It sounds like you’ve been getting some mixed messages. Let’s see if we can clarify a few things. Water is an essential component to all body tissues. In the gut, it’s used to help extract nutrients from the foods we eat; so it’s vital for digestion, absorption, and excretion (Sawka, 2005). But the question here seems to be more focused on when these fluids should be consumed in relation to a meal.
Fluids have a tendency to take up space in the gut. You may be familiar with the old diet trick of drinking a glass of water prior to every meal. The basis for this theory has to do with the displacement of volume. The idea is that you’re replacing some of the room in your stomach with water, thus making it easier to fill up on less food. This is an effective technique if your goal is to lose weight- provided you are able to pay attention to signs of fullness-but it isn’t necessarily an appropriate technique for everyone. Those individuals who have undergone weight loss surgery are usually advised not to consume fluids prior to or during meals because of their diminished stomach capacity. Reserving space for food alone ensures more adequate nutrient intake in a smaller environment. This makes it imperative for these patients to sip fluids throughout the day at times outside of meals to avoid issues associated with dehydration.
However, for the majority of people who haven’t undergone weight loss surgery, drinking fluids (specifically water) during or after a meal can actually aid your digestion and is instrumental in helping you avoid constipation. Of course there are a number of other ways we can enhance digestion such as ensuring that our diet contains plenty of fresh fruits and vegetables as well as fiber through whole grain sources. Regular exercise is also an effective way to assist the digestive process. Activity stimulates muscle-like movement in the GI tract, helping your food continuously process through the system, which also plays a role in preventing constipation.
Source: Sawka MN et al: Human water needs, Nutrition Review 63 (6Pt2): S30-S39, 2005