This is an excerpt from Advanced Exercise Physiology.
Cachexia is not simply weight loss. In some cancer populations, such as patients with early-stage breast cancer, weight gain is actually a more common occurrence that is associated with poor prognosis. Cachexia is a metabolic syndrome linked to an underlying condition (e.g., cancer, heart failure) that results in the loss of muscle with or without fat that impairs function. Although the mechanisms that lead to cachexia are not fully understood, one factor that differentiates itself from sarcopenia (see "Body Composition and Weight Management Across the Life Span") is an increase in metabolic rate. This increased metabolic rate may be attributable to the cancer tumor itself, especially when the cancer cells spread to metabolically active tissue. An example of this was seen inpatients with colorectal cancer, where an increase in liver mass due to metastatic cancer cells was associated with a concurrent decrease in total muscle mass. Regardless of what triggers the increased metabolic rate, the end result is increased muscle catabolism, which places the cancer patient at higher risk of early death.
One misconception about cachexia is that obese individuals are somehow protected because of increased energy reserves. Although the obesity paradox suggests a protective effect with increases in certain cancer populations (e.g., individuals undergoing chemotherapy), in general BMI does a poor job of classifying those patients with a high mortality risk. The obvious reason why is the inability of BMI to differentiate FFM from FM. Using a bioelectrical impedance analysis, Gonzalez et al. showed that FFM index (i.e., FFM divided by the square of height) was a better predictor of mortality regardless of how much FM was present. In other words, it did not matter if a patient's BMI was higher or lower; if the patient had a reduced amount of muscle mass, the risk of death was increased.
The clinical implications for measuring FM and FFM become evident if you compare two obese patients, one with cachexia and one without. Both patients lose 20 lb (9.1 kg) after a diagnosis of cancer. Using body composition methods, a clinician can identify the higher risk patient. Methods such as the CT measurement of lumbar skeletal muscle currently are being used, although unfortunately not widely. Regardless, the identification of cachexia is one potential area where knowledge of body composition methods can be invaluable.
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