AN 83-year-old Indian holy man who claimed he spent seven decades without food or water astounded a team of military doctors who studied him during a two-week observation period.

Prahlad Jani spent two weeks in a hospital in the western Indian state of Gujarat under constant surveillance from a team of 30 medical staff equipped with cameras and CCTV.

During the period, he did not eat, drink or go to the bathroom.

"We still do not know how he survives," neurologist Sudhir Shah said after the end of the experiment. "It is still a mystery what kind of phenomenon this is."

The long-haired and bearded yogi was sealed in a hospital in the city of Ahmedabad in a study initiated by India's Defence Research and Development Organisation (DRDO), the national defence and military research institute.

The DRDO hopes that the findings, set to be released in greater detail in several months, could help soldiers survive without food and drink, assist astronauts or even save the lives of people trapped in natural disasters.

"(Mr Jani's) only contact with any kind of fluid was during gargling and bathing periodically during the period," G Ilavazahagan, the director of India's Defence Institute of Physiology and Allied Sciences, said.

Mr Jani returned to his village near Ambaji in northern Gujarat after leaving the hospital, where he will resume his routine of yoga and meditation. He said he was blessed by a goddess at a young age, which gave him special powers.

During the 15-day observation, which ended on Thursday, the doctors took scans of Mr Jani's organs, brain and blood vessels, and conducted tests on his heart, lungs and memory capacity.

"If (Mr) Jani does not derive energy from food and water, he must be doing that from energy sources around him, sunlight being one," said Dr Shah.

"As medical practitioners, we cannot shut our eyes to possibilities, to a source of energy other than calories."