One of the lessons every school child learns is that you shouldn't believe everything you read. This applies doubly to the Internet, where anybody can play expert and put up definitive-sounding information at Web sites, blogs, discussion groups, and elsewhere.

I just engaged in an exercise that underscores how you also can't be complacent with information at highly reputable Web sites either.

The background: In recent years I've often seen partially hydrogenated oil in the ingredients list of many commercial food products, yet in the nutrition facts box the trans fat content indicated is 0 grams.

Trans fat is a favorite target of public health professionals because it has been implicated in a number of serious and widespread illnesses. Trans fat raises LDL ("bad") cholesterol and lowers HDL ("good") cholesterol, each of which increases the risk of heart disease, the leading killer of men and women in the U.S., according to the Mayo Clinic (www.mayoclinic.com) and other sources.

Trans fat also promotes inflammation, resistance to insulin, and obesity, according to The Nutrition Source, a Web site maintained by Harvard University (www.hsph.harvard.edu/nutritionsource) and other sources. Along with increasing the chances of developing heart disease, partially hydrogenated oils also increase the frequency of stroke and diabetes, according to the American Heart Association (www.heart.org).

Food manufacturers use partially hydrogenated soybean, cottonseed, palm kernel, and other vegetable oils because they extend a product's shelf life. According to the 2004 book Trans Fats: The Hidden Killer in Our Food by Judith Shaw, "Foods made with partially hydrogenated vegetable oils have an indeterminate storage time or shelf life. Sometimes it's years."

Partial hydrogenation is a food processing technique that turns a relatively healthful liquid vegetable oil into an unhealthy semi-solid fat having the consistency of lard or butter. Partially hydrogenated vegetable oils have been used by food manufacturers beginning in the early 20th century, replacing lard and butter, because they're less expensive and because people previously thought they were more healthful.

The Food and Drug Administration lets food manufacturers list the content of any ingredient, including trans fat, at 0 grams if a product has less than 0.5 grams per serving of that ingredient. But how can a product, for instance, that uses partially hydrogenated oil as its main fat list its total fat content per serving at 7 grams and at the same time list its trans fat content at 0 grams?

The answer is that partially hydrogenated vegetable oil isn't the same as trans fat. This is despite the fact that reputable Web sites such as those of the American Heart Association and the Mayo Clinic describe partially hydrogenated vegetable oil as being the same as trans fat. Further, some other Web sites and some magazines and newspapers have copied this misinformation.

I discovered this by interviewing two food scientists teaching at major universities, Eric Decker, professor and department head, Department of Food Science, University of Massachusetts, and Susan Berkow, adjunct professor, Global Community Health, George Mason University.

Both told me that trans fat is only one of the fats that results from the partial hydrogenation of vegetable oil and that other fats, depending on the specific process used, can include saturated fat, monounsaturated fat, and polyunsaturated fat. Decker said that partially hydrogenated oil is typically only about 10 percent trans fat.

I then contacted the American Heart Association and the Mayo Clinic. I heard back from a nutritional scientist or dietitian representing each. Both were eager to help, but they indicated in so many words that their organizations were trying to simplify the point for consumers that they should beware of partially hydrogenated vegetable oils because of their trans fat content.

The larger truth each pointed out was that even relatively small amounts of trans fat in individual servings can add up to unhealthy levels over the course of a day. But the fact remains that partially hydrogenated vegetable oil is not the same as trans fat.

The lesson: Double- or triple-check anything you come across that's both surprising and important. One term that's sometimes used for this is the "principle of triangulation of data." Just as a triangle has three angles, confirming information with three different sources is more reliable than relying on one.

Even this isn't foolproof, as the above case illustrates, when many sources copy information from what they regard as a reliable source. Critical thinking can often get to the bottom of things.

You shouldn't be cynical about information gleaned from the Internet. But a healthy dose of skepticism can only help.