Hydration When You're Eating Less: What the Labels Say and What to Watch For

Hydration is easy to forget when you're not hungry. With a smaller appetite, you often aren't.

There's a knock-on effect. A National Academies report found that about 80 percent of people's total water comes from drinks, and the other 20 percent comes from food. These medications reduce how much people eat: a 2025 joint advisory from four US obesity and nutrition societies reports calorie intake reductions of 16 to 39%. Eat less, and some of the water that used to arrive with your meals goes with it.

Then there's the stomach side. NIH's MedlinePlus lists vomiting and diarrhea as ways the body loses fluid, and nausea as a reason people don't drink enough.

The prescribing information for these three weekly injections each carry a warning called acute kidney injury due to volume depletion. They report postmarketing cases of acute kidney injury, some requiring dialysis, and say most happened in people who had gut side effects leading to dehydration, such as nausea, vomiting or diarrhea. Doctors are told to monitor kidney function in people with these side effects, especially when starting or increasing the dose.

The patient Medication Guides put it plainly: "It is important for you to drink fluids to help reduce your chance of dehydration."

The 2025 joint advisory says dehydration from severe nausea, vomiting or diarrhea can cause acute kidney injury, with or without existing kidney disease, and that efforts should be made to prevent it. Its dietary recommendations include "ensure adequate fluids," and it says follow-up care should regularly re-assess hydration.

As for how much: the National Academies set an adequate intake for women of about 2.7 liters (91 ounces) of total water a day, from drinks and food combined. That isn't a drinking target, and very active people or those in hot climates may need more. Ask your healthcare provider how much is right for you.