Where do you find trans fatty acids?

Widespread in the diet. Main source is from commercially hydrogenated vegetable oils - used in formulating margarines, shortenings, salad and cooking oils. Also from animal fats (beef, lamb) and milk fat.

The hydrogenation process

Hydrogen atoms added to double bonds of fatty acid using very high temperatures and a metal catalyst (e.g. nickel). Main fatty acids in vegetable oils that are hydrogenated are the 18 carbon, oleic, linoleic and linolenic acids (N.B. all have = 1 double bonds). Some shifting of double bonds occurs and many cis double bonds get converted to trans double bonds.
Partial hydrogenation produces a mixture of cis and trans isomers. Complete hydrogenation produces the fully saturated 18C stearic acid. During partial hydrogenation of polyunsaturated oils there is isomerization and migration of double bonds. This results in a distribution of cis and trans double bonds. Thus products containing trans fatty acids invariably have a distribution of isomers which will depend on the conditions employed during the hydrogenation process. A major tFA isomer is - elaidic acid (t9 - 18:1).
Dairy products and meats have appreciable levels of t9 - 16:1 and t11 - 18:1 (vaccenic acid).

Why hydrogenate?

This is done typically to improve plasticity, increase the melting range and improve flavor stability.

Estimates of trans fatty acid consumption

Average per capita consumption
8 g/person/day or 6% of total US fat consumption (Hunter & Applewhite, 1991).
Upto 27 g/person/day or 24% of total fat intake in certain groups (Enig et al, 1990).
9.6 g trans fatty acids consumed in a 1800 calorie diet - or 5% of total fat intake (Litin and Sacks 1993)
3-13 g/person/day (Craig-Schmidt, 2001).
7 g trans fatty acids /day Scottish diet - some cases upto 48 g/day (Bolton-Smith et al, 1995).

Food sources of trans fatty acids

Stick margarines ~ 3 g/serving
Vegetable shortenings ~ 2.5 g/serving
Milk ~ 0.2 g/serving
Butter ~ 0.4 g/serving
Meats ~ 0.1 g/serving
Main contributors - pastries, fried foods (doughnuts, french fries), dairy products and meats

Check trans fatty acid content of foods in USDA data-base and trans fatty acid data-base

Health effects of dietary trans fatty acids

Starting with a clinical report from the Netherlands in 1990, and an epidemiological study from the United States in 1993, there is now strong scientific evidence to support the notion that trans fatty acid (tFA) consumption is detrimental to human health. The accumulated evidence on the effects of tFA consumption on blood LDL ("bad" cholesterol) and HDL ("good" cholesterol) levels, is consistent with the observations associating tFA consumption with increased risk of premature heart disease. Since the initial reports, many clinical studies as well as the results from several epidemiological studies have led various health agencies and governments to impose restrictions on tFA, as mounting evidence shows that they may be even worse than saturated fatty acids with regards to their effects on blood cholesterol levels (specifically the ratio of LDL to HDL cholesterol). Recently, it has been reported that tFA consumption is positively associated with markers of systemic inflammation. In addition tFA has been reported to be an independent predictor of diabetes.