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I've heard from several sources that trans FAs are bad for you and their consumption will lead to cardiac problems, and that they are indigestible. But I also learned from biochemistry that they are digestible. What is the reason for this, or is it true at all?

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    $\begingroup$ I'll try to get to a formal response later. In the meantime, fatty acids are not bad or unhealthy for humans if taken in moderation. We need fat and cholesterol in our diets, but too much raises the risk of poorer health. Some fatty acids are more beneficial than others. $\endgroup$ Mar 28, 2012 at 13:10

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Trans fatty acids are digestible, but they cause an increase in LDL and a decrease in HDL, which is the leading mechanism for atherosclerosis. So, they increase the risk of a cardiac infarct.

LDL means Low Density Lipoprotein, it's one of the 5 major lipoprotein groups. Lipoproteins carries the fatty acids and cholesterol which were absorbed from intestines (because lipids are hydrophobic and cannot be carried within the blood directly like glucose or proteins.)

So, LDL mainly carries cholesterol and fatty acids from intestines to tissues. (fatty acids and cholesterol are essential for cells.) But in excess levels, they tend to accumulate in vessel walls, which also starts and inflammatory response and makes the situation worse. (mainly includes macrophages and T cells) This is the main mechanism for atherosclerosis.

In the other hand, HDLs serve as a reverse cholesterol transport system, which carries the cholesterol back, mostly to the liver (for bile secretion) or steroidogenic organs such as adrenals, ovary, and testes by direct and indirect pathways. So a high HDL reduces the atherosclerotic process.

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    $\begingroup$ Thanks, nice response, but could you give me some resources on why transFAs raise the LDL level more than any other fatty acids? If they don't than this is just an argument against consuming excessive amounts of fat. $\endgroup$
    – zeller
    Mar 29, 2012 at 6:15
  • $\begingroup$ No, it's not an argument just against consuming excessive amounts of fat. Trans fatty acids increase LDL more than cis unsaturated fatty acids, even equal to saturated fatty acids. ajcn.org/content/59/4/861.full.pdf+html You can check the discussion section. $\endgroup$
    – caeruleus
    Mar 29, 2012 at 17:57
  • $\begingroup$ Thanks, I'll read it more in depth on the weekend. Anyway, I'm still a bit skeptic about the topic. Do you (or anybody) have some texts on the physiological background of the phenomenon? $\endgroup$
    – zeller
    Mar 30, 2012 at 7:02
  • $\begingroup$ When you say they are digestible, do you mean that the GI tract can cut off the glycerol and make trans-fatty acids, or do you mean that cells can process trans-fatty acid into other molecules? I thought the problem was lack of enzymes that can handle the trans double bond. $\endgroup$
    – Superbest
    Nov 13, 2014 at 23:40
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Studies have been done in which saturated fats and cis-unsaturated fats in the diet were replaced with trans-fatty acids. In all cases there were increases in LDL cholesterol, blood triglycerides and Lp(a) lipoprotien, and decreases in HDL cholesterol and particle size of LDl cholesterol. All of these changes result in an increase in the risk of cardiovascular disease. Beyond that the incidence of CHD in prospective studies looking at trans fatty acid consumption has been greater than what would be predicted from the changes in blood lipid profiles. Over the past decades it has become clear that trans fatty acids increase the risk of CHD more than do saturated fats. In addition, they have been shown to increase systemic inflammation and decrease endothelial cell function. The FDA suggests that consumption of trans fats should be kept at 2 gms/ day or lower.

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