Colas contain a fair amount of caffeine. It turns out to be 35 to 38 milligrams per 12-ounce can, or roughly 28 percent of the amount found in an 8-ounce cup of coffee. But few know that diet colas -- usually chosen by those who are trying to dodge calories and/or sugar -- often pack a lot more caffeine. A 12-ounce can of Diet Coke, for example, has about 42 milligrams of caffeine -- seven grams more than the same amount of Coke Classic. A can of Pepsi One has about 56 milligrams of caffeine -- 18 milligrams more than both regular Pepsi and Diet Pepsi.

Even harder to figure out is the caffeine distribution in other flavors of soda pop. Many brands of root beer contain no caffeine. An exception is Barq's, made by the Coca-Cola Co., which has has 23 milligrams per 12-ounce can. Sprite, 7-Up and ginger ale are caffeine-free. But Mountain Dew, the curiously named Mello Yellow, Sun Drop Regular, Jolt and diet as well as regular Sunkist orange soda all pack caffeine. So does Kick (58 milligrams) and Surge (53 milligrams).

Caffeine occurs naturally in kola nuts, an ingredient of cola soft drinks. But why is this drug, which is known to create physical dependence, added to other soft drinks?

The industry line is that small amounts are added for taste, not for the drug's power to sustain demand for the products that contain it. Caffeine's bitter taste, they say, enhances other flavors. "It has been a part of almost every cola -- and pepper-type beverage -- since they were first formulated more than 100 years ago," according to the National Soft Drink Association.

But recent blind taste tests conducted by Roland Griffiths at Johns Hopkins Medical Institutions in Baltimore found that only 8 percent of regular soft drink consumers could identify the difference between regular and caffeine-free soft drinks. The study included only subjects who reported that they drank soft drinks mainly for their caffeine content. In other words, more than 90 percent of the self-diagnosed caffeine cravers in this small sample could not detect the presence of caffeine.

Bone Weakening
Animal studies demonstrate that phosphorus, a common ingredient in soda, can deplete bones of calcium. And two recent human studies suggest that girls who drink more soda are more prone to broken bones. The industry denies that soda plays a role in bone weakening.

Animal studies -- mostly involving rats -- point to clear and consistent bone loss with the use of cola beverages. But as scientists like to point out, humans and rats are not exactly the same.

Even so, there's been concern among the research community, public health officials and government agencies over the high phosphorus content in the U.S. diet. Phosphorus -- which occurs naturally in some foods and is used as an additive in many others -- appears to weaken bones by promoting the loss of calcium. With less calcium available, the bones become more porous and prone to fracture.

The soft drink industry argues that the phosphoric acid in soda pop contributes only about 2 percent of the phosphorus in the typical U.S. diet, with a 12-ounce can of soda pop averaging about 30 milligrams.The National Academy of Sciences has set 3 grams (or 3,000 milligrams) per day as the tolerable upper limit of phosphous for children ages 1 to 8 years, and 4 grams per day for those 9 years and older.

To reach that amount would require drinking at least 100 cans of soda pop per day. But there's growing concern that even a few cans of soda today can be damaging when they are consumed during the peak bone-building years of childhood and adolescence. A 1996 study published in the Journal of Nutrition by the FDA's Office of Special Nutritionals noted that a pattern of high phosphorus/low calcium consumption, common in the American diet, is not "conducive to optimizing peak bone mass in young women."