[は] is a topic marker. It sometimes shows Subject as a result, but it does not always show Subject.
●[は and が]
("The true purpose of [は]")
There are 主題(theme, topic) and 主語(subject).
1. John ate that apple.
Speaking about John, (he) ate that apple.
As for John, (he) ate that apple.
Topic(/Theme) is not written here.
That apple, John ate (it).
Speaking about that apple, John ate (it).
As for that apple, John ate (it).
Q1: What did John do?
(Speaking about / As for) John, (he) ate that apple.
Q2: Who ate that apple?
John ate that apple. (/ John did.)
Q3: (I put my apple on this table. But, it is not there.) Where is that apple? (Do you know that?)
(Speaking about / As for) that apple, John ate (it).
I'm ramen. (strange sentence)
I want to eat sushi. What about you? [context]
B: 私はラーメン。(natural sentence)
(Speaking about / As for) me, (what I want to eat is) ramen.
It doesn't mean "Konjak doesn't get fat."
[こんにゃく(Konjak) is a food which has very low calorie.]
(Speaking about / As for) konjak, no matter how much (we) eat (it), (we) don't get fat.