Sample size

I need your help to define the sample size of a study.
We want to study the effect of a food on the expression of a series of microRNAs measured before and after the treatment itself.
In your opinion, what is the minimum number of patients that must we consider for the study?
Considering also the high costs of this experiment, we think that 40 patients (in very homogeneous conditions) are sufficient for analyses.. what do you think?
Thank you in advance!


Omega Contributor
How is "a series of microRNAs formatted? Tell us more about this variable.

What is you hypothesis, how big of an effect do you expect to see?
A miRNA is a small non-coding RNA molecule (containing about 22 nucleotides) found in plants, animals, and some viruses, which functions in RNA silencing and post-transcriptional regulation of gene expression.
Although the expression levels of a miRNA are counts, analysing the expression using a Real-time PCR (a laboratory technique of molecular biology), the final response variable is generally continuous and normally distributed.
We want to know if there is a change in the expression levels of one particular miRNA (for example: mir-100-5p) before and after treatment.
I don't know how big will be the effect (the null hypothesis is that the expression levels not vary before and after treatment), but Real-Time PCR is a very sensible and precise technique, that allows you to see even small changes of miRNA expression levels.