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My Life - December 2010

Decoding the mystery math behind nutrition facts labels
How to make sense of information that is supposed to help you eat sensibly

You are what you eat, but do you know what that is? Thanks to nutrition facts labels on most packaged food products, consumers have access to a wealth of information to help them decide which foods they should put in their mouths and which they should pass over. But understanding these labels sometimes requires some mathematic skill. If you break them down into their most important aspects and stick to real numbers, you can unlock the code.

Consumers have access to a wealth of information to help them decide which foods they should put in their mouths and which they should pass over.First, we need to address serving size, since every number on the label is based on it. Serving size usually is described in common measures like 1 cup, 1/2 cup, etc. Currently, there are no standards governing what serving size is used. It's up to the product's manufacturer. Not surprisingly, manufacturers tend to choose serving sizes that make the rest of the information on the label look better. Foods high in undesirable content, such as fat and calories, may be given unrealistically small serving sizes (e.g., 1-2 cookies, 12-15 chips), and foods low in these may be given impractically high serving sizes to accentuate the healthy nutrients they contain.

Every other number on the nutrition facts label is based on this serving size, so understanding what you are eating requires knowing how much you realistically will eat in a single sitting. You could measure every portion of food you take to determine what your actual consumption is, or you could use a shortcut. Next to the serving size on the nutrition facts label is the number of servings per container. This is the total volume of the package divided by the manufacturer's serving size. Divide this number by the actual number of servings you get out of the package in reality, then multiply each of the other numbers on the label by the result. For example, if a box of cereal says it contains 10 servings and you typically only get five bowlfuls out of it, you need to double every number on the label to get an accurate idea of what you are consuming. On foods that require preparation, it also is important to note whether the serving size is based on cooked or uncooked volume. For example, 1/2 cup of uncooked rice produces 1 to 1 1/2 cups cooked.

Now, we're armed with the mathematical key to understanding the four most important items on the nutrition facts label.

  • Calories. The nutrition facts label contains two numbers for calories: total calories and calories from fat. The label gives you the number of calories that come from fat, but you need to know what percentage of the total calories this represents. To get this, divide the number of calories from fat by the total number of calories, then multiply by 100. For example, if the product contains 260 calories per serving and 45 calories from fat, the percentage of calories from fat would be 16. However, making this calculation for each food you eat can be misleading. Healthy adults should get about 20-35 percent of the total calories they consume in a day from fat, so you need to consider the number of calories and calories from fat of all the foods you eat. Thus, some foods that have a high fat calorie percentage may be fine to eat as long as they account for a relatively small portion of the total fat and calories you consume in a day.
  • Carbohydrates. Most nutrition facts labels will present at least three separate numbers (in grams) for carbohydrates: total carbohydrates, dietary fiber and sugars. Some labels also will give a number for "other carbohydrates," but this is simply the number of carbohydrates not attributable to fiber or sugar. If you are counting carbohydrates for meal planning, you can subtract the grams of fiber from the total carbohydrates. Carbohydrates are your body's main source of fuel and most medical experts recommend that about 60 percent of a healthy adult's daily calorie intake should be from carbohydrates. That means the average person should consume about 300 grams of carbohydrates per day. People who are more physically active than most should consume more carbohydrates, while those who are less active or who have medical conditions, like diabetes, that affect their bodies' ability to process carbohydrates, should eat fewer.
  • Fat and cholesterol. There are five numbers given for fat: total fat, saturated fat, trans fat, polyunsaturated fat and monounsaturated fat. The three most important to watch are total fat, saturated fat and trans fat. A healthy adult should consume about 65 total grams of fat per day, with no more than 20 grams coming from saturated fats. Trans fat has been linked to higher blood cholesterol levels and increased risk of heart disease. Though there are no defined guidelines, most health experts recommend you keep your intake of trans fats as low as possible or avoid them altogether. Cholesterol is listed separately on the label, and a healthy adult should consume less than 300 mg per day.
  • Sodium. A healthy adult should consume no more than 2,400 mg of sodium per day and less than 400 mg per serving of any one food. For comparison, 2,400 mg of sodium is roughly equal to about 1 teaspoon of salt. Most processed, packaged foods (and meals from fast food restaurants) contain high amounts of sodium - as much as an entire day's worth or more in one serving!

In addition to all the above measures, a nutrition facts label also indicates a percentage daily value for many of the nutrients listed. However, this number can be quite misleading since it is based on the manufacturer's serving sizes and assumes a 2,000 calorie diet. Rather than rely on these percentages, talk to your health care professional about healthy levels of calories, carbohydrates, fat and sodium you should consume given your health and activity levels. Then, know how to accurately determine what you are eating.