With the complication of today's manufacturing and food production, it's hard to know what goes into the food we eat. Food labeling helps, but not everything is listed, and what is listed is often a mystery. Most Muslims know to look out for pork, alcohol, and gelatin. But can we eat products that contain ergocalciferol? What about glycerol stearate?
Dietary laws for Muslims are very clear. As outlined in the Qur'an, Muslims are forbidden from consuming pork, alcohol, blood, meat dedicated to false gods, etc. It is easy to avoid these basic ingredients, but what about when the ingredients are disguised as something else? Modern food production allows manufacturers to start out with one basic product, then cook it, boil it, and process it, until they can call it something else. However, if its original source was a forbidden food, then it is still forbidden to Muslims.
So how can Muslims sort through it all?
Some Muslim dieticians have published books and lists of products, from Burger King hamburgers to Kraft cheese, to indicate which things are forbidden and which are permitted. The soc.religion.islam newsgroup compiled a FAQ file using this approach back in the 1990s. But as Soundvision points out, it is nearly impossible to list every possible product. In addition, manufacturers often change their ingredients, and international manufacturers sometimes vary the ingredients from country to country. Such lists often become outdated and obsolete rather quickly.
As another approach, the Islamic Food and Nutrition Council of America has compiled a list of ingredients that is very helpful. You can use this list to check labels for items that are forbidden, permitted, or suspect. This seems to be the most reasonable approach, as the short list is not likely to change over time. With this list in hand, it can be very simple for Muslims to purify their diets and eat only what Allah has permitted.