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I've been trying to learn about cholesterol, lipoproteins, and fats, but unfortunately, there is an ocean of confusing information on the internet. I don't have any solid background in biology, so this is kind of making my head spin.

I know that Low Density Lipoprotein (LDL) and High Density Lipoprotein (HDL) are carriers of cholesterol in blood. LDL carries cholesterol from the liver to the tissues while HDL takes cholesterol from the blood to the liver.

How does saturated fat increase the cholesterol level in the blood?

Some sites explain that saturated fat affects the LDL receptors on the liver, which inhibits the liver's ability to receive and break down excess cholesterol in the blood. But LDL is supposed to carry cholesterol from the liver to other tissues. So why should the liver have LDL receptors in the first place? If saturated fat affected the HDL receptors on the liver, then that would have made sense, right? But many sites say saturated fat increases LDL cholesterol in the blood.

What is it that I'm missing here?

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  • $\begingroup$ Welcome to Biology.SE! Please take the tour and then go through the help center pages starting with How to Ask questions effectively on this site and edit your question accordingly. In particular, each question should be posted separately — this improves the chances that you will get answers for each question and makes the answers more accessible for future users. ——— Please also tell us where you've looked for answers — for example by providing citations (to reliable sources) for the background you've provided. $\endgroup$
    – tyersome
    Commented Dec 24, 2021 at 19:13
  • $\begingroup$ I have found that when learning about a new area starting with a relatively accessible and reliable source like Khan Academy is very helpful. Wikipedia is also generally a good starting point and you can then check their references. Online platforms called MOOCs offer free (or very low cost) courses on a wide variety of subjects — two I am familiar with are Coursera and edX. Finally, textbooks with a good level of detail are also freely available online e.g. from NCBI. $\endgroup$
    – tyersome
    Commented Dec 24, 2021 at 19:13
  • $\begingroup$ I can’t. General normal fat metabolism involving liver, adipose and other tissues, and triglyceride synthesis and transport as lipoproteins is more than enough for me. How diet might distort that and affect cholesterol deposition in arteries is not something I know anything about, and is difficult because it is observational rather than a question of fundamental mechanistic design. Cave men didn’t suffer from arteriosclerosis. You might try asking on SE Medical Sciences. $\endgroup$
    – David
    Commented Dec 24, 2021 at 20:46

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Saturated fat does not increase serum cholesterol. I know you can find lots of studies that say otherwise, but its basically a myth that has been propagated since the 1960s, like claiming that the human genome has 24 chromosomes (yes they used to say that). All of the studies I have seen claiming such things are "observational" studies with no controls. A lot of these studies are just BS generated by nutritionists trying to justify why you shouldn't eat eggs and other crazy things.

The intestines are impervious to sterols which have to be digested into more fundamental packages such as phospholipids before they can be transported across that boundary. Phospholipids are not processed by the liver, nor are they a component of serum cholesterol. Virtually all serum cholesterol is directly synthesized in the liver from fructose. This is easily proven (and has been proven) by guinea pig studies. If you feed a guinea pig a carbohydrate free diet which is heavy in saturated fats, their serum cholesterol drops to very low levels. Conversely, if you feed a guinea pig a high-fructose diet, they will develop high serum cholesterol levels. This is strong experimental evidence that serum cholesterol is almost purely dependent on carbohydrates and on fructose in particular (a carbohydrate derivative).

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    $\begingroup$ Please add references supporting your assertions. As currently written this comes across as excessively opinionated. In addition, I'm confused by you claiming that sterols are "digested" into phospholipids — this suggests that phospholipids are a component of sterols, which is not correct. Finally, since guinea pigs are not humans, it would be great if you could indicate why those results are relevant (with citations). $\endgroup$
    – tyersome
    Commented Dec 28, 2021 at 18:25
  • $\begingroup$ @tyersome onlinelibrary.wiley.com/doi/full/10.1111/eci.12161 To claim that guinea pigs metabolize dietary cholesterol in a different way than humans or other mammals I think is a specious argument. $\endgroup$ Commented Dec 28, 2021 at 18:49
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    $\begingroup$ I think if you were to reread my comment you will see I made no such claim — I am aware that there is evidence supporting guinea pigs being a good model for human cholesterol metabolism, but that is not known to everyone. I'm trying to make constructive suggestions — please respond by making edits to your post rather than posting links in the comments. $\endgroup$
    – tyersome
    Commented Dec 28, 2021 at 19:25
  • $\begingroup$ your answer is contrary to the most reliable hospitals, medical universities and dietary associations in the world such as the harvard heath school, mayo clinic and the American Heart Association. But your criticism of observational studies as evidence is spot on, a lot of laymen have been misguided by these studies' findings $\endgroup$
    – Hisham
    Commented Jul 13, 2022 at 0:53
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I think I understand the picture somewhat now. The liver synthesizes both VLDL and cholesterol. Cholesterol is packed (along with triglycerides) in VLDL and send to tissues through the blood. As tissues absorb triglycerides from the VLDL, it changes to IDL and then to LDL. LDL continues to transport cholesterol to tissues across the body. But when there is excess LDL cholesterol in the blood, it comes back and is received by the LDL receptors in the liver. Meanwhile, the liver also synthesizes another lipoprotein called HDL. HDL travels to the cells across the body and receive excess cholesterol from the peripheral tissues, and take it back to the liver for breaking it down.

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    $\begingroup$ Welcome Curiouser. Whilst you may be correct (personally, I'm not familiar with this), your answer could be improved by references to a reliable source which confirms your explanation. Answers without references risk being downvoted or deleted in extreme cases. $\endgroup$ Commented Dec 28, 2021 at 11:46

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