I'll do one final reply--I think I've spent too much time as it is already.
I am guessing that a lot of the confusion originates from understanding of the method used. A partial spearman correlation is a rank-based (not value-based) correlation which effectively regresses out the effects of other measured variables to try to quantify the
direct influence of one variable on another as much as possible. Unlike other statistical comparisons which might be affected by these confounders (like the tests for between-group abundance differences), it is looking at covariance. This has the following benefits:
1. Confounders which change the abundance of any of the metabolites will not, on their own, change the number of significant partial correlations. The reason is precisely because of the following point, which I also stated previously:
But not if the supplemented progesterone results in slightly different metabolites. Which it can (see my post upthread).
Progesterone (and any other hormone which can be supplemented) is readily converted into other metabolites. So if what you're assessing is the extent to which levels of A directly correlate with levels of B in a relationship where A--> B, having some people with higher levels of A from contraceptives doesn't obscure that relationship. You
still have a situation where the samples with high levels of A will also have high levels of B, and the samples with low levels of A will also have low levels of B. The fact that some of those high levels of A will be due to contraceptives does not change the [edit: downstream] relationship that is being studied here.
[edit: you’d still expect some number of downstream relationships to hold even if the relationship between progesterone and its own upstream metabolite are confounded.]
2. The regulatory effects that certain metabolites will exert on enzymes in this pathway are still going to be the
same regulatory effect regardless of relative abundance. So if metabolite D downregulates the enzyme which mediates A --> B, you're still going to see that correlation between A and D regardless. In a rank-based intra-group correlation, if D has a strong negative influence on A-->B, the samples with high levels of D (even through contraceptive use) are going to have low levels of B, and the samples with lower levels of D are going to have higher levels of B, regressing out the influence of A.
To that point:
Is it possible that a contraceptive changing one hormone level might lead to downregulation or upregulation of these core enzymes so that many more hormones are affected than just progesterone?
Only a few of the hormone intermediaries are bioactive. Yes, supplementing some of those bioactive hormones would probably lead to upregulation or downregulation of some of those enzymes which might affect something at a different point in the pathway. Which is what the "partial" part of partial spearman correlations aims to address--it is assessing the strength of the relationship between A-->B while accounting for the levels of D, E, F, etc. If A-->B is so strongly influenced by D that the relative abundance of A doesn't strongly correlate with the relative abundance of B, then the A--> B relationship is not significant but the D-->B relationship is.
That's what the 52 significant relationships in the control group tells us--that even with the interrelatedness of the system and the general influence of outside factors, there are still a bunch of strong direct relationships in the healthy group. The fact that nearly nothing could be picked up in the ME/CFS group speaks to a strong influence beyond what can be attributed to the confounders that were brought up in this thread.
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So the takeaway is, as I said before, the way that the network analysis was done makes it pretty robust to those particular confounders.
I think a larger sample size would probably make the analysis more robust to even the very slim possibility that the control group happened to be extremely homogenous in terms of age
and time that the samples were taken
and contraceptive use
and diet
and ACTH levels
and any of the huge number of additional confounders that cannot be adequately controlled for (but can be expected to zero each other out in a large enough sample size). The focus on contraceptives and hormone cycles may be because they are the most obvious confounders for absolute hormone levels and ratios, but I'd caution against extending that assumption to the network analysis.
And I do agree that the text does make an inadequately supported jump to HPA axis function and whether this dysregulation would actually be central to the pathophysiology of ME/CFS. Could the analysis have taken into account a few more variables and been more careful in the interpretation? Sure. Can the primary finding be explained away by the confounders listed in this thread? I don't think it actually can.