Lower Calories, More Active Immune System
It is the common wisdom that people who practice calorie restriction don't tend to manifest minor illnesses like colds and the like - but I'm not aware of any study to back up that claim. A while back, research linked hunger hormone ghrelin with increased immune function, and here is another study on the topic of fewer calories leading to a more aggressive immune system: "researchers have discovered an elementary mechanism which regulates vital immune functions in healthy people. In situations of hunger which mean stress for the body's cells, the body releases more antimicrobial peptides in order to protect itself. ... his natural immune defence system is linked directly to the metabolic status via the insulin signalling pathway. ... If we have not eaten for a while or have to climb many stairs, the energy level of our cells drops and with it the level of insulin. The researchers from Bonn have now discovered that in the case of a low insulin level the FOXO transcription factor is activated. A transcription factor can switch genes on and off. FOXO switches genes for immune defence proteins on when energy is needed. These antimicrobial peptides (AMP) - not to be confused with antibodies - are subsequently jettisoned by the body's cells. They destroy possible pathogens by dissolving their cell walls." One would expect calorie restricted people to have more of this going on in their bodies.
Link: http://www.eurekalert.org/pub_releases/2010-01/uob-hig011910.php