Fast food is served at a fast food restaurant, such as Mcdonalds, &c; while junk food a specific type of food such as chips, pop, &c. Obviously, junk food could be served as fast food.
Essentially, the adjective: "fast" modifies the noun "food" in describing how long it takes the consumer to get the food ready to be eaten, that is the food will be ready very soon.
The adjective: "junk" describes the quality and nutritional value of the food.
Here "junk" is considered as that which is not usable, i.e. garbage.
Therefore to answer your question, different aspects of the food are being described. preparation time quality
ell.stackexchange.com/users/1081/skullpatrol
Fast food refers to the assembly process.
Usually, fast food is obtained at a restaurant, where the entire restaurant is designed to get customers their food in matter of a few minutes.
The classic example is the McDonald's hamburger.
Junk food refers to the food's nutritional value.
Junk food is typically processed, containing high amounts of sodium, calories, and/or fat, with little nutritional value, such as bag of Doritos chips, or a candy bar.
There is some overlap between the two terms.
If a hamburger is put together at a fast food restaurant using, say, 70% lean beef, a lot of salt, and a cheap bun made from bleached flour, that burger could be regarded as both fast food and junk food.
However, not all fast food is junk food, and not all junk food is fast food.
Many fast food restaurants, responding to criticism about health concerns and obesity issues, have put healthier choices on their menus in recent years.
A related term is convenience food , which is not generally sold in restaurants, but in supermarkets.
It's designed to be easily prepared or consumed.
Again, there's a lot of overlap between junk food and convenience food, but the two are not necessarily synonymous.
For example, baby carrots that are sold prepeeled and prewashed could be considered a convenience food, but they wouldn't be considered junk food.
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They may refer to the same thing, but I think the distinction is that fast food refers to the way the food is prepared/served, whereas junk food refers to the kind of food.
Fast food was introduced as opposed to traditionally cooked (slow) food: instead of someone preparing a large quantity of food in advance before serving it to people, or people ordering a meal and having to wait for the whole meal to be prepared in a traditional way, fast food was prepared in a quick way (all the ingredients being ready at hand and the classic burger being fried the moment you ordered it.
Junk food can be prepared as fast food , but it simply means food that is not adding any really healthy nutrients to one's diet.
So I could very well prepare “slow” junk food.
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Fast food may be thought of as convenience food .
It is quick.
It can be, but is not always, junk food .
Junk food is food that is not nutritionally-dense, or, put another way, is calorie-dense for the nutrient content.
The confusion comes thus: these terms are sometimes used as synonyms, while they are not.
This is because there is not complete overlap between the two sets: the set of fast food can include a quick green leafy salad with a light oil-and-vinegar home-made dressing, while the set of junk food does not contain the same salad.
Expanded in full, with respect to these two sets:
There are foods that are only fast food: a quick salad made with dark-green vegetables; There are foods that are only junk food: a corn-starch custard topped with meringue; And there are foods that are both: a store-bought hamburger topped with wilted, nearly-white lettuce; Finally, there are foods that are neither: a duck confit served with a side-salad topped with candied walnuts.
Therefore, we see that these two terms do not describe the same item , although they are mistakenly often used as synonyms.
ell.stackexchange.com/users/10141/Chris
A freshly made salad from top quality in season locally produced products can be fast food served in less than 5 minutes.
Junk food is food that people “look down” on, e.g. it has lots of fat, or is low quality etc.
It is often but not always served quickly.
So you can have a “take a way” that takes 1 hour to arrive, but is just a ready made source added to cheap cuts of meat, cooked badly in lots of fat.
ell.stackexchange.com/users/5301/Ian Ringrose
What is the difference between "fast food" and "junk food"?  Are they the same or not?  "Are they used in the same way?"
Difference between "fast food" and "junk food"
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<difference><usage><food>
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ell.stackexchange.com/users/8972/Ice Girl
They may refer to the same thing, but a distinction is that  fast food  refers to the way the food is prepared/served, whereas  junk food  refers to the  kind  of food. Fast food  is obtained at a restaurant, such as McDonald's, where the entire restaurant is designed to get customers their food in matter of a few minutes. Junk food describes the quality and nutritional value of the food. Junk food is  typically processed, low quality and  not nutritionally dense. Fast food refers to the assembly process in restaurants. Junk food can also be fast food, it refers to food's low nutritional value.
Fast food refers to the assembly process in restaurants. Junk food refers to food's low nutritional value. It can also be fast food.