Health effects of partial hydrogenation of vegetable oils
Partially hydrogenated vegetable oils contain trans fatty acids which are different from the natural fatty acids in vegetable oils and animal fat . Fatty acids in partially hydrogenated vegetable oils are 14 cis and trans isomers of octadecenoic and octadecadienoic acids that are formed during hydrogenation. Trans fatty acids cause inflammation and calcification of arterial walls. They also inhibit cyclooxygenase needed for conversion of arachidonic acid to prostacyclin, a compound beneficial for the vascular tree. There are reformulations of hydrogenated fat containing the essential fatty acid linoleic acid which gets converted to arachidonic acid. When trans fat content goes up and linoleic acid content goes down, mortality rate rises as per epidemiological data .
Hydrogenation of vegetable oils were innovated for increasing the shelf life. It is common knowledge that vegetable oils can become rancid on storage. This is tided over by hydrogenation and hence the shelf life increases. But trans fatty acids seen in partially hydrogenated vegetable oils have been shown to damage pancreatic beta cells and increase insulin resistance in animal studies. Trans fatty acids cannot be fully eliminated from diet as they are also present in meat and diary products of ruminant animals [2,3]. Industrial trans fatty acids promote inflammation whereas cis-unsaturated fatty acids are protective against inflammation. Industrial fatty acids also promote fat storage in the liver compared with cis-unsaturated fatty acids and saturated fatty acids. In vitro studies have shown that industrial trans fatty acids, but not cis-unsaturated fatty acids or saturated fatty acids stimulate the cholesterol synthesis pathway by activating sterol regulatory element binding protein (SREBP) 2 mediated gene regulation .
A meta-analysis checked effects of replacement of partially hydrogenated vegetable oils with alternative fats and oils on coronary heart disease. 1% replacement of trans fatty acids, with saturated fatty acid, mono unsaturated fatty acids or polyunsaturated fatty acids decreased total cholesterol/high density lipoprotein cholesterol ratio by 0.31, 0.54 and 0.67 respectively. The replacement with animal fats and vegetable oils could reduce the estimated risk by 65-80% and 50% respectively .
In another systematic review and meta-analysis, total trans fat intake was associated with all cause mortality (34% increase), coronary heart disease mortality (28% increase) and total coronary heart disease (21% increase), but not to ischemic stroke or type 2 diabetes mellitus. This was contributed mainly by industrial but not ruminant trans fat as industrial trans fat was associated with coronary heart disease mortality and coronary heart disease. Ruminant trans-palmitoleic acid was inversely associated with type 2 diabetes mellitus . Industrial trans fat is produced by partial hydrogenation of vegetable oils in the presence metal catalyst, vacuum and high heat. Ruminant animals biohydrogenate unsaturated fatty acid via bacterial enzymes . But ruminant derived trans fat consumption is relatively low in most populations.
Another review mentioned that 2% absolute increase in energy intake from trans fat has been associated with 23% increase in cardiovascular risk. They can increase blood levels of low density lipoprotein cholesterol .