> Results are:
> Low expression - 50 pts are not obese and 12 pts are obese
> High expression - 40 pts are not obese and 23 pts are obese.
>
> so the p value is 0.026.
> so does ths mean the people with a low expression are MORE likely to be non-obese than those with a high expression? or how do you describe it?
Yes.
What you have found is that there is a very low probability that in the population from which your sample was drawn, whether or not someone is obese is unrelated to tumour marker. So you reject the possibility of no association. Which means there must be an association.
And the association is as you've described (low expression more rather than less likely to be non-obese). The probability of the actual pattern (in the population) being the opposite of what you observed in your sample is even less than p = .026.
Whether or not you can infer cause depends on theory, study design and so forth. It's nothing to do with the fact that chi square is a "test of association".