The relationship between food and health is well-documented. Moreover, scientists have suspected for many decades that restricting the diet - specifically, the calorific intake - of certain species of animal enables them to live longer.

But the exact reason why this works - and how far it can be taken - has left most scientists scratching their heads.

Pushing the envelope even further, a research team in East China led by Professor Han Jingdong at the Shanghai Institutes for Biological Sciences, which operates under the Chinese Academy of Sciences, has come up with a new theory based on their experiments with roundworms or nematodes.

Han’s team identified three groups of genes, each numbering over a hundred, that affect how the worms age when their diets are restricted.

By changing how the genes were expressed in cells in all three gene groups simultaneously, the team was able to increase the average lifespan of their wriggling subjects from under 20 days to over 50 days, they said.

The “diet pills” for humans which the team hopes to see developed one day would be filled with chemical compounds that manipulate the expression of these gene groups in a similar way.

“The same genes also exist in humans,” Han said.

“We are now testing on mice a drug which can hit the three groups of genes in one shot. This could [one day] allow us to live more than two hundred years,” she added.

Yet growing doubt has been cast in recent years on whether a leaner diet can really influence a person’s longevity.

Two separate studies published last year on Rhesus monkeys, which are genetically very close to humans, produced contradictory results after the effects of feeding them restricted-calorie diets were monitored over a long period.

Han said this confusion was partly caused by the different kinds of food the monkeys were given, and partly due to a lack of understanding about the underlying mechanism behind how the ageing process can be retarded.

Their study also showed that regular fasting, which is practised by Muslims and various cultures around the world, does not always lead to longer life.

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The team found that the three groups of genes play different roles in how the body functions. The group known as TOR, for example, helps control cell life and ageing by regulating the level of lipids - for example, cholesterol - in the blood and organs.

If TOR - or any of the other two groups - remain inactive, a person’s longevity would not be influenced by how much they eat, the team found. In people, all three groups are usually inactive.

Han said most scientists do not encourage healthy people to diet because of the risk of harmful side effects ranging from malnutrition to organ damage, or stunted growth in children.

It can also be miserable to go on a strict diet as the human body has evolved over millions of years of evolution and foraging for food to favour a high-calorie diet. Many studies show that people put on more weight after they finish their diet than they measured before they started it.

One of the intriguing aspects of Han’s recipe for a special diet pill is that it promises to deliver the nutrition our bodies need, as well as the restrictions required to live longer, without any of the dangerous side effects.

Referring to the worms as a case in point, Han said that even those which were deliberately overfed the specially constructed diet did not die any earlier than regular worms.

“Eat free and live long - all you need is the right pill,” she said.

Well, may not all. The team admitted there are many factors apart from diet that contribute to the ageing process, meaning that more research must be undertaken.

Han said it could be “many years” before a tailored long-life drug hits shelves in China or elsewhere.

However, previous experiments by other scientists in the United States have also proven it possible to make animals live longer, which lends credibility to the argument the same could be done for humans.

In 2013, scientists from the Buck Institute for Research on Aging, which is based in California, used genetic mutations of worms to quintuple their lifespan.

Worms live naturally for one to nine years, depending on the species, but some nightcrawlers have reportedly lived for up to 20 years.