Halliday & Matthiessen (2014: 111:
The fact that we do find clauses such as unfortunately protein you can’t store, with marked topical Theme in such an environment, shows that the experiential element following the interpersonal Adjunct still carries thematic status — otherwise there would be no sense in fronting it. This in turn means that an ordinary unmarked Theme under the same conditions is just that — an unmarked topical Theme.