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A study conducted by Oxford University on Rats who were fed high fat and low protein diets have shown that rats who take in excessive fat not only have heart problems, but apparently show the effects of having decreased mental capacity as well. Because fat makes muscles less effective at effective at using oxygen, this causes the heart to enlarge and also deprives various body systems of oxygen including the brain.
Rats fed this high fat diet made considerably more mistakes when traversing a maze than rats who were kept lean and with optimal protein, low fat diets. These same rats made more frequent mistakes after just nine days of consuming such bad diets, and performed better on the mazes prior to starting the high fat diets.
Bottom line, we all know fast food is bad for you, and time, money and studies continuously show how fast food has nothing but negative impact on our physical health, yet humans continue to eat it. You can complain about people who smoke cigarettes, while you are chowing down on your fast food burger and fries and not even realize you are doing just as much if not more physical damage to your body as smoking.
You can go here to read the full article about Fast Food Makes You Less Intelligent.