Then the Los Angeles-based dietician helps them read the back of their containers for portion size, calories and sugar content. Then one by one, she stacks sugar cubes by their drink so they can visualize what they are swallowing.

Sometimes she has to stack as many as 25 cubes - nearly 400 calories worth. That's far more than a usual serving of one or two cubes most people use in a cup of coffee or tea.

"They don't often equate what they drink with calories. They just equate what they eat with calories, so that has to be explained," said Giancoli, who has taught such classes for more than a decade to a variety of students, some already grappling with weight issues or diabetes. Many are low-income or elderly.

New York City Mayor Michael Bloomberg's recent plan to restrict cup sizes for sugar-sweetened beverages to 16 ounces (47 centiliters) has reignited debate over what health advocates call "liquid candy."

The focus on sugary drinks comes as two out of every three Americans is overweight or obese and policymakers take aim at sugary drinks, from New York City's cup-size restriction to new efforts to impose drink taxes and cut sales in U.S. schools.

Policymakers and health activists argue that consumers still have options but that government intervention is needed to help buyers make better decisions that improve public health and stem the nation's costly obesity crisis. The beverage industry has countered by stepping up its defense of consumer choice.

But nutritionists, marketing experts and others who study people's drinking habits say cutting back on sugary drinks is not so easy. Understanding drink labels and calculating serving size and calories is increasingly tricky, they say.

"It can be confusing," said Angela Ginn, a Baltimore-based dietician.

"The biggest thing is they don't understand 'What's good for me?' or 'What's bad for me.' They read the outside of the packaging but not the back of the label," said Ginn, who like Giancoli is a registered dietician and part of The Academy of Nutrition and Dietetics, a professional group for dieticians.

While sales of soft drinks have declined in recent years, sales of sports drinks, juices and other non-carbonated beverages have grown.

Americans on average drank 714 8-ounce (24-centiliter)servings of carbonated soft drinks last year - about 9 billion cases, according to Beverage Digest, a leading beverage industry newsletter. Sales of other, non-carbonated drinks, including bottled water, reached about 15 billion cases.

STRUGGLING WITH SERVINGS

"We do a lot of education around portion control, front of pack labeling ... we offer consumers a lot of choices," Simon Lowden, chief marketing officer for PepsiCo Inc's PepsiCo Beverages Co, said at a recent industry conference.

Companies have added calorie counts to the front of many cans and bottles but not fountain drinks. Serving sizes also vary: Coca-Cola Co lists both a 20-ounce (60-centiliter) bottle and a 12-ounce (35-centiliter) can of cola as one serving.

In Giancoli's classes, students still struggle, she said.

Some see pictures of fruit on the label and assume the sugary drinks are good for them. Others have a hard time telling how many servings - experts say nearly three - and calories are in a 20-ounce (60-centiliter) bottle.

Giancoli said showing students the actual sugar in drinks helps. Having a 20-ounce (60-centiliter) bottle of soda every day on top of a regular diet can add 26 pounds (12 kg) a year, she tells them.

Part of the problem, experts said, is that the brain doesn't register calories from drinks as filling the way it does calories from food. It's also tough for people to fully grasp the impact of bigger sizes even as portion sizes have swelled.

In the 1920s, Coca-Cola sold its cola in 6.5-ounce (19-centiliter), 85-calorie single-serving glass bottles. Now companies sell 64-ounce (189-centiliter) drinks that pack up to 850 calories, nearly half of the daily calories for a middle-aged woman who exercises moderately or about one-third of those needed for her male counterpart.

Marketing researcher Pierre Chandon recently reviewed 21 studies and found that even if a cup or package size doubles, consumers cannot tell they are getting twice as much.

"Our mind is very bad at geometry," said Chandon, a professor at INSEAD business school in Paris whose work will be published later this year. Fountain drink cups need pictures on the front that show the number of servings, he said, adding labels such as "medium" or "large" are meaningless for buyers.

Policy makers are also trying to steer consumers away.

Bloomberg's plan, if approved, could start next year. Soda taxes, like one on the ballot in Richmond, California, this November, aim to steer people away with higher costs. U.S. officials are also weighing new school rules that could limit or ban the drinks.

Restaurants oppose Bloomberg's proposal. The National Association of Convenience Stores, which represents retailers exempted from the New York plan, said it is not planning voluntary action like changing cup labels but is working on a plan to help members sell healthier items.

Drink makers themselves have long-touted voluntary steps, such as selling smaller sizes and limiting school sales.

Baltimore's Ginn said she agrees with beverage manufacturers that say people want tasty choices. But it is so difficult for her clients -- mostly blacks with type 2 diabetes, a condition linked to obesity -- to navigate them that she tells them to avoid sugar-sweetened drinks altogether.

"I try to keep things as simple as possible," she said.

(Additional reporting by Martinne Geller and Ransdell Pierson in New York; Editing by Philip Barbara)

• A Permanent Economic Emergency