Will drinking more improve my constipation?

A common recommendation to manage symptoms of constipation is to increase fluid intake, often to levels of 1.5-2 litres (or 6-8 glasses) per day.  Despite enthusiasm for this recommendation, there is very little scientific evidence to suggest that the strategy on its own is effective, except in the presence of dehydration or when increased fibre recommendations are provided. 
For instance, the fluid intake of constipated individuals is thought to be similar to that of healthy individuals (1) and while bowel movement frequency decreases with a very low fluid intake (500ml per day compared to 2500ml per day)(2), additional fluid intake (above normal levels) does not appear to increase in stool output in healthy individuals(3). Although one study in people with functional constipation who were consuming adequate fibre intake (25g per day) showed that consuming ~2L fluid per day increased bowel movement frequency and reduced laxative use (compared to a group consuming ~1L per day)(4), increasing fluid intake alone has not been shown to improve constipation. 

The take home message? 

Make sure you are drinking enough fluid to avoid dehydration, but going overboard will not improve your constipation. As far as monitoring your hydration status goes, your thirst and urine colour are simple, reliable measures(5-8). For useful tips on managing constipation, read this previous blog on the topic https://www.monashfodmap.com/blog/managing-constipation-ibs/

References

1.      Lindeman RD, Romero LJ, Liang HC, Baumgartner RN, Koehler KM, Garry PJ. Do elderly persons need to be encouraged to drink more fluids? J Gerontol A Biol Sci Med Sci. 2000 Jul;55(7):M361-5. PubMed PMID: 10898251. Epub 2000/07/18. eng.
2.    Klauser AG, Beck A, Schindlbeck NE, Muller-Lissner SA. Low fluid intake lowers stool output in healthy male volunteers. Z Gastroenterol. 1990 Nov;28(11):606-9. PubMed PMID: 2288138. Epub 1990/11/01. eng.
3.    Chung BD, Parekh U, Sellin JH. Effect of increased fluid intake on stool output in normal healthy volunteers. J Clin Gastroenterol. 1999 Jan;28(1):29-32. PubMed PMID: 9916661. Epub 1999/01/23. eng.
4.    Anti M, Pignataro G, Armuzzi A, Valenti A, Iascone E, Marmo R, et al. Water supplementation enhances the effect of high-fiber diet on stool frequency and laxative consumption in adult patients with functional constipation. Hepatogastroenterology. 1998 May-Jun;45(21):727-32. PubMed PMID: 9684123.
5.    Kavouras SA. Assessing hydration status. Current opinion in clinical nutrition and metabolic care. 2002 Sep;5(5):519-24. PubMed PMID: 12172475. Epub 2002/08/13. eng.
6.    McKenzie AL, Munoz CX, Armstrong LE. Accuracy of Urine Color to Detect Equal to or Greater Than 2% Body Mass Loss in Men. Journal of athletic training. 2015 Dec;50(12):1306-9. PubMed PMID: 26642041. Pubmed Central PMCID: PMC4741257. Epub 2015/12/08. eng.
7.    Armstrong LE, Maresh CM, Castellani JW, Bergeron MF, Kenefick RW, LaGasse KE, et al. Urinary indices of hydration status. International journal of sport nutrition. 1994 Sep;4(3):265-79. PubMed PMID: 7987361. Epub 1994/09/01. eng.
8.    Armstrong LE, Soto JA, Hacker FT, Jr., Casa DJ, Kavouras SA, Maresh CM. Urinary indices during dehydration, exercise, and rehydration. International journal of sport nutrition. 1998 Dec;8(4):345-55. PubMed PMID: 9841955. Epub 1998/12/08. eng.