Last week, we saw a 3-year-old boy for his routine checkup. His weight had skyrocketed and the plot on his growth chart showed that he, like 70 percent of children in North Philadelphia, weighs too much for his height.

His mother insisted he was a picky eater, not a voracious one. Looking in his mouth, we could see several cavities forming in his teeth.

We thoroughly reviewed his diet with his mother and discovered something astonishing. Our patient was downing about 36 ounces - more than a quart - of sweetened fruit drink every day. That added 500 calories to his daily intake. And because, like many toddlers, he still drinks from a bottle, his teeth bathe all day in corrosive sugar.

The medical cost of obesity is estimated to be $147 billion a year in the U.S., approaching 10 percent of the nation's health-care expenses. The costs are apparent even in the early years: increased bone disorders from excessive weight, sleep apnea and its stress on the heart, fatty infiltration of the liver and gallbladder, and early coronary artery plaque formation. Add to all that the psychological toll our young patients face in bullying, depression, and low self-esteem.

Companies spend almost $900 million each year advertising sugar-sweetened beverages such as fruit drinks, soda, and sports drinks on TV, in print media, and on billboards. Some ads even try to make the products sound healthful, with phrases like "100 percent juice," but sugar has the same calories and tooth-decaying power no matter its source.

Messages are focused at poorer and minority communities, and they apparently work. A recent study showed the odds of heavy consumption of sugar-sweetened beverages were almost double among more impoverished children and teens compared to better-off counterparts.

Another study, from the Rudd Center for Food Policy and Obesity, showed that African American youth saw twice as many TV ads for sugar-sweetened beverages than white youth, and that ad spending on Spanish-language TV had increased 44 percent in the last three years.

A 2012 study in Pediatric Annals showed that sugar-sweetened beverages contributed more calories to the diet than any other single type of food or drink, more than 1,200 "empty" calories a week. Children who down these worthless calories were less interested in healthier beverages, such as water and milk. Another study showed that children who drink sugar-sweetened beverages don't find veggies and fruits appealing because they aren't sweet enough for them. And they were more likely to watch at least two hours of TV daily - with all the inactivity that goes with it.

Since the 1970s, kids' daily consumption of soft-drink calories has increased by 230 percent, and fruit-drink calories went up by 170 percent. A more recent study showed higher intake of sugar-sweetened beverages among African Americans, Latinos, and more impoverished children. Another study demonstrated that for every additional ounce of sugar-sweetened beverages consumed daily, the risk of becoming overweight increased by 4 percent.

Besides the extra calories, how do sugar-sweetened beverages contribute to weight gain? It turns out these empty calories don't register like real calories would. The child who drinks them may have consumed hundreds of calories but doesn't feel full. So he eats more, and, wham, his weight goes up.

A 20-ounce serving of cranberry juice has an incredible 17 teaspoons of sugar. A sports drink has 8.5 teaspoons of sugar.

No parent would pour that much sugar into his or her child on purpose, but this is what we do when we fill our children's cups with this stuff.

And it's often not just table sugar, but a more sinister sweetener: fructose, in the form of corn syrup.

The human body needs sugars for energy, but fructose bypasses the regular metabolism of sugar and can wreak havoc on the body when consumed in large quantities over long periods. Fructose increases one's risk of hypertension, elevates sugar in the bloodstream, and decreases one's ability to realize that one is full and does not need more calories. It also increases insulin levels, causing the body to crave more. At the same time, energy levels sink, causing one to become more sedentary.

So where are children getting all these sweet liquids?

More than 60 percent of all sugar-sweetened beverages, in all age groups, come from the children's homes. When there are bans on such drinks in schools, children usually compensate by drinking more outside school.

What can we do?

First and foremost is awareness. Many of the families we serve do not know the effects of sugar-sweetened beverages on their children's health.

Our 3-year-old patients' mother thought she was making a healthy choice by giving him what she called cranberry juice, but it really was "cranberry juice cocktail" made with high-fructose corn syrup, although the amount of sugar on the labels of both was identical.

Even 100 percent fruit juice should be limited because although it is more nutritious than sugar-sweetened beverages, natural sugars also can affect young teeth and waistlines. Whole fruit is best, but if you do give juice, the American Academy of Pediatrics advises:

Birth to 6 months. No fruit juice, unless it's used to relieve constipation.

6 to 12 months. If juice is given, limit it to 4 to 6 ounces and serve it in a cup (not a bottle) to avoid tooth decay.

1 to 6 years. Up to 6 ounces a day.

7 years and older. Up to 12 ounces a day.

Mass-media campaigns can be effective, but the beverage industry spends close to a trillion dollars a year to strategically combat these forces. What about taxes, and, more precisely, an excise tax, borne by the manufacturers? That has great potential, yet only Berkeley, Calif., has been able to get such a tax passed. Mayor Nutter tried twice in Philadelphia and failed.

In 1964, 42 percent of Americans smoked cigarettes. With research, lawsuits, taxes, and mass-media campaigns, the rate today is 19 percent.

The science behind sweetened beverages and health is still in its infancy. But together, as we have done with other harmful consumer products, we must collectively limit the consumption of sugar-sweetened beverages for the health of our children.

Daniel R. Taylor, D.O., is an associate professor at Drexel University College of Medicine and director of community pediatrics and child advocacy at St. Christopher's Hospital for Children. Jennifer Robbins, M.D., is a third-year pediatric resident at St. Christopher's.