The recent study was the work of Dr. David Schneider, associate professor of microbiology and immunology at Stanford University School of Medicine. The findings are to be published in the journal PloS Biology.
A number of studies that have concluded limiting caloric intake extends lifespan have been done under sterile lab conditions and therefore don’t properly reflect reality - where we constantly face numerous pathogens, with many causing infection. To investigate this further, Schneider measured the appetites of infected and uninfected fruit flies – in both cases some flies had been reared on a calorie restriction diet while others had not.
The researchers infected flies with three different types of bacteria, all of which cause death in humans: Enterococcus faecalis, Salmonella typhimurium, and Listeria monocytogenes. They then compared the response of the calorie-restricted flies to normally fed flies.
In the case of Enterococcus faecalis, the flies on calorie-restricted diets ate no less after infection than uninfected flies, and survived as long as normal eaters. In the case of the other two bacteria, the flies with limited caloric intake ate less after they were infected compared with uninfected flies, but their survival patterns differed from normal flies, depending on the type of bacteria they were infected with.
The low calorie flies infected with Salmonella outlived other infected normal eaters – they survived 15 days after infection compared to just 8 days for normal eaters. But low calorie flies infected with Listeria died faster than other infected normal eaters – just 4 days compared with 6 or 7 for normal eaters.
The researchers are not clear what underlying biological reasons might explain the different responses to different bacterial infection. However, they believe the results as a whole suggest that the idea of a calorie-restricted lifestyle should be viewed with skepticism.
Schneider concluded, “The work reported here should raise a cautionary flag, as it demonstrates that diet restriction can have complex effects on the realized immune response of a diet-restricted animal.”