Talk It Over

According to the National Soft Drink Association, most 12-ounce cans of soda contain approximately 45 milligrams of caffeine. By comparison, a 7-ounce cup of coffee contains approximately 100 milligrams of caffeine. Caffeine is also found in tea, chocolate, and some over-the-counter medicines (check the labels). What is caffeine? What effect might it have on how fast the heart beats?

Note: Before you begin, learn how to take a person's pulse. You'll find directions for taking the pulse (counting heartbeats) in many health books and Web sites. Practice recording the pulse rate for 1 minute exactly (using a stopwatch) until you are sure you can do it accurately every time.

Get

• 10 or more adults who are willing to help you
• Personal music player with earphones and soothing music (such as Pachelbel Canon)
• Eye mask
• Comfortable chair
• 10 or more cans of a soft drink that contains caffeine
• Clock and stopwatch

Go

1. Arrange a time for testing each adult. Each test will require about 30 minutes of the adult's time. Ideally, you should test each of your subjects at the same time of day, in the same place, using the same music, and so on—that is, keep all your test conditions as much the same as possible.
2. Ask each of your subjects to eat or drink nothing for at least 2 hours before the test.
3. Test your subjects one at a time. Seat the person in a comfortable chair. Put a mask over the person's eyes. Ask the person to listen to soothing music and relax completely for 5 minutes. Then, disturbing the person as little as possible, take his or her pulse for 1 minute. Record.
4. Then ask the adult to drink the can of soft drink as quickly as possible, while still relaxing in the chair. After the subject finishes the drink, wait exactly 5 minutes, then measure and record the pulse rate for 1 minute.
5. Take the 1-minute pulse rate again every 5 minutes until you have at least 4 readings.

Stay Safe

No special precautions for this project!

Go Easy

Get an adult helper to take pulse rates for you. Take pulse rates only twice: once before drinking the soda and again 15 minutes later.

Go Far

Test different brands of soft drinks. (Visit the manufacturers' Web sites to determine how much caffeine the drinks contain.) You might also try testing different kinds of chocolate or varieties of sports or energy drinks. Or, try testing people who regularly drink sodas or coffee in comparison with people who don't. Do their responses differ?

If you have more time and your subjects are willing to be tested several times, have an adult make instant coffee drinks that vary in caffeine content. For example, you might dissolve ½ teaspoon of instant coffee in ½ cup of water for the first test. You might double or triple that amount for subsequent tests. Use the "Go" procedure to see whether greater amounts of caffeine produce different results than lesser amounts.

Show Your Results

Put pulse rates in a data table like this for "Go":

Make a line graph of the average pulse rates over time. Make line graphs that show the differences among individuals. (You may find big changes in some people and smaller or no changes in others.)

For "Go Easy," use a data table like this:

Make a bar graph for each person you test. Use two bars on each graph to show how each person's after-drink pulse rate compares with the before-drink rate.

For "Go Far," use data tables and line graphs as for "Go," comparing the drinks or amounts of caffeine you test.

Tips and Tricks

Be careful about the conclusions you draw. The drink you tested contains caffeine, but it also contains other ingredients such as sugar or artificial sweetener. Therefore, caffeine may be responsible for any differences you observed, but you cannot be sure, because you did not test pure caffeine.

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