With the super-busy lifestyle that everybody is gravitating to, there is less and less time for everything, including cooking and eating. More and more processed foods are finding their way into the refrigerator, which can just be picked up and eaten anytime hunger pangs are felt. And we do not immediately realize that processed foods are devoid of all the essential ingredients. What we end up eating has less fiber, more salt, more preservatives (generally not very safe for all) - all leading to an unhealthy lifestyle.
Refined flour, white sugar, cheese, juices, canned juices, cheese-laden pizzas and breads, frozen foods - all these sound yummy but not something readily accepted and digested by your stomach. Try eating pizza with loads of cheese for two days and experience the bloated feeling for yourself. Not to scare you but imagine this - you would have to eat an entire medium pizza to get like 2 g of fiber. Your body requires about 25 to 40 g of fiber per day for regular health maintenance.
Biologically, our digestive system requires a lot of fiber which adds to the roughage. This is very important to help in digestion and to aid in cleansing the bowel after that. The function of the large intestine is to remove water and essential ingredients when food passes through it. A junk food passes through quite quickly, thereby leading to loss of water and ingredients. A lot of digestive issues are also related to lack of fiber in the food - polyps, colon cancer, irritable bowel syndrome, high cholesterol, and obesity.
Most of us are aware but tend to ignore the fact that junk food is well as the name says, junk. There is hardly anything of great nutritious value in it. There is a lot of salt and fat and artificial chemicals which all make it very tasty but with not-very-good nutritious value. Try to eat only junk food for about a week regularly, and you will surely feel:
1. Bloated (a lot of gas due to indigestion)
2. Dehydrated (excessive salt needs more water, which we don't realize)
3. Constipated (lack of fiber leading to reduced bowel movement)
4. Weak and lousy (lack of nutrition)
This, of course, does not include the harmful effects on skin, hair, looks, etc.
It does not mean you have to stop eating junk completely and eat only healthy all the time. The human body is adaptive and can take adjust to a lot of variations. It is not wise to try the example above (eating pizza for a week), while binging on junk food once in a week would definitely not hurt. Use your discretion as nobody knows your body as well as you do.