All vitamins are either water-soluble or fat-soluble, so you need both water and fats to absorb many of the nutrients in your diet. Water also plays an important role in your body's ability to digest fats and use them for energy. As a result, although you should try to limit your intake of fats and drink plenty of water throughout the day, a good time to drink a glass of water is after eating fatty foods.
Your body relies on a compound called bile to digest fats. Because water is one of the major components of bile, drinking water after a fatty meal can help your body produce enough bile and replenish your bile stores. In addition, bile acids need water in your intestines to dissolve fat molecules. As a result, drinking water after a fatty meal can make it easier for your body to digest and dissolve large amounts of dietary fats.
One way to classify vitamins is by looking at how well they dissolve in fats or water. The fat-soluble vitamins include vitamins A, D, E and K, while B-vitamins and vitamin C are water-soluble. Unfortunately, fatty foods, such as meats, eggs and dairy products, are among the only sources of water-soluble vitamin B-12. Because other fatty foods contain water-soluble B-vitamins, drinking water can help you absorb all the vitamins in your meal.