A tasty double burger with cheese and bacon and a side of golden fries... Why is this meal appealing? And why would our brains tell us this food is delicious if "junk food" is so bad for us? The problem in this editor's opinion lies in the evolution of humans and how our body prioritizes food needs.
A quick rundown of the human diet consists of primarily 4 types of macromolecules. These are Carbohydrates, Proteins, Lipids (or fats and oils), and Nucleic acids.
The human body uses Carbohydrates primarily for fueling the body's metabolism in the forms of sugars and starches. Sugars are relatively short term or "quick energy," while starches must be broken down into sugars to achieve energy needs. You can evidence carbohydrates by getting a quick energy boost from drinking a sugary soft drink versus eating a baked potato and being energized for hours.
Proteins are used by the body for building new tissue, be it bone, muscle, hair, or nails. The human body is constantly regenerating and uses protein for rebuilding. However, the body can use protein to fuel daily metabolism if carbohydrates are not available.
Nucleic acids are of low caloric value but are still needed in the diet to synthesize new DNA for cells. Diets that eat other living cells with DNA in them (all of them!) will consume nucleic acid needs fairly easily.
If you are still reading this post, you must be thinking "wait, this question was about junk food. Why is this guy talking about macromolecules?" The answer lies in the last group called Lipids, or fats and oils. The Fats and oils in most junk foods are a highly desirable food source for the human body. For every gram of starchy potato or meaty protein, the body can glean about 4 calories of energy. For a the fats and oil on the fries and cheese however, the body can get nearly 9 calories per gram. Evolutionarily, this led to the human brain releasing endorphins (feel good chemicals telling the brain "Good job!") when you consume a particularly fatty meal.
The human diet has evolved from one that contains very little fat in a hunter/gatherer society to one of almost limitless options. The problem with junk food is more about quantity of fats in our diet. The body is responding to a fatty diet by trying to store as much as possible of the "valuable" fat resource. It finds ingenious places to store these fats, be it in your cheeks, backside, or arteries leading into your heart. So your momma was right: you can enjoy your burger, just dont eat junk food all the time!
Mr. Angel Science Guy
While tasting very good for the most part, there are some significant problems with junk food consumption. The first rests with its addictive quality. Junk food can be easily consumed and easily prepared. In all honesty, it takes more time to make a salad than it does to open and eat a bag of chips. This accessibility makes it something susceptible to addictive behavior. Along these lines, junk food's open accessibility and ease can facilitate this addiction. In many urban centers, there are more establishments that would constitute as "junk food" areas than fruit and vegetable areas/ establishments. The high level of fats and indigestible oils in many junk foods also make them a challenge to a healthy lifestyle.
A lot of problems are prominent in junk food. One, much sugar is present, more than a person can handle. If he or she is diabetic, then they are not allowed to have it unless their blood sugar is low. Two, a lot of salt is present. This means getting a clogged artery, which could lead to a stroke, heart attack, or death. Three, sometimes caffeine is in the food, such as chocolate. This can cause increased heart palpitations and could lead to death too. These are some very prominent reasons why junk food is not good. In fact, many healthcare workers recommend eating fruits and vegetables daily instead of junk food.
Junk food got the bulk of its reputation from the documentary Supersize me wich showed what would happen if you ate fast food for one month. the man almost died and is still recovering. the problem is all of the fillers and dyes and calories and just pure nastiness.