Mouse study: When it comes to living longer, it's better to go hungry than go running

• However, when comparing the rats in these two groups that eat the same amount, the longest-lived animals in the exercise group don’t live any longer than the longest-lived rats in the non-exercise group. Taken together, these findings indicate that exercise can prevent an early death from disease in some rats, but does not extend the maximal lifespan of any of the rats.
  
  

• When comparing rats that exercise to those that don’t exercise but eat much less, the longest-lived rats are from the group that ate less.
  
  

Two theories

Taken together, these findings indicate that caloric restriction protects against disease better than exercise does, and has the added benefit of extending the life span of some rats. Physiologists have been trying to unravel the reasons for this, and two major theories have emerged.

One theory is that exercise places stress on the body, which can result in damage to the tissues and DNA. Another theory is that caloric restriction leads to physiological changes which benefit the body.

Huffman and his colleagues designed a study to examine the roles of exercise and caloric restriction, singly and combined. They controlled for factors such as weight and the amount of energy expended versus the calories consumed.

They found:

• Mice allowed to eat as much as they wanted had higher insulin levels, regardless of whether they exercised. That is, how much the mice ate determined their insulin level, while exercise did not have much effect. High insulin levels are associated with a risk of diabetes.
  
  

• The animals that ate as much as they wanted and did not exercise had the highest levels of insulin-like growth factor (IGF-1), which plays a key role in regulating cell growth and cell death. The animals on caloric restriction had the lowest levels of IGF-1. Exercise also seemed to play an important role in regulating IGF-1 levels.
  
  

• There were some elevated levels of heat shock proteins, a measure of oxidative stress and possible tissue damage among the exercising mice. But total protein carbonyls, another stress measure, were not significantly different.