During the summer, John Sadler enjoys a daily Starbucks Grande Iced Coffee Mocha—espresso blended with mocha syrup, milk and ice. The 22-year old Pleasant Hill, Calif., resident knows the drink packs 320 calories, but until recently he had no idea about the caffeine content.

"Do I really want to know?" he asked while sipping his drink at a Starbuck's. "I love my coffee, soda and iced tea. But if I know how much caffeine that adds up to in a day, there goes my blissful ignorance."

Curiosity eventually got the best of him. He looked slightly pained to learn his 16-ounce coffee contained 175 milligrams of caffeine.

"Well, that explains a lot," he said. "When I skip my morning Starbucks run, I always feel run-down. Now I know why."

Calories, carbohydrates and fat are consciously tracked nutritional facts. Caffeine? Not so much. An alkaloid, caffeine acts as a stimulant and is commonly found in coffee, tea, soda and energy drinks. The severity of those stimulating properties, addictiveness and potential health problems are widely debated. However, most agree that because tolerance levels vary, everyone responds differently to caffeine.

The problem, especially for caffeine-sensitive people, is setting limits when the quantity is not transparent.

Although the U.S. Food and Drug Administration requires product labels to list caffeine in the ingredients, it does not mandate specifying the amount. Since 1997, numerous medical, health and research officials have unsuccessfully petitioned the FDA to change that stance.

Some companies, including Starbucks, PepsiCo and Coca-Cola, have decided in the past year to list the amount voluntarily.

That still falls short of the full transparency sought by petitioners such as Brenda Eskenazi, a University of California, Berkeley, professor of epidemiology in the School of Public Health.

"I feel strongly that the public has a right to know what they're eating so they can make educated decisions," Eskenazi said. "There are sensitive subgroups, like pregnant women, elderly people and those with heart problems, who need to limit their caffeine intake."

Eskenazi added that whether caffeine has adverse or beneficial health effects is irrelevant.

"I want to be allowed to make my own informed decision," she said. "When my son was younger, I didn't want him to have extra caffeine. Unfortunately, it's not always clear. Who would have thought Mountain Dew would have caffeine? It's the wrong color."

Wrong assumptions are easy to make. While some soda types are generally caffeine free, there are exceptions, such as Barq's Root Beer (22.5 mg), A&W Cream Soda (29 mg) and Sunkist Orange (41 mg).

Certified nutrition consultants, such as Nori Hudson in Berkeley, advise their clients to be diligent.

"A product may not include what you think it does," she said. "Put it under a magnifying glass. Read the label, and if you can't get the information you need on the label, then go online."

Web sites with comprehensive databases listing caffeine amounts include www.EnergyFiend.com and www.CaffeineAwareness.org.

Marina Kushner, founder of the Caffeine Awareness Alliance, also has published a book on the topic called, "The Truth about Caffeine: How Companies That Promote It Deceive Us and What We Can Do About It."

Competition has driven up the amount of caffeine packed into energy drinks. More than 500 such drinks are on the market and their sales pitches say it all.

Fixx Energy promises it will "outperform all other energy drinks" with 500 mg in its 20- ounce bottle. The company's Web site even cautions to consume no more than half the bottle at once until knowing the drink's effects.

Such fine print warnings don't always work. A study by the Sacramento division of the California Poison Control found nine cases of adverse reactions to the energy drink Redline RTD from January 2004 to March 2006. Symptoms included tremors, nausea, vomiting, tachycardia, dizziness, chest pain and bilateral numbness, which landed patients in the emergency room.

Redline RTD is an 8-ounce energy drink with 250 mg of caffeine. Small type on the label recommends drinking only half a can to "assess tolerance."

Energy drinks are popular with teenagers and college students. One industry study found that 62 percent of high school and college students regularly consume energy drinks.