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In the 1980s, two groups of researchers asked a seemingly simple question: can animals live longer by eating less? A team at the US National Institutes of Health (NIH) in Bethesda, Maryland, and another at the University of Wisconsin–Madison each fed rhesus monkeys (Macaca mulatta) diets with 30% fewer calories than given to their control animals. The Wisconsin-based team found that caloric restriction helped the monkeys to live longer, healthier lives^,; the NIH team observed no such effect.

To resolve the disparity, the researchers looked into the experimental set-ups. They found that even though both studies looked at the same level of caloric restriction, the specifics of the animals’ diets could have contributed to the differing outcomes. “Diet is just another variable,” says Kristin Gribble, a molecular biologist at the Marine Biological Laboratory in Woods Hole, Massachusetts. “If it’s different between experiments, it’s an additional variable to be considered when explaining the results.” ...