The results are in, calorie restriction may not extend your life. Previous studies of rats and mice demonstrated that calorie restriction could extend life span. While human behavior can sometimes be rat-like, there are distinct differences in physiology between rats and humans. A much anticipated calorie restriction study using rhesus monkeys shows that these results do not translate to our close cousins. Being overweight is a tremendously important predictor of life expectancy, but the number of calories you consume, if you are still thin, does not appear to make a difference in this study.

Other studies, namely one being undertaken at UW-Madison, has demonstrated numerous benefits of calorie restriction including an increased life span. Why is there a difference between the studies? A comparison of the NIA monkeys and the WNPRC monkeys brings up a number of possibilities, with the most interesting being diet and weight. The WPNRC monkeys were allowed to truly eat as much as they want, while the NIA monkeys were given more food, but there was a limit. Understandably, the WNPRC monkeys weighed more (males +12% more, females +18%). Their diets were also different. The WNPRC diet was made completely from processed foods, while the NIA diet has more natural sources. Mattison et. al lay it out very clearly in their comparison.

A notable difference between the two studies is diet composition. The NIA-1-87 formulation (Labdiet, PMI Nutrition International) has a natural ingredient base whereas WNPRC diet is purified (Harlan Teklad). Although natural ingredient diets risk having some variation between batches, they contain components that may have an impact on health such as phytochemicals, ultra-trace minerals and other unidentified elements. In purified diets, each ingredient supplies a specific nutrient and each required mineral and vitamin is added as a separate component. Nutrient sources were also different. Protein was derived from wheat, corn, soybean, fish and alfalfa meal for the NIA diet, whereas the WNPRC diet protein source was lactalbumin. The NIA diet also contained flavonoids, known for their antioxidant activity, and fat from soy oil and the oils from the other natural ingredients (that is, corn, wheat and fish). Fish meal contains approximately 8–12% fat and is rich in omega-3 fatty acids. The WNPRC study dietary fat was derived from corn oil. Carbohydrate content was also notably different; although both diets had 57–61% carbohydrate by weight, the NIA study diet was comprised primarily of ground wheat and corn, whereas the WNPRC study diet contained corn starch and sucrose. Indeed, the WNPRC diet was 28.5% sucrose, whereas the NIA study diet was only 3.9% sucrose. This latter point may be particularly important as a diet high in sucrose may contribute to the incidence of type II diabetes.

The take home message appears to be, eat a healthy diet and keep your weight under control. Eat fresh fruits and vegetables and avoid processed foods. More study is needed to sort out all these variable.