From the folks at one of my favorite and most trused websites, WebTV
Understanding Trans Fats

Trans fats seemed like such a good thing once, enhancing the flavor, texture, and shelf life of many processed foods -- from cookies to frozen pizza. Unfortunately, they come with a health risk.
Trans fatty foods tantalize your taste buds, then travel through your digestive system to your arteries, where they turn to sludge. Small amounts of trans fats occur naturally in beef, lamb, and full-fat dairy products. But most come from processing liquid vegetable oil to become solid fat.
Recommended Limits of Trans Fats
Like saturated fats, trans fats raise LDL "bad" cholesterol and increase the risk of heart disease. But unlike saturated fats, trans fats lower HDL "good" cholesterol and may do more damage, says the American Heart Association (AHA). The AHA advises limiting saturated fat consumption to less than 7% of daily calories and trans fat consumption to less than 1%. Given that a gram of fat has 9 calories, the following are the recommended trans fat limits based on calorie intake:
Total calories 1% of total calories = Trans fat limit
2,000 20 About 2 grams
1,500 15 About 1.5 grams
1,200 12 Slightly more than 1 gram
Trans Fats and Foods to Watch Out For
The FDA label ruling and consumer awareness of the dangers of trans fats have led many food manufacturers to reformulate products to reduce or eliminate trans fats. Today you can buy cookies and soft-spread margarine with zero trans fats. But trans fats still exist in some products.
Carefully read nutrition labels on foods in these categories. Chose brands that don't use trans fats and are low in saturated fat in these products:
cookies, crackers, cakes, muffins, pie crusts, pizza dough, and breads such as hamburger buns
some stick margarine and vegetable shortening
pre-mixed cake mixes, pancake mixes, and chocolate drink mixes
fried foods, including donuts, French fries, chicken nuggets, and hard taco shells
snack foods, including chips, candy, and packaged or microwave popcorn
frozen dinners
The Meaning of Zero Trans Fat
Reach for the product whose label shouts "0 Trans Fats!" and what do you get? Maybe some trans fats. That's because the FDA allows that label on anything with 0.5 grams of trans fat per serving.
As a result, keep in mind this advice:
1. Even if you're a conscientious shopper, it's easy to ingest a significant amount of trans fats without knowing it. A bowl of "trans-fat-free" cereal (that actually contains half a gram) plus a slice of birthday cake at the office and some microwave popcorn in the evening add up quickly.
2. Get in the habit of reading nutrition labels, the ones headed "Nutrition Facts." Look at all the fats listed there. Keep in mind that saturated fat is also unhealthy.
If the label lists Trans Fat as 0 g, look at the Ingredients List for the words "partially hydrogenated." Any oil that is partially hydrogenated is a trans fat. So a single serving of cookies could have as much as a half gram of trans fat and be labeled "0 TransFats." Be aware, too, that often a "single serving" is often less than an average person would eat.
Bottom line: When choosing foods with "0 grams trans fats," evaluate the total fat content including the amount of saturated fat.
Choose foods that have the least amount of saturated fat and that use healthy fats such as canola oil in the product.
Here are some examples from the Nutrition Facts on food labels:
Food Trans fats in a single serving
Cake mix 0.5 g
Frozen chicken and noodles 0.5 g
Blueberry muffin mix 1.5 g
Refrigerated crescent rolls 1.5 g
Stick margarine (1T) 1.5 g
Frozen beef pot pie 2 g
Microwave popcorn 6 g
The following are some examples of foods that list 0 g trans fats but contain partially hydrogenated oils, such as soybean or cottonseed oil:
Corn muffin mix
Pizza
Stoned wheat thin crackers
Cookies, including some cartoon-licensed brands
The Costs of Trans Fat-Free Products
Budget-conscious shoppers might be tempted to buy the cheapest brand of pastry, pot pie, or microwave popcorn. But don't make that decision at the expense of nutrition. Reformulating foods to reduce or eliminate trans fats costs manufacturers money. Some "0 trans fats" foods may cost more, although not all do. Again, be sure to read the nutrition label carefully so you know if you're buying a healthier version of the snack, cookie, cracker, or cake.
There's also concern that some food processors will remove trans fats only to substitute low-cost saturated fats -- another contributor to heart disease. But a 2006 marketplace survey published in the Journal of the American Dietetic Association showed that had not occurred except in one category: microwave popcorn.