Thirsty? The beverage you choose to quench that thirst may be one of the most important health decisions you make.

Until recently, nutritionists haven’t paid much attention to what people drink. But lately, a handful of researchers have begun to look more seriously at beverages. And they’ve been shocked by what they’ve discovered.

More than 21 percent of the calories most Americans consume come in the form of beverages, studies show. A typical 12-ounce can of cola packs a whopping 150 calories (the average person should consume about 1800 to 2400 calories a day, depending on their size and activity level). Many people don’t stop at 12 ounces. Some convenience markets now sell beverages in servings of 40-ounces and more. At 7-Eleven, the Double Gulp contains 64-ounces—and an awesome 600 calories.

That’s well over a one-quarter of the calories that most of us should consume during an entire day.

A calorie is a calorie is a calorie, nutritionists like to say. But that’s not strictly true when it comes to beverages. Studies show that calories in liquid form are “stealth” calories. They slip past the sensors that normally tell us when we’ve eaten enough and feel full. In a classic experiment conducted by researchers at Pennsylvania State University, volunteers sat down to a meal that included a calorie-free beverage for some and for others a sugary high-calorie lemonade drink. On average, the volunteers ate the same amount of food, whether they were drinking a high-calorie beverage or a calorie-free drink. In other words, they didn’t compensate for the 150 calorie drink by eating less food.

Some researchers think that the extra calories Americans now drink in the form of sugary soft drinks and fruit drinks may be one of the main culprits behind the nation’s expanding waistline. One recent study found that the risk of becoming obese increases by 60 percent for every sweetened soft drink consumed a day.

Another worry is diabetes. Many beverages that look healthy—like sports drinks and fruit-flavored beverages—are in fact overloaded with refined sugar. A single can of sweetened cola may contain the equivalent of 7 to 10 teaspoons of sugar. Sweetened soft drinks are the single biggest source of sugar in most people’s diets.

“Because sugar is so easily digested, refined sugar comes in and slams right through the digestive system,” explains physician David Ludwig, an expert in endocrinology at Children’s Hospital in Boston. As blood glucose levels soar, the body quickly ramps up production of the hormone insulin, which is used to process glucose and make it available for energy. The result may be what Ludwig calls the “overshoot phenomenon”—in response to surging insulin, blood glucose levels fall below normal.

Over time, the roller-coaster of soaring and plummeting blood sugar levels can lead to type 2 diabetes.

Evidence that sugary drinks increase the risk of this destructive disease is already mounting. In a 2004 study published in the Journal of the American Medical Association, researchers reported that consuming just one soft drink a day almost doubles the risk of developing type 2 diabetes. And there are other dangers. A study by researchers at the Population Health Research Institute in Hamilton, Canada found that people who consumed the sugary soft drinks tend to have low levels of HDL, so-called “good cholesterol,” which normally protects against heart disease. Soft drink guzzlers also had higher than average levels of fats in the blood called triglycerides, another key risk factor for heart problems.

So what should you reach for when you’re parched? Sugar-free versions, of course. But even more convenient, and certainly less expensive, is good old plain water. Replacing one sweetened soft drink a day with a glass of water or a diet soda can shave 150 calories from your daily diet—roughly 5 to 15 percent of total daily calories for most people.

Just that one small and easy-to-make change can help keep weight off and lower your risk of diabetes and heart disease. There aren’t many simple health tips that can offer such a big bang for the effort.

Tags: beverages, calories, sugar-free, water