The work by Dobb's et al. [1] showing an apparent association between taking laxatives and slower progression of PD has been discussed on this forum [2, 3].

The laxatives used were of different types. So, it seems to me unlikely that they can each slow progression directly. One way to explain the results might be to argue that their action is indirect: laxatives reduce constipation and constipation reduces the efficiency of the absorption of exogenous levodopa in the upper intestine. But, as schwad01 has pointed out, the result appears to apply also to PwP not taking levodopa. We could square the circle if there is an endogenous source of levodopa in the gut.

I can't find any references to the production of levodopa in the gut. However, we do know that the biotransformation through fermentation of tyrosine into levodopa is possible [4]. We do know that the gut houses a vast and on-going chemical experiment. We suspect that people with PD have a different microbiota.

To test the endogenous levodopa hypothesis would be simple: is there any levodopa in the faeces of people not taking levodopa?

A potential therapeutic route is to proactively try to produce levodopa in the gut.

References:

[1] "Quantifying rigidity of Parkinson’s disease in relation to laxative treatment: a service evaluation"
Aisha D. Augustin, André Charlett, Clive Weller, Sylvia M. Dobbs,DavidTaylor,IngvarBjarnas and R. John Dobbs
Br J Clin Pharmacol (2016)
Quantifying rigidity of Parkinson's disease in relation to laxative treatment: a service evaluation - Augustin - 2016 - British Journal of Clinical Pharmacology - Wiley Online Library

[2] Who would have thunk it!

[3] Please read!!!!!!!maybe the first drug to be disease-modifying in pd--please read

[4] International Journal of Scientific and Research Publications, Volume 2, Issue 5, May 2012 1, ISSN 2250-3153
"Biotransformation of a single amino-acid L-tyrosine into a bioactive molecule L-DOPA"
http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf

John