fast foods contain trans fatty acids . these can harm you blood vessels, increase your cholesterol level , as well as fat percentage, leaving you to gain weight. it can also cause indigestion, acidity, flatulance. these foods are high in carbs and fat content , can also lead to increase in the blood sugar level. they cause irritbility and sleeplessness on the long run as these foods lack essential nutrients. they can lead to bowel disturbances and even IBS. constipation as it also lacks in the fiber content.