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March 12, 2004
Body mass index (BMI) is a way of characterizing weight status. For example, an adult's weight status is classified as underweight (BMI < 18.5), normal (BMI = 18.5 - 24.9), overweight (BMI = 25.0 - 29.9), or obese (BMI = 30.0). For children and adolescents, somewhat different BMI ranges are used to classify their weight status. The BMI has gained increasing use by health professionals because it is highly correlated with body fat.
The BMI values used to classify adults as underweight, normal, overweight, or obese are based on their ability to predict the effect of body weight on the risk for some diseases. For example, common conditions associated with increased risk in adults classified as being overweight or obese include premature death, cardiovascular disease, high blood pressure, osteoarthritis, some cancers, and diabetes. Although BMI is only one of many factors used to predict the risk of these diseases, it is an important factor and one that can be modified by individual changes in eating and physical activity behaviors.
For adults, BMIs are calculated from mathematical formulas that take into account an individual's height and weight. BMI can be calculated using pounds and inches with this equation:
BMI = (weight in pounds/(height in inches x height in inches) x 703
A calculator that automatically estimates the BMI for an individual is available on the CDC Web page (http://www.cdc.gov/nccdphp/dnpa/bmi/calc-bmi.htm).
BMI values for children and teens are used to assess their body fatness changes over the years as they grow. Unlike adults, where the same BMI ranges are used for both men and women and across all ages, gender- and age-specific BMI values are used to classify the weight status of children and teens. This is necessary because children's body fat levels change over the years as they grow. Also, girls and boys differ in their body fat levels as they mature. BMI decreases during the preschool years and subsequently increases into adulthood. BMI-for-age tools are useful for children and teens because they compare well to laboratory measures of body fat levels and can be used to track body size throughout life. More information on BMI values for children is available on the CDC Web page (http://www.cdc.gov/nccdphp/dnpa/bmi/bmi-for-age.htm).
For some individuals such as athletes who have a muscular body with relatively small amounts of body fat, the use of BMI values may inappropriately classify them as overweight. For these individuals, the additional use of other estimates of body fat such as waist circumference may help to more accurately estimate their weight status. For example, a waist measurement greater than 40 inches in men and 35 inches in women is usually indicative of excessive abdominal fat, which is an independent predictor of risk factors and ailments associated with obesity.
Overweight and obesity result from an imbalance that occurs when the calories consumed exceeds the calories expended. Even small imbalances over time can result in weight changes. For example, a difference of one 12-oz soda (approximately 150 calories) or 30 minutes of brisk walking most days can add or subtract approximately 10 pounds of body weight per year.
There are many physiological factors (e.g., gut hormones) that operate to maintain body weight at a constant level even though calorie intake often varies considerably from day to day and week to week.(2) The physiological factors regulating food intake tend to be more effective in defending against weight loss than against weight gain. This is thought to be an adaptive mechanism that protected humans from the adverse effects of famine and starvation. However, the physiological factors that tend to maintain calorie balance can be overwhelmed by environmental and behavioral factors that favor high calorie consumption or low physical activity. When weight gain occurs, a person's energy balance thermostat is reset to achieve calorie balance at the new, higher level of body weight. Thus once weight gain occurs, a new calorie balance level is established. The body then tends to defend against weight loss from this new, larger weight status.
Although the tendency for overweight and obesity is a product of complex interactions between physiological, genetic, environmental, and behavioral factors, the rapid increase in rates of overweight and obesity in the United States over the last several decades has occurred too rapidly for changes in genetic or physiological mechanisms to be solely the cause. Therefore, the emerging obesity epidemic is almost certainly due to changes in consumer food choices and physical activity levels resulting in an overall positive calorie balance and weight gain.
Total calorie intake refers to all energy consumed as food and drink. Proteins, carbohydrates, fat, and alcohol provide 4, 4, 9, and 7 calories per gram, respectively. Some calories (e.g., approximately 1.5 calories per gram) are obtained from dietary fiber that undergoes bacterial degradation in the large intestine to produce volatile fatty acids which are then absorbed and used as energy in the body. Physical activity such as walking 2 miles in 30 minutes burns approximately 150 calories. Because of limited capacity to convert excess calories to protein or carbohydrate, the body stores excess calories as body fat, regardless of whether the excess calories are caused by inadequate physical activity or excessive intakes of calories from any of the nutrient sources of calories. Reductions in large body fat reserves, which have often accumulated gradually over long periods of time, and subsequent maintenance of healthy body weight, will likely require long-term commitments to changes in eating and physical activity.
(1) The term "energy balance" is commonly used to describe the relationship between the number of calories consumed from foods and the calories used by the body. For purposes of this document, however, the term "calorie balance" is used in place of "energy balance" since calories are the unit of energy measurement used for nutrition labeling and best understood by consumers. Therefore, in this document, the terms "energy balance" and "caloric balance" are used interchangeably.
(2) Among the factors affecting body weight are body size and fat-free mass (i.e., the weight of the body less the weight of its fat mass) and also to a lesser degree age, gender, body composition, nutritional status, inherited variations, and/or differences in the hormonal status. Physical activity is the most variable of the calorie expenditures among individuals. For some individuals, physical activity is only a small proportion of the total calorie requirements; for very active individuals, it can be a significant proportion of daily calorie needs. Body weight is a major determinant of the calorie expenditure from physical activity. For example, the calorie cost of walking a mile at a moderate pace is 69 calories for a 140 pound individual and 58 calories for a person weighing 114 pounds. The intensity of physical activity can also affect calorie expenditure. For example, more calories are expended when jogging than when walking for the same amount of time.
Macronutrients are the components of food that provide energy (i.e., calories). There are three categories of macronutrients: carbohydrates, proteins and fats. Carbohydrates represent over half, and fats about a third, of the energy intake of typical Western diets. Understanding the caloric contribution of macronutrients to the diet requires knowledge of their chemical composition.
Carbohydrates - Carbohydrates (sugars, e.g., glucose, sucrose; and starches) provide energy to cells in the body and glucose is a primary source of energy for the brain. Sugars and starches are broken down to glucose and the energy provided is 4 calories per gram. Other types of carbohydrates such as sugar alcohols (e.g., sorbitol, maltitol) and dietary fiber are not well absorbed by the small intestine and are fermented by bacteria in the large intestine. Carbohydrates that are fermented in this manner provide a lower energy value per gram.
The rapidity and extent of carbohydrate absorption by the body directly influence the speed and extent of the rise in blood glucose (i.e., glycemic response), which, in turn, triggers an insulin response. The glycemic index of carbohydrate-containing foods has been proposed as a way to quantify the blood glucose response following their consumption (Jenkins et al., 1981). Many factors can affect the glycemic index of a single food, especially when the food is consumed in a meal.
Foods or meals that have a high glycemic index trigger the release of insulin into the blood. Elevated blood insulin levels stimulate the uptake of fat from the blood into fat cells, and inhibit the breakdown and release of stored fat from fat cells. Some scientists believe that consuming a high glycemic index food (e.g., a food that contains sugar or starch) can result in an increase in stored body fat.
Weight loss plans based on greatly restricting carbohydrate intakes have been promoted for more than a decade. "Low" carbohydrate products are being promoted as a way to reduce weight and to assist diabetics in their control of carbohydrate intake; however, not all carbohydrates raise blood glucose levels, nor deliver the same energy value per gram. In addition, when one macronutrient is restricted in a food product, it is often replaced with another macronutrient. For example, when "low" fat products were introduced several years ago, carbohydrates often were the replacement macronutrient. In many of the current "low" carbohydrate products marketed today, fat is often the replacement macronutrient. Also today many of the low carbohydrate products replace the high glycemic index carbohydrates (e.g., sugars and starches) with other carbohydrates such as sugar alcohols, which have no measurable glycemic index and may provide fewer calories per gram. Thus, it is important to look at the NFP to determine the calorie content of and the type of carbohydrate in a product.(1)
Fats (lipids) - A major source of energy for the body is derived from fats (lipids). Fats aid in the absorption of fat-soluble vitamins and carotenoids. There are two essential fatty acids, a-linolenic and linoleic. Fats contribute 9 calories per gram. There are three major components: saturated fatty acids, trans fatty acids and unsaturated fatty acids (monounsaturated fatty acids and polyunsaturated fatty acids). All yield the same caloric value, but may affect metabolism differently. Saturated fatty acids and trans fatty acids raise blood lipid levels, especially cholesterol and low density lipoprotein cholesterol, which have known adverse health effects. There is no known requirement for trans fatty acid for specific body functions.
Acceptable Macronutrient Distribution Range (AMDR) has been estimated for individuals. The AMDR is the range of intake for a particular energy source that is associated with reduced risk of chronic disease while providing adequate intakes of essential nutrients. The AMDR for carbohydrates and fats is estimated to be 45 to 65 and 20 to 35 percent of energy, respectively, for all adults. Consumption of carbohydrates and fats within these ranges reduces the risk for obesity, as well as certain chronic diseases such as coronary heart disease and diabetes.Proteins - Proteins make up the major structural components of cells and are composed of amino acids. There are 20 essential amino acids. Proteins function as enzymes, hormones, and have other important functions in the body. Proteins provide 4 calories per gram. Animal protein sources (e.g., meat, milk, eggs) generally contain balanced amounts of the essential amino acids whereas vegetable protein sources frequently have a limited amount of one of the essential amino acids. Foods that are low in fat tend also to be low in protein; foods that are low in carbohydrate tend to be high in protein and fat.
(1) FDA has received petitions requesting that the agency provide for nutrient content claims related to the carbohydrate content of foods. As discussed in section V.A.3.b., the OWG recommends that FDA file these petitions and publish a proposed rule to provide for nutrient content claims related to the carbohydrate content of foods, including guidance for the use of the term "net" in relation to carbohydrate content of foods.
