A small but revealing study suggests that even mildly stressful intellectual challenges change our emotional states and behaviors, even if they do not profoundly alter brain metabolism. Fourteen female Canadian college students either sat around, summarized a passage of text or completed a series of computerized attention and memory tests for 45 minutes before feasting on a buffet lunch. Students who exercised their brains helped themselves to around 200 more calories than students who relaxed. Their blood glucose levels also fluctuated more than those of students who just sat there, but not in any consistent way. Levels of the stress hormone cortisol, however, were significantly higher in students whose brains were busy, as were their heart rates, blood pressure and self-reported anxiety. In all likelihood, these students did not eat more because their haggard brains desperately needed more fuel; rather, they were stress eating.
Messier has related explanation for everyday mental weariness: "My general hypothesis is that the brain is a lazy bum," he says. "The brain has a hard time staying focused on just one thing for too long. It's possible that sustained concentration creates some changes in the brain that promote avoidance of that state. It could be like a timer that says, 'Okay you're done now.' Maybe the brain just doesn't like to work so hard for so long."
Essential fat is the level below which physical and physiological health would be negatively affected. Whether a particular body fat percentage is better for one's health; athletic performance can also be affected. The leanest athletes typically compete at levels of about 6–13% for men or 14–20% for women. Body-builders may compete at ranges even lower than these levels. Certified personal trainers will suggest to male body-builders that they aim for a body fat percentage between 2% and 4% by contest time. However it is unclear that such levels are ever actually attained since (a) the means to measure such levels are, as noted below, lacking in principle, are inaccurate, and (b) 4–6% is generally considered a physiological minimum for human males. ( see wikipedia )