Science Buddies: "Ask an Expert"

Hi!
I just finished doing my experiment with glycemic index. I had 3 participants and 6 different test foods. I tested the participants blood glucose levels every 30 minutes for 2 hours after eating the foods. I have the graphs plotted and now am having trouble calculating the area under the curve. Everything I found is using calculus and I am not at that level and I don't understand it. I know that you can use area of rectangles to get the area under the curve but I need some help with that.
Thanks

Hi,

I'm assuming you have access to MS-Office and Excel, so here's a video that shows how to calculate the AUC by using an approximation method called Riemann sums: http://www.statisticshowto.com/how-to-f ... oft-excel/

You don't need to know calculus. All you have to do is plug your x and y data into the spreadsheet and follow the instructions for setting up the formula and doing the calculation.

Let us know if this works OK for you.

Good luck!

Sybee

Hi again
I have a question regarding the data I collected. With certain foods, the blood sugar did not return to its beginning value. Does that matter? I had blood glucose readings at 0, 30, 60, 90 and 120 minutes after eating the food. Do I just calculate the area under this curve or do I need to extend my line so that it goes back to the initial value?
Thanks

Hi,

The blood glucose level will not necessarily come down to the zero-time value after a certain length of time. I would calculate the AuC for just the curve from t = 0 to t = 120 minutes. It might be worthwhile to calculate the AuC for the intervals 0-30, 30-60, etc and see how they compare. There may be some early effects that diminish by 120 minutes. Post again after you do the calculations and let us know how it came out.

Sybee

This is getting really confusing. My results are all over the place and I'm assuming this is because I only tested three people.
The way I did my graphs is that I plotted the time the blood was read as (0, 1, 2, 3, 4) which corresponded to (0, 30, 60, 90, 120) I just wanted to work with smaller numbers so I don't think it matters how what values I use for the x axis
On the y-axis I plotted the change in blood glucose sugar. I did this so that my graph would start at (0,0)

I'm going to give an example
Corn Flakes was one of the foods tested. Here is the blood glucose readings I got
0 30 60 90 120
97 155 146 122 92

I plottede
0 1 2 3 4
0 58 49 25 -5

When I calculated the area under the curve using the change in blood glucose and the (0,1,2,3,4) as I explained before I got a value of 139.5
I tried to attach the excel sheet I got a message that the board attachment quoata has been reached.
Let me know if you don't understand what I explained and Ill send the attachment a different way

You don't have to start at 0,0 on the graph. Just plot the mg of glucose per dL on the y-axis and time in minutes on the x-axis from 0 to 120. It is better to use the actual values rather than 1, 2 ,3 etc so that a person looking at the graph will be able to understand it more quickly.

I don't understand what you mean by a 'board quota' on Excel. Try it again with the actual times and glucose values and let me know what happens. If Excel doesn't work you can just plot your data on a piece of graph paper and count the squares under each curve. It's not as accurate but it should be good enough to show a difference. In order to prove or disprove a hypothesis you should have at least three subjects for each test. Then you can take the average, get the standard deviation and do a statistical test. Keep this in mind for your next science project.

Good luck!

Sybee

Good morning
So I did the area under the curve by drawing out each of the curves and usings triangles/trapezoids. It finally worked out.
My new question is that I want to compare the values I got with published values. However, I used bread as a reference and most data uses glucose as a reference. Do you know if there is published data with white bread as a reference?

Glucose is used as the reference because it is a sugar that can be absorbed without needing the body to break it down further like sucrose or starch. You can use bread as the reference but you will have to adjust your glucose readings because bread does not have a GI of 100 as glucose does. White bread has a GI of 70 (http://nutritiondata.self.com/topics/glycemic-index).

Find the ratios of the change in blood glucose levels of your other foods to the change in BG level from white bread and multiply it by 70 to get the GI of the other foods. Let me know if this works out ok.

Sybee