Uses and Limitations of the Balance Technique

Classical balance techniques are a powerful, sensitive, and usually accurate technique for assessing the nutritional or metabolic response to changes in nutritional intake or to metabolic or physiological perturbations. Balances are particularly sensitive for examining transient or short‐term responses to nutritional or metabolic stimuli. A major factor responsible for the precision and sensitivity of this technique is the precise control of the activities and environment of an individual during a classical metabolic balance study (eg, the carefully defined dietary intake, degree of exercise, and environmental temperature to prevent sweating); these same factors may enhance the precision and sensitivity of other metabolic or nutritional investigations that may be carried out concurrently with a balance study. Finally, for nitrogen balance studies, the measurement of nitrogen (eg, by the Kjeldahl technique) can be very accurate and sensitive. Despite these advantages, there are important limitations and errors that are inherent in the balance technique. The errors tend to overestimate intake and underestimate output, thereby leading to erroneously positive balances. These errors include losses of food on cooking and eating utensils and dishware, losses of feces or urine on toilet paper or in collection containers, and losses through sweat, exfoliated skin, hair and nail growth, saliva, menses, blood sampling, toothbrushing, semen, and, for nitrogen, from flatus and respiration. Cumulative balance measurements are particularly likely to be falsely positive. The magnitude of unmeasured losses vary among healthy individuals, with the magnitude of the nitrogen intake (for nitrogen), in altered environmental conditions (eg, with sweating), and possibly in disease states. Balance studies are also expensive and time consuming. Finally, the fate and intermediary metabolism of the compounds ingested or infused into the subject and the sources of the output of nitrogen or minerals are poorly defined by the balance technique. Balances are probably most helpful when they are performed in conjunction with other techniques for studying nutrition or metabolism, when the intake is not too high (eg, equal to or less than 12–15 g/day for nitrogen), and when the same individual is studied before and after a perturbation in their nutritional or metabolic status. Classical balance techniques are so complicated and expensive that they are primarily of value for research studies. (Journal of Parenteral and Enteral Nutrition 11:79S‐85S, 1987)