We're all familiar with the standard Nutrition Facts label that appears on all packaged foods sold in this country. The label is mandated by the U.S. Food and Drug Administration (FDA) in accordance with the 1990 Nutrition Labeling and Education Act (NLEA) and requires that values for specific nutrients be reported in a standardized format. This law was intended to provide clearer, more transparent information to consumers, and enable them to better compare the nutritional merits of products.

Unfortunately, Nutrition Facts labels are not always factual. For starters, the law allows a pretty lax margin of error—up to 20 percent—for the stated value versus actual value of nutrients. In reality, that means a 100-calorie pack could, theoretically, contain up to 120 calories and still not be violating the law. The same margin of error goes for other nutrients as well, which doesn't bode well for diabetic carb counters, folks with high blood pressure who are watching sodium intake, or moms looking to boost the iron content of their babies' diets. The FDA has never established a systematic, random label-auditing process, and compliance with the law is expected to be self-enforced by food manufacturers.

A 2008 report by the Government Accountability Office (GAO) found that, within a small sample of 300 Nutrition Facts labels that were randomly audited in the mid-1990s, values for the vast majority (roughly 90 percent) of nutrients reported fell within the acceptable 20 percent variance to actual levels. But the most problematic nutrients were iron and Vitamin A.

The GAO's report stated that one third of the audited labels were unacceptably inaccurate with regard to iron content. And now a decade after that audit, I routinely find erroneous overstatements of iron content that wouldn't have passed a sanity check by a first-year nutrition student. An organic brand of 3.5 ounce baby food pouches I've purchased sells two fruit and veggie purees claiming to have 45 percent and 120 (!) percent of the daily value for iron, respectively. Absurd! Separately, an organic brand of plain instant oatmeal (ingredients: oats, salt) lists 20 percent of the daily iron value per envelope—or, the same as a 4 ounce piece of steak. And oddly, the label of a leading brand of cereal states that the product's iron content jumps from 10 to 15 percent once you add milk. (Did I mention that milk contains no iron?) Here's a rule of thumb: unless a packaged, non-meat food is iron-fortified—in which case, the word "iron" would appear in the ingredient list—any label's claim to have more than 10 percent of the daily value for iron per serving should be viewed with great suspicion.

In some cases, stated levels for vitamins appear erroneously low. According to the GAO report, almost half of all items analyzed had reported Vitamin A levels outside the permitted 20 percent variance. Indeed, my own experience is consistent with this finding. A package of dried apricots I bought last week listed its Vitamin A value at 2 percent per ounce; the proper value should be 20 percent. And a supermarket brand's package of freeze-dried mangos, made from "10 ounces of perfectly ripe, fresh cut mango," lists its daily Vitamin A value at 0 percent. That's curious, since that quantity of fresh mangoes would contain 60 percent of the daily value. Where did the vitamins disappear to?!? As a general rule: snacks or foods containing a hefty dose of dark, leafy greens or orange fruits or vegetables—carrots, pumpkin, butternut squash, sweet potato, apricot or mango—should also have a hefty dose of Vitamin A, which stands up quite well to the rigors of food processing. But low Vitamin A numbers on labels of foods like dried-spinach fettuccine or spinach wraps/tortillas are likely accurate; the negligible amount of powdered spinach added to these foods are generally to provide color and a healthy halo, not actual nutrition.

Knowing how flawed nutrition labels often are, it's tempting to just ignore them altogether. However, a 2010 study published in the Journal of Consumer Affairs suggests that this instinct might be a mistake. Among 3,700 people ages 37 to 50 who were trying to control their weight, those who read nutrition labels (but did not exercise) were more likely to lose weight than those who did not read labels but did exercise. In other words, the awareness of a food's (approximate) nutritional content and portion size does appear to influence eating behaviors in a beneficial way.

The best guidance I can offer is to minimize the number of foods in your diet that even have a nutrition label; eating more whole, unpackaged, minimally-processed foods is likely to bump up the quality of your diet and minimize the risk of hidden calories. There are numerous, credible lists of the most nutrient dense foods available—some examples include foods that score high on Dr. Joel Fuhrman's Aggregate Nutrient Density Index ("ANDI") or those found on the George Mateljan Foundation's "World's Healthiest Foods" list (www.whfoods.org). If your diet contains multiple foods from these lists on a daily basis, you're likely to be well-covered in the vitamin and mineral department. For the most accurate nutrition information on foods you eat regularly—both unpackaged and packaged—consult the USDA's excellent, searchable online database (ndb.nal.usda.gov).

As for those foods that do come in a labeled package, choosing those with short, recognizable ingredient lists featuring whole grains, nuts, seeds, fruit, vegetables, and/or beans are most likely to deliver the most nutritional benefits … regardless of what the label states.

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