THE HUMAN METABOLISM IS NOT SUITED FOR OUR SEDENTARY LIFESTYLE. Driving to work, you have a near-miss accident. Your adrenaline is released, fats mobilized, which for the cave man fleeing a predator would have fueled a flight or fight muscle response. But for you in the car, your fat released is just laid down in the arteries and ultimately, liver. Stress triggers that once served a vital function, now are harmful to our health. Our metabolic evolution is not matched to our social evolution.
http://www.frontiersin.org/Neuroscience/10.3389/fnins.2013.00047/abstract