Usage of が in this paragraph

Question

Once again I have been stumped over the usage of particles in a passage from my textbook.

先生: 最近、アメリカでも魚や豆腐を食べている人が増えてきたらしいです。

学生: はい。

先生: アメリカでも、ヘルシーな食べ物[が]{L}人気[が]{L}出てきたということです。

The paragraph continues but I don’t see the point in typing out the rest as it won’t be relevant. In the fourth line the there are two が‘s. I asked my japanese friend why this is and if one could be a は, they said it’s definitely が but they couldn’t explain why. Please if someone could explain why it’s a double が and maybe a little additional work information that will help me to use が and は correctly. Thank you !!

Answer 1

There is a concept called topic-prominent in Linguistics field. I think this theory has the key to understand the difference.

In many cases, you can translate “Nは” to “As for N”, “In case of N”, or “Now I’m going to talk about N”. It’s like setting a topic.

When you introduce yourself you should use “は” because you have to set the topic of you.

わたしは東京出身です

As for me, I’m from Tokyo.

If you use “が” in the former example, it sounds like you aren’t describing about yourself (probably because of topic-less).

わたしが東京出身です

I (not him) am from Tokyo.

This sounds natural as long as answering for a question like “Who is from Tokyo?” or “Mike is from Tokyo, isn’t he?” This is not to introduce “myself” but to clarify a contrast, difference or misunderstanding.

One important point here. To understand the difference between は and が, we have to clarify the difference between “introducing” and “describing (from 3rd party’s view)”.

(1) マイクは日本語を勉強している

(1) (We’re going to talk about Mike,) he studies Japanese.

(2) マイクが日本語を勉強している

(2) Mike studies English.

(1) sounds like introducing Mike. So if you were asked “What did you see in the library?”, マイクは日本語を勉強していた is bad because now the topic is “what you saw” and not “Mike”. わたしが見たのはマイクが勉強しているところです is perfect instead.

(2) has no topic. It sounds like describing Mike from 3rd party’s view. So if you were asked “What did Mike do yesterday?”, マイクが日本語を勉強していた is bad because it sounds like irrelevant story from “what Mike did”. Obviously started to talk about “Mike” so マイクは日本語を勉強していた is good.

(3) 人気が出てきた

(3) It’s getting popular.

(4) 人気は出てきた

(4) As for popularity it’s getting better. (But sales is not good…)

(3) is very common. we usually talk about “popular things”. When you want to set “healthy food” as a topic, you can say ヘルシーな食べ物は人気がある. You can also say ヘルシーな食べ物が人気がある it sounds more like describing healthy food fairly.

On the other hand (4) is not common because we usually don’t specifically refer to “the popularity of something”. So it has strong meaning like “As for popularity” or “In case of popularity”. Thus in this case (4) leads to contrasting effect against the example of "わたしが東京出身です".

Conclusion. In your example I don’t think ヘルシーな食べ物は人気が出てきた is bad. Because they have already talked about healthy food like fishes and tofu. So it’s natural to set “healthy food” as next topic by using “は”.
However ヘルシーな食べ物は人気は出てきた this is probably also possible but it sounds confusing because topic is multiple. If you want to emphasize the contrast against ‘sales’ for example, you can say ヘルシーな食べ物の人気は出てきた.

Answer 2

In your sentence, 食べ物 is the subject, and 人気が出てきた is modifying, describing 食べ物. It is what I would call an "adjectival phrase" in English (I don't know if the term is linguistically right in Japanese, but hopefully it helps to understand the function of the clause). Therefore, you are to parse the sentence like this:

(ヘルシーな食べ物) が (人気が出てきた) ということです。

Let's compare some examples:

この食べ物が美味しいです。

この食べ物が大好きです。

この食べ物が人気が出ている。

All of the three sentences above follow the same structure:

A　が　B

where B is describing the noun A. All the three highlighted parts are modifying the noun in the same way, but while the first two are pure adjectives, the third one is a clause, a sentence itself, and therefore there is an extra が.

Another way to look at it. Consider the sentence:

部屋が空いている。

Here, despite having a verb instead of an adjective at the right side of the sentence, that verb is marking a state because it is in progressive tense, and therefore it is effectively describing the noun 部屋. The whole expression 人気が出ている, also a verb clasue in progressive tense, could be thought of the same way if it helps.

Note that despite jisho.org has an entry for 人気が出る as an expression or set phrase as a whole, I think that this pattern can be generalised, i.e. it is perfectly fine to have subclauses using extra が that act as a noun modifier as a whole, even if they are not set phrases.