By the way vs. As a matter of fact
A reader from Brazil asks for clarification on the expressions “by the way” and “as a matter of fact,” having heard that they are interchangeable expressions to be used when a person wants to change the subject of a conversation.
by the way
As one of its earliest meanings, the expression “by the way” had the literal meaning of “along the way” in the sense of something happening in passing on a journey. For example, “I lost my money by the way.”
The use of “by the way” in conversation is figurative. A person thinks of something else while in the process of talking:
We went to the Crystal Theater over the weekend to see Titanic. By the way, while we were there, we bumped into Leroy and his new girlfriend.
The expression does introduce a new subject, but not as a deliberate ploy to change the subject. The new subject has been suggested by something already being talked about.
as a matter of fact
This expression means, “in fact, really, actually.” Its strongest use is to correct a falsehood or misunderstanding, as in this example:
Most people probably believe that the actress Judy Holliday, who acted the part of a dumb blonde in Born Yesterday, was really of low intelligence. As a matter of fact, she had an IQ of 172.
In its weakened use, “as a matter of fact” still means, “in fact” or “actually,” but not in the sense of correcting a falsehood. It seems to be used as one might say indeed for emphasis or contrast.
Here are some examples from the Web:
I asked my sister if she drank diet soda. She told me that she did. As a matter of fact, she was getting ready to crack one open that moment.
At 12 years old, I wasn’t a fat kid. As a matter of fact, I was pretty skinny.
The weight isn’t going to instantly fly off in the first week. As a matter of fact, I gained weight before I started losing.
I am doing so well in my health. As a matter of fact I have dodged several colds that my husband has come down with.
Yes, you have to deal with co-workers. As a matter of fact, you have to interact with them daily.
It is possible that someone uncomfortable with the turn a conversation has taken might use one of these expressions to introduce an entirely new topic, but that is not their general function.