Hunt for new obesity pills
Two promising approaches to finding effective weight loss drugs have been presented at the annual meeting of the Endocrine Society.
Animals given one experimental drug showed "dramatic" weight loss, according to scientists.
In another early study, in men, the love hormone oxytocin appeared to help reduce appetite.
Experts said a weight loss pill would be like finding the Holy Grail, but they also raised concerns about safety.
Much more work is needed to check these approaches will be safe to use in humans, they told the conference in San Diego.
First, a team at Houston Methodist Research Institute used a drug that manipulates the amount of energy burned by mimicking hormones produced by the thyroid.
"It raised metabolic rate and lead to dramatic weight loss," said lead researcher Dr Kevin Phillips.
The chemical, GC‐1, worked by converting white fat, which is just an energy store, into brown fat.
This type of fat helps keep weight off by burning calories and turning them into heat. It is more common in babies.
In mice experiments, heat production kicked in and raised the animal's temperature by four degrees.
In the space of two weeks, obese mice became thin as they lost half their body fat.
Dr Phillips added: "Our data demonstrate that GC‐1 is a novel fat‐browning agent that may have use in the treatment of obesity and metabolic disease."
It is too early to know if the chemical could be used safely in people.
Raising the internal body temperature too much could be deadly. There are also concerns that a drug that has such a dramatic impact on fat would be dangerous elsewhere in the body.
Dr Jason Wexler, an endocrinologist in Washington, DC, commented: "This is the Holy Grail - can you take a pill to lose weight? This area is obviously very exciting.
"I would continue to highlight the safety issues and off target effects, that really needs to be hurdled before therapeutic conversion of white fat to brown fat is going to be realised."
Meanwhile, an early set of human trials on oxytocin was presented by a team at Harvard Medical School.
This hormone is normally released during childbirth and helps a mother bond with her baby.
A single dose of the hormone, delivered by nasal spray before breakfast, seemed to reduce appetite and the number of calories consumed in a test on 25 men.
The men consumed 122 fewer calories at breakfast after using the nasal spray. If this effect was sustained over three meals a day for a whole year, it could lead to 17kg of weight loss, the researchers calculate.
Previous animal tests suggest there could be a longer‐term effect from multiple doses, but this is still untested in people.
Researcher Dr Elizabeth Lawson told the BBC News website: "It is absolutely early work and to me it is incredibly exciting because we're now translating what was found in animals to humans.
"I think it certainly has potential in the treatment of obesity, but we really can't extrapolate to say what would happen over time, those studies need to be done."