Caffeine: Best friend you ever had or your worst enemy?

Some people can’t start the day without a cup of coffee, knowing that the caffeine will see them through at least a few hours. Learn about the pros and cons of this popular stimulant in a live Web chat with registered dietitian Ruth Frechman Monday, Aug. 1 at 11 a.m. PDT (1 p.m. CT, 2 p.m. ET). Frechman is a spokeswoman for the American Dietetic Assn. and the founder of “On the Weigh,” a nutrition consulting business in Burbank.

Considering the number of people we see chugging coffee at the gym, we wondered whether it was OK to have a caffeinated beverage just before a workout. A typical 8-oz. cup of brewed coffee has about 100 to 200 milligrams of caffeine.

“Caffeine has been shown to enhance athletic performance,” she said. “It may help you go longer and faster and increase alertness and concentration.” People who consume little or no caffeine (under 50 milligrams a day) may see a stronger reaction compared with those who drink a large cup or more regularly. “You need to know how your body reacts to caffeine,” she added.

For those who are used to the stuff, Frechman recommends trying this formula for deciding how much caffeine to take in to boost performance for endurance exercises such as running or cycling: 3 milligrams of caffeine per 1 kilogram of body weight. For example, a 125-pound person might take in about 170 milligrams of caffeine—or less, again depending on one’s sensitivity.

The body can react to caffeine in a number of ways, Frechman said, including temporarily raising blood pressure, causing jitteriness or gastroinestinal problems, and even anxiety. “Experiment with the amount,” she added. “Slowly increasing the amount before a workout would be prudent.”

Do you have a question for Ruth Frechman? Email jeannine.stein@latimes.com and join the chat to see the answer.