Stealth Assault on Health: Beverages Pack Calorie Punch (Op-Ed)
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Katherine Tallmadge, M.A., R.D., is a registered dietitian, author of "Diet Simple: 195 Mental Tricks, Substitutions, Habits & Inspirations" (LifeLine Press, 2011) and a frequent national commentator on nutrition topics. This article was adapted from one that first appeared in the Washington Post. Tallmadge contributed this article to LiveScience's Expert Voices: Op-Ed & Insights.

Juicing is all the rage these days, with juice cleanses, celebrity-sponsored juicers and Starbucks opening its first juice bar.

I've been drinking orange juice every morning of my life. You'd think I'd be sick of it by now. But every morning, I look forward to my "sunshine in a glass,"and it never disappoints — especially when it's fresh-squeezed. I could live on the stuff. Just thinking about it makes me salivate!

But I limit my juice to four ounces in the morning because, while it packs a nutritional punch, it also puts on pounds — and fast! Here's an example of how.

One of my clients, Caroline, was successfully losing weight, but in recent weeks was disappointed that she wasn't losing weight as well as she was before. It didn't make sense to either of us. Her food intake was stellar. She was even a little more physically active than usual. Not until we reviewed her food diary thoroughly did we discover the culprit was liquid calories, and they added up in a way that surprised her. In her case — as is the case with many people — that extra glass of wine or mixer, or juice as a snack here and there can add up in ways we don't expect.

Liquid calories in just about any form — alcohol, juice or soda — are stealth calories. They come in undetected under the radar screen, but have an impact that can be enormous. Scientific evidence confirms that although such liquids count as calories, the body doesn't detect them the same way as it would detect solid food.

When people eat calories in the form of solid food, they naturally compensate by reducing the rest of their food intake. But when people ingest liquid calories, they don't compensate for them by eating fewer calories, studies show.

"Fluid calories do not hold strong satiety properties, don't suppress hunger and don't elicit compensatory dietary responses," said Richard Mattes, M.P.H, R.D.,a professor of foods and nutrition at Purdue University. "When drinking fluid calories, people often end up eating more calories overall."

This may help explain the results of a study by researchers from Harvard University and Children's Hospital in Boston, conducted over eight years, with nearly 50,000 women. The researchers behind that study found that women who increased their intake of sugar-sweetened beverages, such as sodas or fruit punch, from one per week to one or more per day added 358 calories daily and gained significant weight. Women who reduced their intake cut 319 calories per day and gained less weight. Earlier studies demonstrated that consumption of sugar-sweetened soft drinks increased the likelihood of obesity in children, but this is the first finding from a long-term observational study in adults.

The mechanisms controlling hunger and thirst are completely different, and liquids — even if they contain calories — don't seem to satisfy hunger even if they quench your thirst. Physiologically, your thirst is quenched once your blood and cell volume is increased by water. This signals to your brain that you are no longer thirsty.

Hunger is regulated in your stomach and intestines. While you're eating, nerves in the stomach wall detect that it is stretching and send satiation signals to the brain. The intestines also release nerve regulators and hormones. At the same time, the hunger hormone, ghrelin, which is released by the stomach when it is empty, decreases — all of which help you feel satiated.

There are several theories explaining why liquid calories cause lower satiety and increased overall calorie intakes, but the process is still not fully understood. First, cognitively, people have a harder time realizing that liquids count. Also, the mouth-feel of a liquid versus a solid may generate different signals, less time and involvement with food, and reduced psychological satisfaction. Finally, because liquids travel more quickly through the intestinal tract, they alter the rate of nutrient absorption, which can affect satiety hormones and signaling. It's likely that all of these reasons are relevant.

Emerging research is finding the hunger hormone ghrelin may play a key physiological role in the weight gain associated with juices and other drinks.

"When the number and type of calories are the same, the calories in liquid form won't suppress ghrelin as effectively as if the same calories were in solid form," says David Cummings, associate professor of medicine at the University of Washington and the Veterans Affairs Puget Sound Health Care System.

Although Cummings hasn't tested many types of fluids and their varying effects on ghrelin, other researchers have found that drinking fluids may produce varying degrees of satiety, depending on what the fluids contain.

Research has established fairly well that alcoholic beverages and sugary liquids, especially sodas and fruit drinks, aren't completely registered or compensated for by the body and simply add extra calories.

"Some beverages cross over the line into being a food," says Barbara Rolls, professor of nutritional sciences at Pennsylvania State University. She conducted studies that found people felt more satiated and consumed fewer calories when they had milk-based drinks at the beginning of a meal. The high protein levels, in addition to cognitive beliefs about milk being a food, may make it more satiating. Also, fluids with food in them, such as soups, are very satiating.

But most caloric fluids Americans consume are not satiating. When you consider that an appropriately sized meal is anywhere from 400 to 700 calories, and one Big Gulp is more than 300 calories, you understand the scope of the problem! A Starbucks Frappuccino can total anywhere from 300 to 500 calories. One glass of wine contains at least 100 calories, and one mixed drink can set you back 300 calories or more. Double or triple these numbers at any given party, tack on the calories in your meals, and you can understand how weight gain is the inevitable result.

Those of my clients who became aware of liquid calories have achieved impressive results. Take Bob Levey, a former Washington Post columnist. Levey wrote about the importance of cutting out his daily lemonade in his successful weight-loss effort. Another client, Julie, easily switched her daily Frappuccino to a skim-milk latte and saved 250 calories per day. A friend of mine, Linda, slowly phased out her daily soda by adding more and more ice to it each week until she was only drinking water. She lost 30 lbs. over a year.

Most people find reducing liquid calories is an easy change. Since liquid calories don't contribute to feelings of satiety, cutting them doesn't lead to feelings of deprivation or hunger. And there are so many great substitutes.

The one liquid that's important to keep drinking is water. In the wintertime, I love sipping water as teas throughout the day. In the summer, it's seltzer with a twist of lemon or lime, and — in moderation — the occasional diet soda.

Of course, if people are mindful of their calorie intake, a moderate daily dose of wine or other caloric beverage won't hurt once in a while. The key is mindfulness and moderation.

Katherine discussed juicing on National Public Radio's Kojo Nnamdi Show on WAMU 88.5 FM. Tallmadge's most recent Op-Ed was 13 Tips for Staying Hydrated in the Summer Heat, and her additional contributions are available on her profile page. Her latest book is "Diet Simple Farm to Table Recipes: 50 New Reasons to Cook In Season". The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on LiveScience.com.