Yeah there must be something going on that she gained that much weight in such a short time.

But, yes, be honest with her, but be nice about it.

Encourage activities that are more physical(besides sex) when the two of you get together. Take a long walk, work out together, go hiking, ride bikes, offer to help coach her when working out.

I had an ex who encouraged the same activities with me and I thought it was really sweet. I mean at least he offered to help me. Sometimes it takes some motivation.

Thats a lot better then blurting out that you think she's fat and don't find her physically attractive anymore.

Thinker is right in ways too. I know you have to be somewhat physically attracted to a person, but what about who she actually is? Her personality? Isn't that attractive to you?
Beauty fades. People change. No one is perfect forever. So maybe you should keep that in mind.

I think though that you can possibly help her by motivating her in a positive way. Also, be there to talk to find out whats going on that shes put on that amount of weight. Maybe she's under a great deal of stress and is turning to food for comfort. Maybe she does have an actual medical condition. Point is, be there for her emotionally and physically and at least try to work thru this.

It would be shallow of you if you didn't at least try.

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