For the first time, scientists have discovered a connection between a cell's fat content and its lifespan, which could explain the 'obesity paradox' that's been puzzling scientists for decades: why obese people have the lowest all-cause mortality rates, whereas the rate for those who are fit and lean is higher.

A new study has found that yeast cells with increased levels of triacylglycerol (TAG) – the main constituent of body fat in humans and animals – lived longer, but when the cells were prevented from synthesising TAG and became leaner, they ended up dying earlier.

The team from Michigan State University also found that the fat, long-living yeast cells had no obvious growth defects, and were able to reproduce as normal. This puts the new method in contrast with previous approaches to extending cell life (such as caloric restriction and gene deletion), which tend to leave the cells stunted or more sensitive to environmental pressures.

And yep, it's some leap from yeast cells to human cells, but researchers think there could be a connection here.

"Via sophisticated analyses, we demonstrated that increasing TAG reproduction preserves life through a mechanism that is largely independent of other lifespan regulation pathways common in yeast as well as humans," said study author Min-Hao Kuo. "Our paper likely will stimulate a new wave of research that has broad and deep impacts, including potential advances in human medicine."

The obesity paradox is a working hypothesis that being obese can actually protect certain groups of people and help them live longer. Of course, putting on too much weight is associated with a whole range of health problems – hence the term "paradox" – but some scientists believe all that extra body fat does come with certain benefits in certain cases.

A New York Times article from 2013 explains, referring to this study:

"The report on nearly 3 million people found that those whose BMI ranked them as overweight had less risk of dying than people of normal weight. And while obese people had a greater mortality risk over all, those at the lowest obesity level (BMI of 30 to 34.9) were not more likely to die than normal-weight people. The report, although not the first to suggest this relationship between BMI and mortality, is by far the largest and most carefully done, analysing nearly 100 studies, experts said."

As yet, there's no consensus on the issue, which is why the new TAG and yeast cell study could be so invaluable for future study. But Kuo himself admits that, "the obesity paradox baffles scientists across numerous disciplines".

At the centre of the experiments run by Kuo and his team was a process to delete enzymes crucial to the breakdown of TAG into smaller molecules. This meant the yeast cells were unable to use TAG, and increased cell fat in response. The scientists boosted this fat production by increasing the enzyme used for TAG synthesis.

Their study has been published in the journal PLOS Genetics.