By a "natural" experiment you mean that you do not control, by randomization, say, the amount of chemical X that each subject takes. This is also often called an observational study. Do you know the difficulties in drawing conclusions about cause and effect from such data?
It's really not a question about statistical tests - they are blind to causality. You can use standard methods to test if intake of chemical X is associated with weight loss correcting for gender if you like, but that does not in itself prove that intake of X causes weight loss. An association might be viewed as evidence in the direction of a causal effect, but how strong the evidence is, and how serious it will be taken, is much more a question of understanding the subject matter than the statistical test.
There is a literature on causal inference, which I am only superficially familiar with, which gives a more nuanced picture on what you can say about causal effects and how you can do it, but a basic premise is a set of untestable assumptions.
If you can provide more details about what you know and what you want, I might be able to give you some appropriate references. There are also other, related, questions with answers here and here.