Even slight dehydration can produce a small shrinkage of the brain that can impair neuromuscular coordination and abilities to concentrate and think.

We can go more than three weeks without food, but deprive our bodies of water, and we can survive only a few days at best. Keeping hydrated is even more important as the summer heats up. More than 300 Americans die each year from dehydration or heat-related illnesses, according to the U.S. Centers for Disease Control and Prevention.

Our brains need water to manufacture hormones and neurotransmitters. Water also acts as a shock absorber for our brains and spinal cords. Water regulates our body temperature, carrying heat away from internal organs. We need adequate fluid to aid in digestion and conversion of food to the nutrients our bodies can use. Water keeps our joints lubricated and our mucous moist and delivers oxygen to various parts of our bodies. Of course, water is required to flush wastes and toxins from our bodies, too.

Hydration is important for heart health as well. Water helps the heart more easily pump blood through vessels to the muscles. With proper hydration, the heart doesn’t have to work as hard and the muscles also work more efficiently.

If you don’t drink enough water, the body’s reaction is to retain what water it does have for its most important functions. Even slight dehydration can produce a small shrinkage of the brain that can impair neuromuscular coordination and abilities to concentrate and think. A mere five percent drop in your body’s water level can cause a 30 percent drop in energy. Signs of dehydration may be subtle and include fatigue, loss of appetite, flushed skin, heat intolerance, light-headedness, a dry cough, headache, swollen feet or dark urine. Not sweating during vigorous activity is another red flag. When there is severe dehydration, blood volume drops, causing a potentially fatal drop in blood pressure.

How much water do you need to stay hydrated? The old eight-glasses-of-water-a-day rule doesn’t apply to everyone. A more accurate estimate can be had by dividing your weight by two: The resulting number is the number of ounces of fluid you need daily. Remember, though, that many factors can change your water needs from one day to the next. The weather, your clothing, your activity, the duration of the activity and how much you sweat all play a role in how much fluid you need. Sitting in the hot sun or in a humid environment can cause you to lose more fluid than usual. People with certain conditions, such as diabetes and heart disease, may require more fluids. Those most at risk for dehydration include children, athletes, outdoor workers and the elderly.

Water is the best thing to drink to stay hydrated, but you also get fluid from foods and other beverages. Fruits and vegetables naturally have a high water content, so be sure to include plenty of those in your diet. Sugary drinks can be hard on your stomach when dehydrated, so be cautious with those. Caffeine-loaded drinks and alcoholic beverages provide fluid, but they tend to act as diuretics, eventually resulting in fluid loss.

If you are planning on enjoying a day in the sun, drink 16 to 20 ounces of water before you go. Drink frequently throughout the day — six to 12 ounces every 15 to 20 minutes — and make sure to have another 16 to 20 ounces of water within a couple of hours of returning indoors. If your kids are outside playing for hours at a stretch, make sure they have fluids readily available and encourage them to drink frequently.