I'm not sure what you are asking. Are you trying to estimate the number of obese and overweight people you will need to isolate stem cells from for study?

Yes. I can not estimate the number of obese and overweight subjects I need to isolate stem cell from for studying some metabolic aspects. No other research has studied these factors in human stem cells so I have no Standard Deviations to calculate the number of subjects. A colleague has suggested that I plan a pilot study to determine the SDs and then estimate the whole study sample size. do you have any other suggestions?

I think I'd be inclined to do a pilot study.

Alternatively, just because they're stem cells does that mean they are so different from other cells that you can't extrapolate a little from other cell lines, or from stem cells from animals?

Also have you tried looking in the metabolite database at HMDB.ca? They usually deal in body fluids, but there are some cell cytoplasm values.

I agree with Terri Grassby. Can you do a pilot study? That would give you some idea of the parameters you'd need to estimate a population size for your study. Perhaps there is a similar cell line that you could extrapolate from as Terry suggests. I'm not sure what factors you are evaluating but you might even try extrapolating from human studies, ie how many patients were used? how large was the difference? standard deviation? Do you know the parameters for non-stem cells in obese and overweight people? I think you might be able to use those as a rough estimate. But overall I think the pilot study is the best approach