Calorie restriction has such a strong effect on health and longevity in laboratory animals that any study failing to account for it - which is pretty much everything run prior to the mid 1990s - is potentially tainted. You think your treatment is providing benefits? No, it just makes the mice feel unwell, and so they eat less. Similarly, within the realm of deliberate calorie restriction studies, those that fail to correctly control feeding are probably producing incorrect results. No-one said that science was easy, and fly studies - where liquid foods are used - are particularly troublesome: "Recent studies have indicated that flies respond to dilute food solutions by compensatory feeding. The existence of compensation mechanisms calls for a reconsideration of the relationships between diet, feeding behaviour and longevity. This study shows that flies fed on liquid diets, sense sucrose and yeast nutrients and adapt to changes in the quantity and presentation of the two nutrients. ... Compensatory feeding suppresses the beneficial action of dietary restriction on longevity when flies are fed on liquid diets supplemented with yeast extracts. Flies which are given the choice to feed on separate yeast and sucrose food sources were longer lived than flies fed on nutrient mixtures. We conclude [that] food presentation is a major factor which determines the sensitivity of flies to dietary restriction."