What is the best practice to abstract out the body handler?

One approach is to create 

Route

 extension function....something like

fun Route.apiRoute() {

    route("/api") {
        get("/interests") {
            call.respondText("hello world")
        }
    }
}

then you'd have

routing {
    apiRoute()
}

aha..wow

A bit late but I usually do this in my projects: Types.kt

typealias RouteHandler = suspend PipelineContext<Unit, ApplicationCall>.(Unit) -> Unit

Handlers.kt

val helloWorld: RouteHandler = {
    call.respondText("hello world")
}

Routes.kt

routing {
    route("/api") {
        get("/interests", helloWorld)
    }
}