In non mammalian models, feeding rate is difficult to control, and often food is diluted with water without giving them a separate source of water. Therefore, the specimens could be dehydrated and this could impact lifespan. In mammal models of calorie restriction, water is given ab libitum.
The first experiment used groups of Drosophilia, calorie restricted and not (a rich medium of yeast extract/sucrose), both with access to ab libitum, radio labeled water. They noted that the non restricted group drank 5 times as much water as the restricted group, perhaps a compensatory mechanism to the increased energy intake; water intake did not change food intake. However, when the food was diluted, this stimulated food intake and less consumption of the independent water source and vice versa. Subsequent experimentation revealed that restricting access to the independent water source resulted in a lower lifespan than flies granted ad libitum access to the water. This effect was greatest in the non restricted groups; in the restricted groups the the lifespan increase was lessened. So the lifespan extension effects of calorie restriction were essentially abolished without access to unlimited water.
When yeast extract/sucrose/and cornmeal was used, independent water consumption was low for both restricted and non restricted groups, suggesting it satisfies water requirements. They found however that a reduction in yeast level stimulated food consumption, and an increase in yeast level correlated with an increase in fecundity of female flies. Independent water consumption did not influence lifespan significantly in these experiments. The dramatic reduction in fecundity upon calorie restriction, which is classically observed in mammalian studies, was only evident in a group allowed unlimited water.
Other findings from the experiments corroborate the existing theory that the protein:carbohydrate ratio is the main influential factor on fly lifespan. When the ratio is kept similar, lifespan does not vary significantly, but when yeast was altered and the ratio was changed, lifespan was dramatically different.
Clearly, it seems that this is further support that there are interspecies differences in response to different dietary manipulations which aim to study the effect of calorie restriction on longevity. All of the previous research on invertebrates now requires reinterpretation, as the results are not applicable to mammalian models that differ in hydration status. Much still remains to be answered.
1. Ja WW, Carvalho GB, Zid BM, Mak EM, Brummel T, & Benzer S (2009). Water- and nutrient-dependent effects of dietary restriction on Drosophila lifespan. Proceedings of the National Academy of Sciences of the United States of America, 106 (44), 18633-7 PMID: 19841272