Water and digestion: 5 hydration myths, checked

"Drink more water" might be the most common advice anyone gets about digestion. It comes up for constipation, bloating, skin, energy and almost everything in between. Some of it holds up. Some of it is closer to folklore.

Here are five common beliefs about hydration and your gut, and what the studies behind them actually found.

Myth 1: Everyone needs eight glasses of plain water a day

Fact: there's no solid evidence for that exact number. A physiologist at Dartmouth set out to find the scientific origin of the "8 x 8" rule, meaning eight 8-ounce glasses a day. He found no studies supporting it for healthy adults living in a temperate climate and mostly sitting. Surveys of food and fluid intake suggested that such large amounts aren't needed, and caffeinated drinks can count toward the daily total (Valtin, 2002).

The same review stressed the other side, too. People doing vigorous work or exercise, especially in hot weather, and people with certain medical conditions may need more. So the useful question isn't "Did I hit eight glasses?" It's "Am I getting enough fluid for my body and my day?"

Myth 2: Coffee and tea don't count because they dehydrate you

Fact: in moderate amounts, they count. In a UK trial, 72 healthy men drank a liter of one of 13 common beverages, and researchers collected their urine for the next four hours. Coffee, hot tea, iced tea, cola, sparkling water, orange juice and several others produced about the same urine output as still water. Milk and an oral rehydration solution were actually retained better than water (Maughan et al., 2016).

Two caveats: all the participants were men, and they drank a set amount at one sitting. Very large amounts of caffeine weren't tested. But your morning coffee isn't erasing the water you drink.

Myth 3: Drinking extra water will fix constipation

Fact: extra water alone may do little if you already drink enough. In a small Texas study, 15 healthy volunteers added first 1 and then 2 extra liters of fluid a day, either water or a sports drink. Their urine output went up significantly. Their stool output didn't change significantly (Chung et al., 1999).

In other words, when you're already well hydrated, the extra fluid mostly ends up in the bladder, not the colon. That doesn't mean fluid is irrelevant, as the next myth shows.

Myth 4: If you eat more fiber, the water part doesn't matter

Fact: fluid seems to help fiber do its job. In an Italian trial, 117 adults aged 18 to 50 with chronic constipation followed a diet with about 25 grams of fiber a day for two months. One group drank as much as they liked. The other group was asked to drink 2 liters of mineral water a day (Anti et al., 1998).

The water group ended up drinking about 2.1 liters a day, compared with 1.1 liters in the other group. Both groups had more frequent bowel movements and used fewer laxatives, but the improvements were significantly larger in the group that drank more.

So the combination is what matters. Water alone did little for healthy volunteers, while fiber seemed to work better with enough fluid. If you're increasing fiber, raise your fluids along with it, and choose a type your gut tolerates. Here's why some fibers are harder on a bloated gut than others.

Myth 5: Sparkling water is always bad for digestion

Fact: it depends on the person. Carbonation adds gas, and some people notice that as burping or fullness. But in a small double-blind trial in Naples, 21 people with indigestion and constipation drank either carbonated water or tap water for about 15 days. In the carbonated water group, indigestion and constipation scores improved, while the tap water group didn't change significantly (Cuomo et al., 2002).

That's one small study, not a reason to switch everything to seltzer. It does suggest there's no universal rule. If bubbles make your bloating worse, skip them. If they don't bother you, there's no need to avoid them.

A simpler way to think about fluids

• Count all your drinks. Water, tea, coffee in moderation, milk and sparkling water all contribute.
• Drink more when you need more. Heat, exercise and a higher-fiber diet are good reasons to top up.
• Spread it out. A glass with each meal and one or two in between is easier to keep up than catching up in the evening.
• Don't expect water to work alone. For constipation, fluid seems to help most alongside fiber, movement and regular bathroom habits.

If you have heart failure, kidney disease or another condition for which your doctor has limited your fluids, follow that advice rather than general tips. And constipation that's new, severe, or comes with blood in the stool, unexplained weight loss or significant abdominal pain deserves a medical check.

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. Maughan RJ, Watson P, Cordery PA, et al. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. Am J Clin Nutr. 2016;103(3):717-23. PMID 26702122
3. Chung BD, Parekh U, Sellin JH. Effect of increased fluid intake on stool output in normal healthy volunteers. J Clin Gastroenterol. 1999;28(1):29-32. PMID 9916661
4. 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
5. Cuomo R, Grasso R, Sarnelli G, et al. Effects of carbonated water on functional dyspepsia and constipation. Eur J Gastroenterol Hepatol. 2002;14(9):991-9. PMID 12352219

This article is for general information and isn't medical advice. If you have a heart or kidney condition or have been told to limit fluids, follow your healthcare provider's guidance.