填空题

Questions 47 to 51 are based on the following passage.
You could fill a store with all the energy drinks now available. They promise to make people feel more energetic and think more clearly. These products have names like Red Bull, Monster, and 5-Hour Energy. They appeal mainly to young people and are fueled mainly by caffeine.
Chad Reissig at Johns Hopkins University in Baltimore, Maryland, is a medical researcher who studies drug dependence. He says the team he works on got interested in studying energy drinks because of the explosion in their popularity in the last few years.
He says the researchers found three major things. One: there are hundreds of brands of energy drinks in most major countries. Two: the drinks are not clearly labeled with enough information. And three: the amount of caffeine varies greatly. Some contain as little as 50 milligrams, others as much as 500. By comparison, a cup of Starbucks brewed coffee contains 330 milligrams in 473 milliliters.
Some energy drinks contain a mixture of ingredients listed as an "energy blend." Some ingredients are naturalsubstances. But Chad Reissig says scientists do not know a lot about them and how they interact with each other and caffeine. And worse, he says, there is no listing of the amount of each ingredient.
Some people combine energy drinks with alcohol. They think they can drink more alcohol that way and not be affected. Studies, however, suggest that they are still under the influence of the alcohol even if they do not feel that way.
Some makers of energy drinks do provide warnings. For example: a popular energy shot warns against use by people who are pregnant, nursing a baby or under the age of twelve. Energy shots are small bottles of liquid. The directions also advise people to limit caffeine products and to drink only half the bottle which is regarded as a moderate amount.
The team at Johns Hopkins University School of Medicine published a report in the journal Drug and Alcohol Dependence. They suggested better labeling with the amount of caffeine and other ingredients clearly listed on the drink. The researchers are continuing to study energy drinks. They arc currently seeking young people who have had an unpleasant experience after drinking them.
For energy shots containing caffeine, the right amount to drink is only ______.