Fiber and water: myths and facts about how much to drink

You've probably heard that fiber without enough water "turns to cement." Or that everyone needs eight glasses a day, and coffee doesn't count. These rules get repeated so often that they sound like settled science.

Some of them hold up. Some don't. Here's what the research actually says about fluids, fiber and staying regular.

Myth: Everyone needs eight glasses of water a day

Fact: there's no solid evidence for that exact rule. A widely cited review went looking for the origin of "8 x 8" and the science behind it. It found no scientific studies supporting the rule for healthy adults in a temperate climate who are mostly sedentary (Valtin, 2002).

The same review stresses that more fluid is advisable in some situations, including certain medical conditions, vigorous exercise and hot weather. So the message isn't "drink less." It's that one number doesn't fit everyone.

Myth: Coffee dehydrates you, so it doesn't count

Fact: in moderate amounts, it appears to count. In a controlled crossover study, 50 men who regularly drank coffee had four 200 mL servings of coffee a day for three days, and the same amount of water for another three days. Across a wide range of hydration measures, including total body water and 24-hour urine volume, there were no significant differences (Killer et al., 2014).

That study only included men who were used to caffeine, so it isn't the final word for everyone. But it's consistent with the review above, which concluded that caffeinated drinks may be counted toward your daily total (Valtin, 2002).

Myth: Fluids have nothing to do with constipation

Fact: too little fluid can slow things down. In a small experiment, eight healthy young men drank more than 2,500 mL a day for one week and less than 500 mL a day for another. During the low-fluid week, bowel movements dropped from 6.9 to 4.9 a week, and stool weight fell from 1.29 to 0.94 kilograms a week (Klauser et al., 1990).

National survey data point the same way. In an analysis of 9,373 US adults, where constipation was defined as usually having hard or lumpy stools, low liquid intake was linked with constipation in both women and men. Constipation was also more common in women, at 10.2% compared with 4.0% in men. Interestingly, dietary fiber intake wasn't a predictor in that analysis (Markland et al., 2013).

Myth: If some water helps, a lot more water helps more

Fact: extra water seems to matter most when intake is low. The researchers behind the low-fluid experiment noted that there's no evidence that increasing fluids within realistic limits improves chronic constipation on its own (Klauser et al., 1990).

Combined with fiber, though, fluids may make a difference. In a randomized trial, 117 adults with chronic constipation ate about 25 grams of fiber a day for two months. Those asked to drink 2 liters of mineral water daily averaged 2.1 liters, compared with 1.1 liters in the group that drank as usual. Both groups improved, but stool frequency rose more and laxative use fell more in the group that drank more (Anti et al., 1998).

Myth: Fiber without water turns to cement

Fact: the dramatic version isn't something we found good studies on, but the practical advice behind it is sound. Fiber supplement powders come with directions to mix them with plenty of liquid, and those directions are worth following exactly.

If you're raising your fiber intake from food, it's also sensible to raise your fluids a little at the same time, especially if you know you tend to drink very little during a busy day.

What to actually do

• Drink to thirst, with a little structure. A glass with each meal and with any fiber supplement is an easy pattern to keep.
• Count most drinks. Water, tea, moderate amounts of coffee, milk and soups all contribute.
• Drink more when you sweat more, such as in hot weather and on active days.
• If you're constipated and know you drink little, drinking more is a reasonable first experiment, ideally alongside a gradual increase in fiber.
• Ask first if you've been told to limit fluids, for example because of a heart or kidney condition.

And if constipation is new or persistent, or comes with blood in your stool, unexplained weight loss, anemia or severe pain, see your doctor rather than troubleshooting on your own.

Sources

1. Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"? Am J Physiol Regul Integr Comp Physiol. 2002;283(5):R993-1004. PMID 12376390
2. Killer SC, Blannin AK, Jeukendrup AE. No evidence of dehydration with moderate daily coffee intake: a counterbalanced cross-over study in a free-living population. PLoS One. 2014;9(1):e84154. PMID 24416202
3. Klauser AG, Beck A, Schindlbeck NE, Müller-Lissner SA. Low fluid intake lowers stool output in healthy male volunteers. Z Gastroenterol. 1990;28(11):606-9. PMID 2288138
4. Markland AD, Palsson O, Goode PS, Burgio KL, Busby-Whitehead J, Whitehead WE. Association of low dietary intake of fiber and liquids with constipation: evidence from the National Health and Nutrition Examination Survey. Am J Gastroenterol. 2013;108(5):796-803. PMID 23567352
5. Anti M, Pignataro G, Armuzzi A, 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;45(21):727-32. PMID 9684123

This article is for general information and isn't medical advice. If you've been told to limit fluids, or constipation is new or persistent, talk to your healthcare provider.