When do I use "aid" as a transitive verb and when do I use "aid in" (aid as an intransitive verb)?

According to Macmillan English Dictionary

[INTRANSITIVE/TRANSITIVE] to help something to happen more easily or with fewer problems

  1. Gentle exercise aids the circulation of blood around the body.
  2. Vitamin C aids in the absorption of iron.

The two examples look similar to me. Why does the first one use "aid" and the second one use "aid in"?

One thread I have checked doesn't directly address my question. Difference between aid and aid in

  • I mean “aid”, not “add”. May 10, 2019 at 21:06
  • Sorry, a typo. I have corrected it. May 10, 2019 at 21:35

1 Answer 1


The difference is subtle, but it's one of specificity and whether you are describing what or how the action is taken.

In the first sentence, "circulation" is the object of "aids" as a transitive verb, meaning circulation is the thing being aided.

In the second sentence, "in the absorption" is a prepositional phrase that modifies the intransitive verb "aids," indicating how the aiding is done. There is no explicit object in this sentence, so the reader is left to assume what is being aided (your body, your mother, teenagers, etc).

As stated in the comments on the page you cited, it's more specific to give the object of the transitive verb than to use a prepositional phrase and leave the object missing or implied.

  • I didn't downvote your answer. Are aid and aid in interchangeable in my two sentences? May 10, 2019 at 21:41
  • Aid as an intransitive verb seems pretty common in dictionaries. Is it possible that "aid in" usually goes with noun which describes a process? May 10, 2019 at 21:48
  • They are basically interchangeable in these statements, yes. Whether something is aiding the circulation itself, or aiding [your body, presumably] in its circulation, the end result is the same. May 11, 2019 at 2:12

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