Liz Neporent is a diet and fitness expert and author of 12 fitness bestsellers. She regularly appears on national TV programs and is the president of Wellness 360, a New-York based wellness provider.

Last week, I referenced a popular energy bar that touted net zero carbs, even though the whole concept of net carbs is a fiction invented by marketing departments to sell more product. While we're on the topic, the real question you need to ask is: "Does that tasty 'engineered food' actually contain the 220 calories it claims on the label?" This isn't a question you might necessarily think to ask. Most of us assume that Nutrition Facts labels -- required by law on all processed, packaged food products and relied on by millions to make healthy food choices -- reflect reality.

Turns out, many of them may not. In a test conducted by the Florida Department of Agriculture and Consumer Services laboratory, 75 percent of diet products tested -- most of them regional brands -- were shown to have erroneous information on their labels. More than 10 percent of all bakery products and candies tested were mislabeled, as were 25 percent of dressings and condiments. Some of the labels were off-the-charts misleading, like a vanilla éclair said to have 2 grams of fat but actually containing 17 grams. An entire line of "sugar-free" baked goods -- more than 20 products -- from a regional company were found to contain sugar, as many as 16 grams per serving. (The baker admitted he added sugar to give the chocolate products more flavor and to enable the cookie dough to brown better. Well, duh.)

Tests commissioned by the Center for Science in the Public Interest, a nonprofit consumer organization, found that Laura's Lean-brand steaks contained, on average, more than twice as much fat and saturated fat and 40 percent more calories than the company stated. Laura's strip steaks were certified to use the American Heart Association's (AHA) "heart-check" logo on their labels, yet all but one of the 14 strip steaks tested failed to meet the AHA's guidelines, which require a serving of meat to contain less than 5 grams of total fat and less than 2 grams of saturated fat. The steaks were later, um, stripped of their certification.

Although smaller, regional brands tend to be the worst label fake offenders, even national corporations have, on occasion, been caught duping the public. Not long ago, CSPI busted McDonald's for low-balling numbers for its vanilla reduced-fat ice cream. In this case, the inaccuracy was related to serving sizes. The McDonald's Web site only offered nutrition information for a 90-gram ice cream cone, purported to have 150 calories and 3 grams of saturated fat. But the smallest cone that CSPI researchers found in the Washington, D.C. area exceeded McDonald's official size by 49 percent and contained an average of 225 calories and 4.5 grams of saturated fat. Most states don't do food-product testing, the federal government rarely makes random checks and organizations like CSPI can only do so much. So what's a consumer to do?

Your best bet is to assume a modest overestimation of calories and fat grams and make your choices accordingly. Keep in mind that labels that "pass" the accuracy test may not be entirely on target. Federal law allows most products a 20 percent variance from the label; in other words, a bagel that claims to contain 200 calories legally can contain 240 calories. In addition, the government allows for a 10 percent margin of error in testing, so some labels must be as much as 30 percent off without being considered misleading. Not surprisingly, manufacturers never seem to err on the side of overestimating calories and fat.

Have you ever suspected your devotion to a low-fat, low-cal food caused a surprise attack of chubby thighs? Are there any food labels you think are just plain telling lies? I'd like to hear what you think.