How does が mark the subject in this sentence?

Question

I'm trying to understand the way が is used in the following sentence:

すると、難しい講義を聞いた後では平均１２３ミリグラムも上がった血糖値が、面白い話を聞いて笑った後では、平均７７ミリグラムしか上がらなかったという結果が出たそうです。

In this case I believe it is a "subject " marker, but I am not sure what the "subject" is interacting with.

From what I know, when the form X が Y is usually used, X is the subject and Y is the action, descriptor, etc.:

あの犬は目が茶色です。That dog has brown eyes.

X = 目 (eye) Y = 茶色 (brown)

私はあのケーキが食べたいです。I want to eat that cake.

X = あのケーキ (that cake) Y = 食べたい (want to eat)

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But in the sentence above, i identified X and Y as:

X = 難しい講義を聞いた後では平均１２３ミリグラムも上がった血糖値

The blood sugar level that rose by an average of 123 milligrams after listening to the difficult lecture

Y = 面白い話を聞いて笑った後では、平均７７ミリグラムしか上がらなかった

rose only by an average of 77 milligrams after listening and laughing to a funny story.

"The blood sugar level that rose by an average of 123 milligrams after listening to the difficult lecture, rose only by an average of 77 milligrams after listening and laughing to a funny story."

It doesn't sound right to me. The が makes it sound like the blood sugar level that rose by 123 milligrams, rose again by 77 milligrams.

So I'm not sure how to interpret this particle.

Here is the full paragraph:

糖尿病という病気は、血液中の血糖値が高くなることによって起こる病気ですが、ある科学者が、面白い話を聞かせた後の患者と難しい講義を聞かせた後の患者の血糖値を比べるという実験をしてみました。すると、難しい講義を聞いた後では平均１２３ミリグラムも上がった血糖値が、面白い話を聞いて笑った後では、平均７７ミリグラムしか上がらなかったという結果が出たそうです。

Answer 1

At the very core of your sentence, the parsing is as follows:

血糖値が[Number]しか上がらなかった。The blood sugar only rose by [Number].

All the other stuff is just details which modify that core statement in some respect. Let's add in some extra details to the core statement.

血糖値が７７ミリグラムしか上がらなかった。 The blood sugar only rose by 77mg.

Now you can add in a clause which modifies the first noun in the core statement - it modifies 血糖値.

１２３ミリグラムも上がった血糖値が７７ミリグラムしか上がらなかった。The blood sugar which had previously risen by 123mg only rose by 77mg.

Does it make sense now? The final word in your X clause is 血糖値 because it is being modified. But it also serves as the subject in your Y clause. So you might translate your sample as:

"The results indicate that blood sugar levels, which had risen by an average of 123mg after listening to a challenging lecture, only rose by an average of 77mg after listening to and laughing at a humorous story."

By the way, it isn't clear from the context of the study whether the same patients were being tested under both conditions. Therefore, it is not clear whether the exact same person's blood sugar (ie the same blood sugar) was tested under both conditions or whether different people were tested under the different conditions. This means that there is a little ambiguity in the text no matter how you translate it.