Let's start with the basics. I assume the original IV was continuous. If not, what was it? Now, why did you make quartiles? BTW what is the original IV and DV? Best, David

Thank you for your reply. 

The original IV was indeed continuous, we just wanted to see whether results would change if we categorize the IV. the original IV is the amount of a substance present in food (measured in mg/day), the DV is beta cell function (we used indices based on data we got from the OGTT).  

Categorizing a continuous predictor like this is generally a very bad idea. You lose information and therefore statistical power. You can also detect spurious effects that are artefacts of the categorization. See paper linked.

http://www.psychology.sunysb.edu/attachment/measures/content/maccallum_on_dichotomizing.pdf

I agree with Thom, and recommend the conference presentation linked below.  (I know the author.) 

;-) 

Presentation Free Advice for Power-Hungry Researchers: Do Not Categorize ...

What you are looking for is called linear regression. Good news is that linear regression is quickly done and easy to interpret. It will also give you more statistical power, as by categorization you loose information.

And don't worry, I've seen this categorization non-sense done by seasoned professors (and sometimes forced upon their students). That's from a past era, where you literally had to crunch the numbers with pencil and paper, which is easier with categories.

https://schmettow.github.io/New_Stats/lm.html#lrm