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I am interested in the biochemistry of diet and the way we have been advised to eat for the past forty years.

In researching an explanation for the relationship between dietary fat and blood cholesterol I came across the following: "It is clear that eating too much saturated fat increases the levels of cholesterol in the blood, especially our "bad" cholesterol which doctors refer to as LDL". This is from heartuk.org.uk.

"It is clear ..." - really?

I am mystified! I understand that dietary fat, other than very short chain fatty acids, does not immediately enter the blood stream but is transported via the lymphatic system as triglycerides packaged in protein assemblies called chylomicrons, finally entering the blood via lymph nodes under the armpits. Once in the blood the triglycerides are hydrolysed into fatty acids and transported to appropriate cells in albumin complexes.

Where does LDL feature in this transportation? And, by the way, why just saturated fats? Surely the small intestine doesn't differentiate between saturated and unsaturated fats, or does it?

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In short: It may not be saturated fat by itself but high saturated/unsaturated fat ratio and high animal/plant fat ratio in the diet that increases the risk of heart disease.

A systematic review of the effect of dietary saturated and polyunsaturated fat on heart disease (NMCD Journal, 2017)

Reducing saturated fat and replacing it with carbohydrate will not lower cardiovascular events or CVD mortality although it will reduce total mortality. Replacing saturated fat with mono- or poly-unsaturated fats or high-quality carbohydrate [from whole-grain food] will lower cardiovascular events.

Saturated Fatty Acids and Cardiovascular Disease: Replacements for Saturated Fat to Reduce Cardiovascular Risk (PubMed, 2017)

Replacing dairy fat with carbohydrates from refined starches and added sugar was not associated with increased or decreased risk of coronary heart disease (CHD), stroke or total cardiovascular disease (CVD). However, replacing dairy fat with carbohydrate from whole grains reduced the risk of CVD, CHD and stroke. Relative to other animal fats, dairy was found to have less impact on CVD. When 5% of energy from dairy fat was replaced with animal fat from non-dairy sources, risk of CHD increased by 6%. However, when 5% of energy from dairy fat was replaced with an isocaloric amount of polyunsaturated fats, risk of CHD was reduced by 26% and CVD risk was reduced by 24% [41].

A basic explanation of the underlying mechanisms:

The liver produces low-density lipoproteins (LDL) and high-density lipoproteins (HDL). Lipoproteins are composed of triglycerids, phospholipids, cholesterol and proteins.

LDL caries cholesterol from the liver to the peripherial tissue, including arterial walls, where it can accumulate and cause atherosclerosis. This is why LDL is called "bad cholesterol" (even if LDL is only a carrier for cholesterol). HDL clears excessive cholesterol from the blood and carries it to the liver for disposal, so it is called "good cholesterol. (Heart.org.uk)"

High LDL levels are associated with an increased risk of coronary heart disease (Bentham Open).

One possible mechanism by which saturated fats increase (LDL) cholesterol (PLOS):

Saturated fat is thought to increase LDL-C primarily through down-regulation of hepatic LDL receptor activity, leading to reduced clearance of LDL particles [from the blood].

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  • $\begingroup$ Why is it saturated and trans fats are bad and unsaturated cis fats are good? The only difference between the two isomers is shape: saturated and trans fats are essentially linear whereas cis fats are bent. If indeed there is a causal link between linear fats and vascular disease then surely it can only be due to their conformation? $\endgroup$ – adlibber Nov 26 '18 at 8:15
  • $\begingroup$ In medicine and nutrition, it may be enough to know what is the net result of high consumption of a certain nutrient without knowing the mechanisms. I'm sure, the exact effect of various types of fatty acids on LDL production is described in detail somewhere, but this is too much for this site, at least for me. They could know more on chemistry.stackexchange.com . The metabolism of LDL occurs in the liver, so you do not need to concentrate on what happens in the intestine and how fats come to the liver. $\endgroup$ – Jan Nov 26 '18 at 12:10
  • $\begingroup$ @adlibber Trans-fat are quite stable because the molecules are tightly packed due to trans structure. And saturated fats do not have a double bond (which makes the molecule reactive) , so they too are stable. $\endgroup$ – user237650 Nov 26 '18 at 13:01
  • $\begingroup$ Interesting point user237650. So does stability = ‘bad’? And if so why so? $\endgroup$ – adlibber Nov 26 '18 at 14:49
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    $\begingroup$ @user237650 many thanks for the link. Is Kendrick correct is the big question of course! If he is then he is in agreement with my understanding obtained elsewhere and hence my original question. The BMJ came out with this a few months ago illustrating the abject confusion on the subject. My gut (excuse the pun) tells me Kendrick is right! $\endgroup$ – adlibber Nov 26 '18 at 18:17

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