Comment Re:Is there a safe amount of air to breathe? (Score 1) 164
And it's a meta-analysis paper, according to the description, and they described the correlation as somewhat questionable. I automatically assume that meta-analysis papers are going to be weak.
Nature MedicineArticle https://doi.org/10.1038/s41591... studies adjusted their effect size measure for age and sex. All studies except one adjusted for smoking. Other common adjustment variables included energy intake (n=13)28,30–35,37,38,40–42, alcohol consumption (n=12)27–30,32,33,36–38,40–42 and BMI (n=14) 27–30,32–36,38–42.
So not all of the original studies adjusted for income.
These study-level covariates included length of follow-up period (10years and >10years), precision of the exposure and outcome definitions, study design (that is, RCT or prospective cohort study), reported measure of association (RRs or ORs), outcome measures (incidence or mortality), number of exposure measurements (single or repeat), method by which outcomes were ascertained (administrative records, self-reports, biomarkers or physician diagnosis) and level of adjustment for relevant confounders (for example, age, sex, smok-ing, education, income, calorie intake, BMI, physical activity, alcohol intake, saturated fat intake and other dietary factors). We adjusted for these covariates in our meta-regression if they significantly biased our estimated RR function.
So basically, it sounds like nowhere near all studies adjusted for income, and they think they took that into account, but because this is a meta-analysis, there's a certain degree of garbage-in-garbage-out involved. The only way to really be sure is to exclude studies that don't adjust for everything you care about.
Also, because this is a meta-analysis, the papers you exclude are also kind of important.
Reports Excluded:
Duplicates n=5
Not study design of interest n=39
Not outcome of interest n=45
Not outcome of interest n=54
Not measure of interest n=2
I'm not sure why "not outcome of interest" excluded both 45 and 54 papers, but that sort of discrepancy raises some red flags, particularly when there are only 16 included studies.
But the real red flag for me is the confidence interval. If I'm understanding this correctly, without compensating for heterogeneity, the effect on colorectal cancer and heart disease are statistically indistinguishable from zero. This intuitively feels like the sort of study where after a few more studies, you'll see regression to the mean.
And type 2 diabetes tends to be strongly correlated with obesity, and there's no mention of the original studies having adjusted for that. If obese people are more likely to eat processed meat because of it being a quick way to get the calories that they need, then it is also possible that the correlation with type 2 diabetes is entirely spurious.
I'm not seeing a whole lot of actual evidence to go from "we combined a bunch of studies with weak-to-zero correlation and got weak-to-zero correlation" to "eating processed meat likely causes an increase in these conditions".