Never work with children and animals, or so they say. Even simple concepts such as calorie restriction (CR) can produce all sorts of challenges when it comes to designing experiments. Here, a researcher uses more rigor to show that protein restriction works to extend fly life while calorie restriction doesn't. "recent studies investigating the role of nutrients have challenged the role of CR in extending longevity. Fuelling this debate is the difficulty in experimentally disentangling CR and nutrient effects due to compensatory feeding behaviour. We quantified compensatory feeding by measuring the volume of solution imbibed [in the] Queensland fruit fly ... We restricted flies to one of 28 diets varying in carbohydrate:protein (C:P) ratios and concentrations. On imbalanced diets, flies overcame dietary dilutions, consuming similar caloric intakes for most dilutions. The response surface for lifespan revealed that increasing C:P ratio while keeping calories constant extended lifespan, with the maximum lifespan along C:P ratio of 21:1. In general, lifespan was reduced as caloric intake decreased. ... Our results 1) demonstrate that compensatory feeding can overcome dietary dilutions, 2) reveal difficulties with methods presenting fixed amounts of liquid diet, 3) illustrate the need to measure intake to account for compensatory feeding in DR studies, and 4) highlight nutrients rather than CR as a dominant influence on lifespan." I'd wait for further confirmatory studies before taking this as read.

Link: http://www.ncbi.nlm.nih.gov/pubmed/19558564