Scientists are straying from the mouth and looking to the other end of the digestive system for a weight loss solution.

Freeze-dried "poo pills" are the latest weight loss strategy under the microscope in the United States.

In a controlled clinical trial starting later this year, researchers will test the fecal formula as a possible treatment of obesity, by looking at how gut bacteria can impact weight loss.

The trial will also investigate how the human microbiome can play a role in health and metabolism and insulin sensitivity, by replacing an obese person's intestinal microbes with those from the skinny donor via their faeces.

In the trial — led by Elaine Yu, an assistant professor and clinical researcher at Massachusetts General Hospital — 20 obese patients will be given capsules filled with freeze-dried faeces from healthy, lean donors. Some capsules will be a placebo.

Participants will be required to take the poo-packed pills every week for six weeks, while also following a healthy diet and exercise regime.

In the future, Ms Yu hoped microbe treatments could go hand-in-hand with dietary interventions to treat obesity and metabolic disorders.

She told Ars Technica scientists were in the early stages of microbiome work.

"We can do all of the detail-oriented work to try to figure out which microbial communities are being affected to give more information for targeted treatments in the future," Ms Yu said.

Skinny donor 'good bacteria' won't last in gut forever

University of Queensland Centre for Superbug Solutions director Matt Cooper told the ABC the concept of faecal transplant therapy was not new, however the US study would be the first time the benefits were examined in a controlled clinical trial.

He said experts first noticed a potential link between faecal transplants and weight loss when treating patients for the nasty gut infection clostridium difficile or "C diff.".

"During faecal transplant therapy we have seen fat people who had received poo from a lean donor getting thinner and it was an interesting observation, but we need clinical trials to link cause to effect," Professor Cooper said.

"The digestion of food is not done by entirely by stomach acid and our human enzymes, but also by the bacteria that live inside us— there's more than a kilogram and a half of bacteria inside the gut.

"Fibre is good for us, we know that, and it encourages good bacteria that produce short chain fatty acids.

"Those acids can regulate the energy balance in the liver and in fat cells, so there's a connection between bacteria converting food into energy in different ways."

If the trial is successful, Professor Cooper said it could prove a tool for helping obese people, but he said there was no such thing as a "quick fix".

"While this may help people with weight loss, it all comes down to exercise and the food you put in your mouth," he said.

"The good bacteria from the pills will not last forever without a balanced diet with a lot of fibre.

"If this trial works, we must remember that if you keep eating bad food, the bad bacteria will return."

Professor Cooper said the best way to enrich short chain fatty acids was to include fibre-rich foods in the diet such as legumes, lentils, oranges, apples, pears, beans, peas and cucumbers.