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lose some weight

gain some weight

put on some weight

go on a diet

shake off some weight

I decided to go on a diet before my holiday.

我决定在谈恋爱之前增点肥

I decided to gain some weight before having a relationship.

I decided to put on / gain some weight before I started a relationship.

...before I go out with someone.

... before I started seeing someone.

... before I started dating someone.