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?
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.
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.
You are right the biology text books claim that trans-“unsaturated” fatty acids can be metabolized. Note : - The text books make it clear that “Trans” means the fatty acid is unsaturated and can be monounsaturated or polyunsaturated.
Trans fatty acids are generally being blamed so as to keep to the unfortunate false “theory” that certain poly unsaturated fatty acids are essential for humans. This false “theory” is the basis for the epidemic of obesity and most of the strange and lifestyle diseases. The actual culprits of the epidemic of lifestyle diseases is the high levels of polyunsaturated fatty acids (both trans and cis) we consume through the processed unsaturated vegetable fats.
Check from your text books, atherosclerotic plaque is made up of oxidised polyunsaturated fatty acids and cholesterol (which has been labelled LDL cholesterol).
Prostaglandins, which are produced in the body by polyunsaturated fatty acids, have been identified by many different research studies as the molecules underlying diseases and ailments such as diabetes, asthma, cancers, dysmenorrhea (menstrual pains), baldness, cramps and muscle pull, migraine, glaucoma, delivery and labour problems, etc.
Aspirin according to the research results, reduces the chances of cancer occurrence or dying early from cancer. Aspirin stops the production of prostaglandins in the body.