I think the confusion lies in what "it" refers to for an English learner. One would think that "it" refers to the weight or the act of losing weight in the sense of maintaining it with "keep it" or "keep it fit." To clarify the meaning, we can use "keep it fit" or "keep it off" in the sentence. Another thing to remember while learning set phrases is that the terms may carry different meanings in different contexts. Following is a scene from Seinfeld.

(Claire passes the table; George wants to ask her opinion on something.)

GEORGE: Claire, Claire, you’re a woman, right?

CLAIRE: What gave it away, George?

It's highly recommendable to learn set phrases in context to distinguish different usages of words and even pay attention to minor detail, such as a little sarcasm in Claire's speech. Linquee and Reverso are two websites I turn to for reference in learning languages from time to time.

No, 'keep it off' here is a set phrase which means 'staying slimmer' or 'not putting more weight back on'. So in other words, lots of people diet, get slimmer, but then almost always they put weight back on eventually, because long term dieting is proven to not work.

It should also, in this context, be 'keeping it off'.

It might help to realize that "it" is a pronoun and its antecedent (the noun it refers to) is "the weight."

So if you were to say "keep it", it would be like saying "Very few people succeed in losing weight and keep the weight." It sounds like you are saying that you actually didn't lose the weight, y ou kept it. (But the phrase keep it, in this context is very non-idiomatic and no native speaker would say this, in this context.)

I think there's another ambiguity. The verb "keep" has two senses. One, is holding/saving an item (in this case, more metaphorically, weight.) Another is a sense of continuity. It suggests you not only do something, but can continue to do it for an extended period of time.

For example-- "I can not only run for ten minutes, but keep running for over an hour."

"anyone can start a habit for a day. But can you keep up the habit for a lifetime?"

In both those sentences, keep has the sense of continuity, of continuing action.

You can also use the phrase "keep in on" or "keep it off" with regard to clothing:

I took off my jacket because it was hot, and I kept it off all day.

I put on a sweater because it was cold, and I kept it on the rest of the evening.

We have to save energy-- turn the light off, and keep it off.

I think that perhaps those those examples will help you to show in what sense "keep it off" is being used.

But to answer your question, you do need word "off," and it wouldn't make sense without it.

I think it would be confusing to just say "keep it". Without "off" would it mean...they fail to lose weight and keep it (maintain their original weight). Or would it mean they lose the weight and maintain that state (of the lower weight.

Saying "keep it off" means they don't put the weight back on. To say it another way...people might lose weight to begin with, but they put it right back on. i.e. They go back to their original weight.