A nasal spray made of synthetic oxytocin -- the hormone released during breastfeeding and sex -- worked as an appetite suppressant in a group of healthy men in Boston.

"Our results are really exciting," says lead investigator Elizabeth Lawson, MD, MMSc, assistant professor of medicine at Harvard Medical School, Boston. "Further study is needed, but I think oxytocin is a promising treatment for obesity and its metabolic complications."

To test the nasal spray, which is approved in Europe but not in the US, the participant group consisted of 25 healthy men of an average age of 27 and 13 of them were at a normal weight while the remaining 12 were either overweight or obese.

They were assigned at random and unbeknownst to them to self-administer a single dose containing 24 International Units of either the oxytocin-based spray or a placebo.

One hour later, the men sat down to breakfast, and ordered from a menu of meals containing double portions.

In a separate visit, the research team repeated the experiment, reversing the treatments to make sure that those given the placebo in the last experiment received the real thing this time.

The men had reported how much they ate during the three days before the experiment, and the research team says they had eaten similar amounts of food before both of the experiments.

Receiving the oxytocin nasal spray reduced their caloric intake by an average of 122 calories and their fat intake dwindled by nine grams, when researchers compared how much they had eaten at the two breakfasts.

Under the effects of oxytocin, the men's body fat was used as fuel for energy, according to the researchers, yet exactly how it affected caloric intake is unclear because the men reported no difference in their appetites.

What's more, blood tests for the hormones that control appetite did not reveal any difference between the oxytocin and the placebo spray, say the researchers.

No serious side effects were observed and Lawson says the same minor side effects occurred from the placebo.

She will present her work on Sunday at the Endocrine Society's 97th annual meeting in San Diego.