disorders of nutrition
Overview of Nutrition
Evaluation of Nutritional Status
To determine whether people are consuming a proper amount of nutrients, doctors ask them about their eating habits and diet and do a physical examination to assess the composition and functioning of the body. Height and weightare measured, andbody mass index (BMI)is calculated. BMI is calculated by dividing weight (in kilograms) by the square of the height (in meters). A BMI between 18.5 and 24.9 is usually considered normal or healthy for men and women. In the United States and other countries with low rates of food insecurity, many people have a BMI that is higher than 24. Maintaining an appropriate weight is important for physical and psychologic health. A standardized height-weight table can be used as a guide, but BMI is more reliable. BMI, however, does not account for differences in Body composition . Waist circumference can be measured instead; the fat in the midsection is sometimes a more accurate measure of excess weight or harmful fat that is deposited in the internal organs and that tends to predict risk of heart disease and metabolic disorders. Levels of many nutrientscan be measured in blood, inside some cells, and sometimes in tissues. For example, measuring the level ofalbumin, the main protein in blood, may help determine whether people are deficient in protein. Nutrient levels decrease when nutrition is inadequate. However, whether these measures reliably indicate nutritional status may depend on what the measurement reflects (for example, whether inside cells or in the blood) as cellular levels of nutrients may be more reflective of usable or available nutrient as opposed to the amount that is carried in the blood. Body composition usually refers to how much of the body is fat and how much is muscle, typically expressed as the percentage of body fat. Body composition is sometimes estimated by Measuring skinfold thickness Doing bioelectrical impedance analysis More accurate ways to determine this percentage include weighing people under water (hydrostatic weighing) and doing a dual-energy x-ray absorptiometry (DXA) scan, computerized tomography (CT) scan, and magnetic resonance imaging (MRI). However, these more accurate methods are not easy to use, can be expensive, expose people to unnecessary radiation (CT scans), and are not always readily available. They are used mainly in research. Skinfold thickness:Body composition can be estimated by measuring the amount of fat under the skin (skinfold thickness). A fold of skin on the back of the left upper arm (triceps skinfold) is pulled away from the arm and measured with a caliper. A skinfold measurement of about 1/2 inch in men and about 1 inch in women is considered normal. This measurement plus the circumference of the left upper arm can be used to estimate the amount of skeletal muscle in the body (lean body mass). Bioelectric impedance analysis:This test measures the resistance of body tissues to the flow of an undetectable low-voltage electrical current. Typically, people stand barefoot on metal footplates, and the electrical current, which people cannot feel, is sent up one foot and down the other. Body fat and bone resist the flow much more than muscle tissue does. By measuring the resistance to the current, doctors can estimate the percentage of body fat. This test takes only about 1 minute. Hydrostatic weighing:People are weighed underwater in a small pool and that weight is compared to their weight on dry land. Bone and muscle are denser than water, so people with a high percentage of lean tissue weigh more in water and people with a high percentage of fat weigh less. Although this method is considered the most accurate, it requires special equipment that is not readily available, as well as considerable time and expertise to do. Dual-energy x-ray absorptiometry (DXA):This imaging procedure accurately determines the amount and distribution of body fat. DXA uses a very low dose of radiation and is safe. However, it is too expensive to use routinely. CT scan and MRI, although not routinely available for health enhancement alone, provide the most detailed and accurate body composition analysis because they can determine more precisely how much fat is in the tissues, including inside muscles and organs, and can differentiate the more harmful abdominal and internal organ (visceral) fat from the less harmful fat under the skin (subcutaneous fat). 