I found this sentence:
It has in it fat, which gives energy.
I can't figure out the usage of the part "in it fat". Can anyone kindly explain it or maybe give some examples please?
It has in it fat, which gives energy.
This is an example of what is called object postposing.
In English, we normally expect to see direct objects occurring directly after the verb:
Other complements of the verb, including Adjective Phrases or Preposition Phrases functioning as predicative complements, will come after the direct object.
We also expect to see adjuncts occurring after the direct object or other complements of the verb:
Sometimes, however, the direct object, usually a Noun Phrase, may be very long (the traditional term for this is that the Noun Phrase is heavy, and linguists sometimes refer to object postposing and similar phenomena as heavy NP shift).
When the direct object is very long like this, it can sometimes move to the end of the clause so that it occurs after any adjuncts or complements of the verb.
Here the direct object, a baboon who can recite the entire works of Johann Wolfgang von Goethe has moved to the end of the sentence and appears after the Preposition Phrase in the attic. Note that we do not require any commas around in the attic.
If we move a direct object that is short, or 'light' then object postposing like this will be very awkward and may seem entirely ungrammatical:
Compare the sentence above with:
Here the object postposing is more acceptable, purely because the Noun Phrase is very long.
In the Original Poster's example, the direct object of has is not fat, but fat, which is energy saving. This very heavy, or long, Noun Phrase has moved to the end of the sentence and occurs after the Preposition Phrase in it. We could rearrange the sentence so that the phrases occur in their normal, canonical positions:
The particular arrangement of the Original Poster's example however is rather unfortunate. The reason for this is that the non-defining relative clause, which gives energy, could refer just to the fat, or to the situation of the food having fat in it. In other words it could be modifying just the word fat or the clause It has fat in it. If we give it the last reading, then the object postposing won't seem acceptable as the direct object fat on its own is too light to warrant moving.
A writer's reasons for using object postposing will have many factors. The end of the sentence is the part that carries the most focus, and so if the direct object is the most salient or important part of the sentence, then it will often be moved to the end. A second reason to move direct objects to the end of the clause is to link with an upcoming topic:
The one constraint on moving long direct objects to the end of the sentence is that the Noun Phrase must be new to the current conversation:
Very long or heavy direct objects often move to the end of the clauses in which they occur. The reasons for doing this are often complex and relate to considerations of information packaging. Short, light direct objects can't normally do this, and speakers are likely to judge sentences where short direct objects are postposed as ungrammatical.
[Epilogue for serious grammar junkies
Aficionados of the Cambridge Grammar of the English Language (Huddleston & Pullum, 2002) may wonder why I've used the term non-restrictive relative clause as opposed to supplementary relative clause. Well, the reason is that I disagree with such writers that non-restrictive relative clauses are supplementary or non-integrated. One of the very important pieces of evidence for them not being supplementary, is that absolutely any syntactic constituency test, such as extraposition from noun-phrase movement, or indeed object postposing - as demonstrated in the Original Poster's example - will show the Noun Phrase including the non-restrictive relative clause to be a single constituent. With regards to object postposing, what this means is that we can move the relative clause to the end of the sentence along with the noun. This, theoretically, should not be possible unless the noun and clause together were a single constituent. In addition to this, it is also clear that non-restrictive clauses count in terms of contributing to the weight of the Noun Phrase and making any subject postposing acceptable. True supplements or parentheticals, on the other hand, do not. I argue, therefore, that although restrictive clauses are not always restrictive, non-restrictive clauses are never supplementary - in the grammatical sense of the word.]
Perhaps the reversing of "fat" and "it" (i.e. "it has in it fat" instead of "it has fat in it") helps to put "fat" instead of "it" next to the comma, and that makes it clearer which word "which" is referring to: