Fast food has a few intended consequences, such as being cheap yet filling, but usually a lot more unintended ones. In fact, a lot of people who knew the inside information would probably never eat at a fast food restaurant again.
To begin with, regardless of what those at the fast food joint might tell you, and despite the advertisements you see, rather than being good and healthy food, what is served is normally reject grade. In other words, they are foods larger chain full service restaurants and food outlets wouldn't buy even if there were a choice.
Even in the case of produce like lettuce, tomatoes and other vegetables, they buy the produce that is the most discounted. Make no mistake; in some cases, the produce may be good, especially if the fast food place has access to local suppliers. However, often they are not. Insects simply washed off the produce aren't uncommon, though the produce is normally not sanitized as that removes flavor. Any feces left on the produce will probably be eaten right along with the food (worse in the case of salads, but still a problem with other meals).
We might ask, doesn't the health department check all this? The answer is yes. However, as a general rule, a warning of an inspection is given well in advance. By the time the health department inspects the restaurant, it is deemed 'clean', because they've switched lower grade food for better grade food.
Ground beef, the most commonly sold item in a fast food restaurant, has problems too. The rule is the higher the fat content, the cheaper the meat. We can all see that in the local store. How do you suppose dollar burgers can be sold and still maintain a profit margin? They use the worst beef possible, with the highest fat content. The commercials like to hint that it is otherwise, but catch phrases like "100% ground beef" really have little meaning. Beef fat is still beef.
Consider that during processing, many parts of a steer are not usually sold for consumption by humans in the market place. Ordinarily, most of those meat scraps would end up as dog or cat food. This doesn't mean it can't be eaten, just that it is far from choice, so it is sold for less when used as filler in ground beef used in fast food establishments.
Fast food may mean a lot more than burgers and fries, but in all but isolated cases, the same applies. You get less for your money, and the fast food lacks nutrients, but often contains fats or additives that make them particularly unhealthy.
It is a safe bet that fast food establishments will probably not admit to any of this. It is likely that they will also ignore the fact that incidences of diseases like heart disease, hypertension, stroke, and heart attack have increased at virtually the same rate that consumption of fast foods have increased. In areas where fast foods still aren't a large part of the diet of the average person, the same diseases still have low rates. Is there a connection?
It is up to the reader to decide. It might be considered, though, that Japan has historically had a lower incidence of heart disease, stroke, and heart attack than that experienced in the western world. But while food was primarily fish and various rice dishes in the past, in the last 20 years fast food has taken hold in Japan. Their rates of cardio-pulmonary disease are now approaching those of the western world, at roughly the same rate that fast food consumption increases. I'd say that was food for thought, no pun intended.