The research adds new complexity to the established view that dietary restriction works to extend life span by reducing calorie intake. This report reveals that the nutritional source (ie fat, protein, carbohydrate) of the calories is more important to longevity than a reduction in the total calories consumed.

The study published on 30th May 2005 in PLoS Biology, shows that fruitflies - Drosophila ™- can live longer without reducing calories but instead by consuming proportionally less yeast (which is the flies ™ only source of protein and fat). The team, led by Professor Linda Partridge of UCL ™s Centre for Research on Ageing, Department of Biology, believes that this demonstrates that calorie intake is not the key factor in the reduction of mortality rate by dietary restriction in this species.

The authors restricted the diet of the fruitflies by diluting the nutrients in the fly ™s standard lab diet of yeast and sugar to a level known to maximize life span. Since both yeast and sugar (carbohydrates) provide the same calories per gram, the authors could adjust nutrient composition of the yeast/sugar mixture without affecting the calorie count, allowing them to separate the effects of calories and nutrients.

Reducing both nutrients increased the flies ™ life spans, but yeast had a much greater effect: reducing yeast in the mixture from 150 grams to 65 grams increased median life span by over 60%, compared to just 9% when calorie intake was reduced to the same extent by lowering sugar levels. Late-starters on the restricted-yeast diet gained the same benefits as the rest of the yeast-deprived group. Forty-eight hours after being switched from normal diets to yeast-restricted diets, flies were no more likely to die than flies fed the yeast-restricted diet throughout their adult life.

Professor Linda Partridge said: Altogether these results make a strong case that calories per se are not the salient factor in prolonging life ”at least in fruitflies. The dramatic impact of reducing yeast suggests that protein or fat plays a greater role in fly longevity than sugar. This suggests that yeast and sugar trigger different metabolic pathways with different effects on life span. The caloric-restriction/longevity paradigm is not as simple as current scientific thought believes and needs more rigorous review.