Can anybody help me out? I have had my head in books for ages now and I feel like I am going a little mad! They are either too basic or too complex to answer my questions.

As part of my MSc I have performed a cross-sectional survey and I am looking to analyse the following variables using MANCOVA through SPSS:

Predictor (IV): Supplement user status (Supplement taker vs non-supplement taker)

DVs: BMI, alcohol score, exercise score, no. of fruit/veg, and smoking status (which given it is categorical I will analyse separately).

I took measurement of the following as possible covariates: age, gender, number of supplements taken.

1) How do I know if the covariates are suitable? I have performed separate ANOVAs with each possible covariate as the DV and my original IV as the fixed factor (supplement status). I would be looking for a non-sig difference to prove independence and hence use as a covariate? Is what I am doing right?

2) Can I use gender as a covariate given it is categorical? If not how would I go about controlling for it?

Thanks.