This is an interesting project idea and is appropriate for your level if you are still interested in doing it.
One thing that is known to affect blood pressure is the intake of water and/or electrolytes (ions dissolved in water). As you know, drinks like Vitamin Water and Gatorade have electrolytes that could affect blood pressure. One option is to modify the Science Buddies Electrolyte Challenge project to make it your own. The great thing about this project is that you can do it at home, and Science Buddies offers a kit that gives you all of the materials you need (http://www.sciencebuddies.org/science-f
... #materials). If you wanted to combine it with your original idea, you could see how the electrolyte content of different drinks correlates with blood pressure. For this, you would also need to purchase a sphygmomanometer (blood pressure cuff), in addition to the electrolyte kit.
You could pick a few different drinks for your test, and use the Electrolyte Challenge procedure (http://www.sciencebuddies.org/science-f
... #procedure) to determine the electrolyte content in each drink. Then you could have people drink the different drinks and look for an effect on their blood pressure. If you don't have time to do the Electrolyte Challenge portion of the experiment, you could just look at the effect of different drinks on blood pressure, as described below.
Let's say you chose 3 drinks to compare. For the experiments, you will want to take resting blood pressure prior to the drink, then ask the person to drink a set amount of liquid (which should be calculated as a fraction of their body weight, such as 10mL of liquid per kilogram of body weight). Then take their blood pressure at set intervals (say, every 30 minutes) for 2 hours after drinking the liquid. You should also have a control group that drinks no liquid, but still has blood pressure taken every 30 minutes for 2 hours.
Thus, you will have four groups: (1) no liquid, (2) water, (3) electrolyte drink #1, (4) electrolyte drink #2. In each group, you will take resting blood pressure, then have the person drink (or not), and repeat blood pressure measurements every 30 minutes for 2 hours thereafter. It will be important to get as many people to participate in your study as possible, so you have a large enough sample size to compare your groups statistically. Once you have the data, I can help you with the statistics.
You will not be able to control for other substances found in energy drinks (such as sugar and caffeine), but you can acknowledge that they, too, affect blood pressure. I think the measurements of the electrolytes will give you a more quantitative
(numerical) way of interpreting your results.
As Donna said, the Science Buddies Topic Selection Wizard (http://www.sciencebuddies.org/science-f ... gister.php
) can help you find another project if you no longer wish to pursue your original idea.
Let us know if you need more help along the way!