Can someone tl;dr? Not that the paper is too long, but I think I'd need to go back to university and major in biology or statistics to understand what they discovered and more importantly what it might mean.
When you have TC (total cholesterol) < 200 mg/dL (milligrams per deciliter) - each 1mmol/L (millimoles per liter) reduces mortality.
When you have TC > 200 mg/dL - each 1mmol/L increases mortality.
The confidence interval was high.
> In the age groups of 18–34, 35–44, 45–54, 55–64, 65–74, and 75–99 years, each 1 mmol/L higher TC increased mortality by 14%, 13%, 8%, 7%, 6%, and 3%, respectively (P < 0.001 for each age group), for TC ≥ 200 mg/dL, while the corresponding TC changes decreased mortality by 13%, 27%, 34%, 31%, 20%, and 13%, respectively, in the range < 200 mg/dL (P < 0.001 for each age group). TC had U-curve associations with mortality in each age-sex group. TC levels associated with lowest mortality were 210–249 mg/dL, except for men aged 18–34 years (180–219 mg/dL) and women aged 18–34 years (160–199 mg/dL) and 35–44 years (180–219 mg/dL). The inverse associations for TC < 200 mg/dL were stronger than the positive associations in the upper range.
Speculation in the discussion section is that while lower TC (below the limits cited above) is correlated with lower cardiac risk, it seems to be correlated with a higher risk of death. So the implication is there's something else bad for your health about being out of the sweet spot they observed in the data.
Imagine you have container ships named HDL and it is loaded with containers called triglycerides. You don't know how many containers you have. You just know you have this many container ships. Do you have too many containers? Do you have too many ships? That question cannot be answered with just one absolute number.
This is why the HDL to triglycerides ratio exists.
Implies a causal relationship. In reality, it's much more likely that causality points the other way, and an underlying disease like cancer is causing low cholesterol: https://www.ahajournals.org/doi/10.1161/01.cir.92.9.2396
Is it correct that 230 mg/dl is 5.6mmol/l? We've always been told that we must keep the number below 5 at all costs for health, but this seems to be as bad as being above 6.3. Am I reading this correctly? If so, current medical advice is way off.
TLDR: this observational study replicated the known phenomenon that (seemingly paradoxically) low cholesterol is correlated with increased risk of mortality. It's an ongoing debate whether low cholesterol is directly bad for you, or it could itself be a marker of underlying disease like cancer. The authors admit this study is unable to shed further light on that debate due to lack of data on specific cause of death.
When you have TC > 200 mg/dL - each 1mmol/L increases mortality.
The confidence interval was high.
> In the age groups of 18–34, 35–44, 45–54, 55–64, 65–74, and 75–99 years, each 1 mmol/L higher TC increased mortality by 14%, 13%, 8%, 7%, 6%, and 3%, respectively (P < 0.001 for each age group), for TC ≥ 200 mg/dL, while the corresponding TC changes decreased mortality by 13%, 27%, 34%, 31%, 20%, and 13%, respectively, in the range < 200 mg/dL (P < 0.001 for each age group). TC had U-curve associations with mortality in each age-sex group. TC levels associated with lowest mortality were 210–249 mg/dL, except for men aged 18–34 years (180–219 mg/dL) and women aged 18–34 years (160–199 mg/dL) and 35–44 years (180–219 mg/dL). The inverse associations for TC < 200 mg/dL were stronger than the positive associations in the upper range.