Restricting the amount of yeast in the diet can increase life expectancy by 50%, research into fruit flies has shown.
Many baked products are made with yeast
It is not yet clear whether the same might be true in humans.
But the authors say their findings hint it might be what you eat rather than total calorie intake that influences longevity, contrary to current belief.
The University College London team told PLoS Biology how it could be down to metabolic pathways triggered by foods.
Quality not quantity
Lead researcher Professor Linda Partridge said: "Yeast and sugar trigger different metabolic pathways with different effects on life span.
"The dramatic impact of reducing yeast suggests that protein or fat plays a greater role in fly longevity than sugar."
She said the results made a "strong case" that calories per se are not the important factor in prolonging life - at least in fruit flies.
A spokesman from the International Longevity Centre UK said: "At this point in time, it is difficult to state whether or not diet, rather than simply calorific intake, has a major bearing on life expectancy in humans."
But he added: "Diet is an important issue we have largely ignored until recently.
"It is well established that obesity can be both highly unpleasant and harmful to life expectancy.
"It is interesting to note however that studies (such as Healthy Ageing: A Longitudinal Study in Europe) appear to confirm that a Mediterranean diet, rich in plant foods and fish, low in meat and dairy products, and with a high ratio of monounsaturated fatty acids to polyunsaturated fatty acids, strongly correlated with a lower mortality risk for cardiovascular disease and cancers.
"It seems likely, therefore, that calorific intake per se is not the only factor in assessing diet.
He said that a Mediterranean diet combined with exercise, moderate alcohol consumption and abstaining from smoking gave a person more years of good heath than any of these lifestyle factors alone.
He stressed that too little food could also be harmful.
For example, because metabolism slows with age, this often means the appetite for calories falls behind the nutritional needs for fluids, vitamins, minerals and protein, leaving the older person vulnerable to unnecessary malnutrition and disease, he said.