Metabolic differences among individuals

And yet our response to caffeine is highly individualized, both experts said. There are genetic differences in how people metabolize caffeine. Also, research shows that smokers tend to metabolize caffeine more rapidly. In contrast, during pregnancy, caffeine is metabolized more slowly, so doctors advise limiting intake.

Federal government guidelines recommend that adults consume no more than 400 milligrams of caffeine a day, which the Food and Drug Administration estimates to be about two to three 12-ounce cups of coffee.

Nevertheless, “if you have a 130-pound female and a 210-pound male, that is a very different relative dose per day,” Arent said. “Under 400 [milligrams] seems to be very safe, but that same 400 can impact people very differently, especially if you get very differently sized people.”

Some habitual coffee drinkers can also become more tolerant of caffeine and would need to drink more to get the same effect, Lieberman said.

The amount of caffeine in a cup of coffee can also vary, Arent said: “Generally speaking, there are 80 to 120 milligrams of caffeine in a 6-to-8-ounce brewed cup of coffee.” How the coffee is brewed can make a significant difference, he said. For example, one study showed that espresso extracts the largest amount of caffeine.

In contrast, tea has a lower caffeine content, about 71 milligrams in a 12-ounce cup of black tea, 37 milligrams in 12 ounces of green tea and usually no caffeine in herbal teas.

Caffeine also exists in energy drinks, colas and chocolate.

Overdoing the caffeine can cause unwanted effects such as high blood pressure, insomnia and other sleep disruptions, headaches, and anxiety, according to the FDA.

Given the high variations in caffeine sensitivity and metabolism, people should see what is best for them to improve alertness and attention, both experts said.

“There is individual variability, so we should give some fairly reasonable and robust guidelines and sort of estimations of about when caffeine will hit its peak, about how long you’ll feel that peak,” Arent said, “but that’s not going to be precise for everybody.”

When to use or avoid caffeine

There are ways, in general, that most people can use caffeine to their benefit, experts said.

Time your caffeine intake for mental sharpness, for instance, if you have an important meeting or presentation. “Caffeine starts to work very quickly, in 15 minutes, and reaches its maximal effectiveness in about half an hour to an hour,” Lieberman said. “Depending on the dose, it will continue to work for four hours or more if you’re taking 200 milligrams, which is about two cups of coffee.”
Drink a little more caffeine, if you tend to have an afternoon crash. There is advice on some social media sites urging people to delay their caffeine consumption by up to two hours after waking up to prevent an afternoon crash. “Honestly, most of that is garbage,” Arent said. “It’s a poor understanding and misinterpretation of how we function physiologically.” If you delay your morning intake and still need an afternoon intake, “you’ve probably pushed it back, so now you potentially interfere with sleep,” he said.
Stop caffeine at least six hours before bedtime. People differ in sensitivity to caffeine, but experts offer this general guideline to reduce difficulty in falling or staying asleep. Experiment to find the cutoff time that works best for you, perhaps one that’s earlier in the day.
Take a coffee nap. One small study showed that drinking coffee and taking a brief nap immediately afterward can improve alertness - more than either measure alone. Napping for about 20 minutes can be refreshing but also gives the caffeine time to take effect. Coffee naps work best after lunch, but one study found that combining caffeine and napping also worked well to improve night-shift workers’ alertness and performance.
Consider avoiding caffeine if you’re already feeling nervous. Caffeine stimulates the central nervous system, leading to a release of adrenaline that can make some people feel anxious or nervous. | The Washington Post

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How to use caffeine to boost alertness