[BaO]

what is the difference between them?

[Will]

Trans fat is a type of unsaturated fat, where at least one of the double bonds is of the (E) configuration. There is more information (and more differences) on wikipedia- look at the types of fat on the right hand side menu.

I know this may seem obvious and I may sound like an idiot for taking things to literally, but trans fat is still fat!
If you're looking at food labels- trans fat is included in the fat content.

[Donaldson Tan]

nutrition-wise, differentiation of fat to saturated and unsaturated types is sufficient.

[Yggdrasil]

Not really, while trans fats are unsaturated, they hold no nutritional value because our bodies have no means of converting them into useful biosynthetic precursors (which makes sense because they are found in very small quantities in nature; significant amounts of trans fats came into our diet only when catalytic hydrogenation of vegetable oils became widespread).

In general, unsaturated fatty acids are much more useful to the body than saturated fatty acids (which makes sense from an organic chemist's point of view; it's easier to functionalize an alkene than an alkane).  For example, unsaturated fatty acids are important precursors to prostaglandins, which play important signaling roles within the body.  Because trans-fats cannot be functionalized using the same molecular machinery which functionalizes cis-unsaturated fatty acids, they do not provide the same nutritional benefits of cis-unsaturated fatty acids.

Although trans fats are structurally unsaturated fatty acids, they are functionally different than other unsaturated fatty acids and thus their classification as unsaturated fatty acids is misleading to the consumer.

[Donaldson Tan]

Because trans-fats cannot be functionalized using the same molecular machinery which functionalizes cis-unsaturated fatty acids, they do not provide the same nutritional benefits of cis-unsaturated fatty acids.

Although trans fats are structurally unsaturated fatty acids, they are functionally different than other unsaturated fatty acids and thus their classification as unsaturated fatty acids is misleading to the consumer.

In that case, are trans unsaturated fat present in significant proportion in our food?

[Yggdrasil]

It depends what you eat.  Since trans fats arise from the catalytic hydrogenation of unsaturated fatty acids, anything which contains partially hydrogenated vegetable oil will contain trans-unsaturated fatty acids.  If you look at the McDonald's website, you can see that a Double Quarter Pounder with cheese contains 3g of trans fats (280g total) and a large order of fries contains 8g of trans fat (170g total).

[Borek]

It depends what you eat.  Since trans fats arise from the catalytic hydrogenation of unsaturated fatty acids, anything which contains partially hydrogenated vegetable oil will contain trans-unsaturated fatty acids.  If you look at the McDonald's website, you can see that a Double Quarter Pounder with cheese contains 3g of trans fats (280g total) and a large order of fries contains 8g of trans fat (170g total).

The way you have wrote it it looks like it is 8 g of trans fat out of the total of 170g of fat - which is not exactly reality