Your body can retain water for many different reasons. And if it does, that translates into higher numbers on the scale. One big cause of water retention is the type of food you eat. Fatty and salty foods, in particular, can cause your body to hold on to water. And the amount you retain is tied to how much activity you get during that same day. If you ate a lot of salty foods, but exercised an hour or two, you might not retain any excess water. But if you ate those same salty foods and didn't exercise (where you can sweat out water), you might end the day up a few pounds. In fact, your body can store as much as 5 extra pounds (2 kilograms) of water per day [source: Everson].
So how can you tell if you're retaining water versus gaining weight? Water retention pounds can come on in a single day, so if your weight suddenly shoots up within 24 hours, that's a likely culprit. You may also notice a swollen abdomen, and/or swelling in the ankles, fingers, feet and face.
If you're prone to water retention, limit the amount of sodium you ingest to 1,000 milligrams per day. You can also try using natural diuretics, which include lemon water and grapefruit. But most important, make sure you drink eight glasses or 2 liters of fluid a day (not just water but juices, food and fruits also count toward the total). Paradoxically, the more water you drink, the less your body will retain [source: Everson].