This paper [1], full version, is by some of the same researchers that our late member John Hutton worked with.

They report:
"5.5% per year increase in flexor rigidity before maintenance laxatives were introduced, and plateauing after."

I suffer badly from constipation and feel a marked increase in performance after defecating. I've put this down to this event freeing up the absorption site for levodopa in the upper colon.

This paper, however, considers a wider range of possible mechanisms as to why constipation can affect PD, including alpha-synuclein misfolding.

The perceived importance of the gut in the pathogenesis of PD seems to be increasing.

It is also interesting that they used objective measuring techniques, rather than relying on UPDRS.

Reference:
[1] "Quantifying rigidity of Parkinson’s disease in
relation to laxative treatment: a service evaluation"
Aisha Augustin et al.
BJCP, April 2016
http://onlinelibrary.wiley.com/doi/1...bcp.12967/epdf

John