked to “wanting” the food; this is indicated by the brain activity. The researchers say that these results show how leptin is involved in “modulating spontaneous eating behaviour”.

What does the NHS Knowledge Service make of this study?

This is an interesting scientific study. However there are several limitations and care is needed in drawing any conclusions from these results.

• This was a study of only two patients, both of whom had a congenital leptin deficiency. The authors state that this is a rare condition. Even if further research were to be carried out in more subjects with the same deficiency, any insight into whether there was a possible link between hormone levels and brain activity in response to food images would be valid only in people with congenital leptin deficiency. 
• This study can only suggest a possible link between hormone levels and whether liking a food was linked to brain activity; it cannot prove that the hormone leptin is the cause. Also the strength of the positive link that has been demonstrated may be questioned.
• We do not know from this experiment how the increased levels of food desire – how the researchers interpreted the increased brain activity – would actually translate into whether the person then went on to eat the food if they had the opportunity.
• Importantly, from the results of this research it cannot be assumed, as the news reports seem to have suggested, that this hormone deficiency could explain the mechanism behind the current obesity epidemic. Also, it cannot be interpreted, at this stage, that this finding could lead to the development of new anti-obesity drugs. Much further research in this area is needed before any conclusions can be made.