Consider how much better a well-lubricated car engine runs compared to one that’s dry. Your body’s “engine” isn’t that different.

“We are made of 70 percent water — everything that works inside the body works on a water base,” says Dr. Bill Rawls, M.D., Medical Director of Vital Plan. So, when it comes to feeling your best, drinking enough fluid is right up there with eating well and getting plenty of sleep and exercise.

Water is key for functions on a cellular level, but is also important when it comes to the big picture. It helps your heart pump blood throughout your body more easily. It facilitates detoxification by filtering and removing waste via your lymphatic system and kidneys. It also lubricates joints, and ensures your skin barrier is strong and resilient. A recent review in the journal Skin Research and Technology even found that if you don’t drink enough water, increasing your intake can improve skin hydration and reduce skin dryness and roughness.

Water is also crucial for maintaining body temperature, and it may play a role in the regulation of blood sugar levels. Those who drink only about two glasses of water or less a day are more likely to have blood sugar levels considered to be pre-diabetic, according to a study in the journal Diabetes Care. And H2O helps keep your brain clear. Research suggests mild dehydration could impair cognitive performance, including the brain functions involved in concentration, mood, alertness, and short-term memory.

Your digestive system is especially sensitive to fluid levels, Dr. Rawls says: All of your beneficial bacteria, enzymes, and digestive juices need fluid to do their job of breaking down food and delivering nutrients. In fact, digestive issues can occur not only because of the lack of nutrients and load of sugar and fat in junk food, but also because of the lack of fluid in that junk.

“If you look at the typical Western diet, we eat a lot of dry food. Your average cheeseburger and fries has no water in it — the burger is mostly protein and oil, the bun is dry,” Dr. Rawls says. “Dump that into your stomach, and you’re not going to have any water for digestion. As a result, your system will have to pull water from elsewhere in your body, and it’s not going to do a good job.”

The other scenario is that you feel so thirsty after your dry, salty meal, that you’re inclined to wash it down with a supersized, icy drink. “That’s another huge problem,” warns Dr. Rawls. “Pouring all that liquid into your system at once isn’t hydrating you, it’s diluting the digestive enzymes and stopping the process of digestion cold.” Literally, cold: Chemical reactions that need to occur for healthy digestion rely on heat to work, he says. In other words, it’s like throwing a bucket of water on the campfire.

So what are the best ways to ensure you stay hydrated throughout the day? Dr. Rawls shares seven simple tips here.

1. Monitor your urine

It’s the best way to determine whether you’re getting enough fluid. “If your urine looks like lemonade, you’re fine,” Dr. Rawls says. If it looks dark, like apple cider vinegar, or if you’ve gone several hours without needing to use the bathroom, you may be dehydrated.

On the other hand, clear urine and going more frequently than usual can be a sign you’re overhydrating. Your body can usually handle some extra fluid — you just pee it out. But if you drink too much water at once, you could end up not absorbing what you need, or diluting your body’s delicate balance of salt and other electrolytes.

Overhydration is rare – it’s typically seen in endurance athletes who over-consume water before and after competition – so don’t worry too much about overdoing your fluid intake. But if you’ve had excess fluids and are experiencing confusion, nausea, and vomiting, see your doctor: Severe cases of water intoxication can cause seizures, coma, and even death.

2. Consider how you feel

If you’re dehydrated, chances are you’ll feel tired, worn out, and foggy, and you’ll probably have a headache. “You just feel yuck, but drinking water should solve the problem rapidly,” Dr. Rawls says. Thirst also sometimes masquerades as hunger. If you’re craving food, but have recently eaten, try drinking a glass of water or having some fruit before filling up on more calories.

3. Forget the eight-glasses-a-day rule

“That recommendation is so arbitrary it’s to the point of being absurd,” Dr. Rawls says. The amount of fluid people need is highly variable — from person to person, but also from day to day, and sometimes even from hour to hour. Your diet, activity level, whether you’re sweating or ill, your size, and the environment all impact your hydration needs. “There are no hard and fast rules except to drink when you’re thirsty,” he says.

Indeed, a report from the Institute of Medicine states that the vast majority of healthy people meet their daily hydration needs by simply letting thirst dictate fluid intake. That’s because your body is really good at keeping itself adequately hydrated.

When you need fluid, receptors in your brain get a message that cells are starting to shrink and are in need of water. That triggers hormones that then make you thirsty. Once you start drinking, taste buds send the message to the brain that water is on its way, and thirst gradually dies down. Follow your body’s natural signals, and you’ll do a fine job of maintaining healthy hydration levels.

4. Avoid sugary drinks

People tend to drink throughout the day, often when they’re bored, Dr. Rawls says. We also drink for pleasure. That’s one reason you may not actually feel thirsty that often: you may never get to the point of cells contracting because you are consistently well-hydrated. It’s a good position to be in – just be careful that your drink of choice isn’t high in sugar and unnecessary calories.

If plain water isn’t appealing, try unsweetened tea (fruity herbal varieties can often satisfy a sweet tooth if you’re trying to cut back on sugar) or carbonated water with a small amount of fresh-squeezed lemon or other fruit. Caffeinated beverages are also okay in moderation, but they shouldn’t be your main source of fluid.

5. Drink more when you sweat or are sick

If you’re losing fluid rapidly through sweat, vomiting, diarrhea, or because of a fever, you need to increase your intake to replace what’s lost. (Drinking extra fluids may also help loosen mucus if you have a stuffy head.)

• For a moderate-intensity workout, keep plain water on hand and let your thirst guide you. However, some research suggests thirst alone may not always help you replenish enough fluid, so be mindful of how you feel after exercise, and consider taking in some extra water during or after your workout. It also helps to be well-hydrated before you start exercising.
• If you exercise intensely or it’s very hot, and you’re sweating profusely or exercising for a long stretch, drink throughout your workout. Aim to take in 16 ounces of additional fluid or more, depending on the length and conditions of your activity, Dr. Rawls says.

  Also consider fluids with added electrolytes — sodium, potassium, magnesium and calcium — which help your body absorb fluid more quickly. Just be sure to read the label on sports drinks: some are loaded with sugar, so opt for a low-calorie, low-sugar option like nuun Hydration Electrolytes or Powerade Zero. Even better, make your own!

• When you’re sick, sip small amounts of water, but do so frequently, especially if you are vomiting. Eating popsicles or ice cubes can be a good strategy. Electrolytes are also helpful in this case.

6. Eat your water

Research suggests at least 20 percent of our hydration needs come from food, and fruits and vegetables are the best sources. All fruits and veggies are packed with water, but melon, cucumbers, spinach, peppers, strawberries, broccoli, and oranges are all at least 90 percent water, plus they’re full of fiber, protective antioxidants, and other nutrients.

“Fruit and vegetables also contain the type of fiber that holds water in our digestive tract — grain fiber doesn’t,” Dr. Rawls says. “The water is trapped inside the cells of produce, and it’s slowly released throughout the digestive process.”

7. Filter your water

While much of America’s tap water is monitored for bacteria, certain pesticides, and other pollutants, recent headlines and scandals make it clear that the system isn’t failsafe. “We don’t always know what’s in the water coming out of the kitchen faucet,” Dr. Rawls says.

Pollutants can originate from a number of different sources and in different forms, including lead from old pipes and chemical byproducts from manufacturing plants released into waterways. Even pharmaceuticals, including antibiotics, hormones from birth control pills, and antidepressants, can get into water from unused drugs being disposed of improperly (thrown away or flushed), or from recycled wastewater (some of the drugs we take are released in urine, which sewage treatment systems may not filter out).

Reverse osmosis filters do the most thorough job of cleaning water, Dr. Rawls says, however they also remove healthy and beneficial minerals like calcium and magnesium. Other filters, including pour-through, faucet-mounted, and under-the-sink filters also effectively remove lead and other toxins. Check to see if a model you are considering is certified by NSF International — a non-profit public health organization that sets standards and rigorously tests products, including filters — to learn what contaminants they remove.

While we tend to worry about hydration most in the summer, risk of dehydration can rise in cooler months when we’re not feeling as naturally thirsty due to higher temperatures, and that can increase susceptibility to cold and flu. So try to keep these simple tips in mind year-round for the best health protection.

Discover more in Dr. Bill Rawls’ new #1 Bestselling book: The Cellular Wellness Solution: Tap Into Your Full Health Potential with the Science-Backed Power of Herbs.