False dichotomies: serum cholesterol level vs all-cause mortality. Cause or effect?

Here are some plots from the MRFIT study.

From http://sph.bu.edu/otlt/MPH-Modules/PH/PH709_Heart/PH709_Heart5.html

Although the relative risk (RR) for coronary heart disease (CHD) and cardiovascular disease (CVD) mortalities increase with serum total cholesterol (TC) level, all-cause mortality follows a U-curve.

According to Low Serum Cholesterol and Mortality: Which Is the Cause and Which Is the Effect?, certain illnesses that increase mortality lower TC levels. This is the Iribarren hypothesis.

According to Cholesterol and all-cause mortality in elderly people from the Honolulu Heart Program: a cohort study, TC that's low and is still low 20 years later results in a 64% increase in the RR for mortality relative to TC that's intermediate and is still intermediate 20 years later.

Table 4 Relative risk for mortality based on change in cholesterol between examinations three and four

Is low TC level the cause of, or the effect of fatal illnesses? I think that it's both. Cholesterol is an important substance, as a severe lack of it is bad news, as per Smith–Lemli–Opitz syndrome. If certain illnesses result in a depletion of cholesterol and cholesterol synthesis is too low, there's insufficient cholesterol to allow recovery.

Interestingly, TC that's low but is intermediate 20 years later results in a 30% increase in the RR for mortality, whereas TC that's low but is high 20 years later results in a 5% increase in the RR for mortality.

P.S. There's a false dichotomy for vitamin D level vs illness. Ditto for carbohydrates vs calories.