Fat free mass

When a drug has been absorbed it enters the systemic circulation and gets distributed throughout the body. The volume of distribution is the theoretical space in a patient that a substance occupies. This is influenced by the proportions of lean body mass versus fat. There is a decline in fat-free mass and body water, whereas there is an... 

Distribution Decrease in the volume of distribution of hydrophilic drugs and increase in the volume of distribution of lipophilic drugs Decline in fat-free mass and increase in body fat with aging Important for some drugs Diazepam and flunitrazepam may have increased plasma half-life. Water soluble drugs as digoxin and teophylline have higher peak plasma concentrations and shorter half-lives... 

A further problem is, on what basis should the REE be reported, e.g. per kilogram body weight or per kilogram fat-free mass The former has the disadvantage of being influenced by the body composition, since adipose tissue has a lower metabolic activity than that of the fat-free component of the body. Per kilogram fat-free mass is now preferred because there is a linear increase in the value of the REE with the fat-free mass (Figure 2.4). There is a... 

Figure 2.4 A plot of resting metabolic rate against fat-free mass in men. The line is a regression Line based on 38 data points. The data are from Elia (1992). The Largest contributor to fat free mass, is muscle.

Table 15.7 Loss of body mass, fat and fat-free mass during a prolonged Antarctic expedition and in one individual fasting...

At death no fat was left and 40% of body protein was lost (assuming no loss of water or bone). RF and MS lost about 20% of body protein. FFM - fat free mass (see Chapter 2). 

The decrease of renal function with age is independent of the decrease in creatinine production. Because of the difficulty of obtaining complete urine collections, creatinine clearance calculated in this way is at least as reliable as estimates based on urine collections. Fat-free mass (equation [14]) should be used for obese patients, and correction should be made for muscle wasting in severely ill patients. 

A review of the effects of obesity on drug pharmacokinetics briefly mentioned that the steady-state volume of distribution of lithium correlated with ideal body weight and fat-free mass but not with total body weight (514). Lithium clearance was greater in those with obesity than in lean controls, suggesting that obese patients may require larger maintenance doses to maintain target serum concentrations. 

Since the doubly labeled water technique can be used to determine total body water, the method can be used to estimate the fat-free naass, as defined in the Obesity chapter. In brief, with a knowledge of the amount of total body water, and the assumption that normal subjects have a fat-free mass that is about 73% water, one can calculate the amount of fat-free mass. Further details are in the Obesity chapter... 

This equation rests on the assumption that the density of fat is 0-9 g/cm, and that of fat-free mass is 1.1 g/cm. Fat-free mass is composed of water, protein, mineral, and carbohydrate. Most of the fat-free mass in the human body is water, which has a density of 1-0 g/cm - About 75% of the fat-free mass is water. About 70% of the fat-free mass is protein (1.34 g/cm ). About 7% of the fat-tree mass is mineral (3.00... 

Kyle UG, Pichard C. Dynamic assessment of fat-free mass during catabolism and recovery. Curr Opin CUn Nutr Metab Care 2000 3 317-322. 

Kyle UG, Genton L, Mentha G, et al. Reliable bioelectrical impedance analysis estimate of fat-free mass in liver, lung, and heart transplant patients. JPEN J Parenter Enteral Nutr 2001 25 45-51. 

Engelen MP, et al. Skeletal muscle weakness is associated with wasting of extremity fat-free mass but not with airflow obstruction in patients with chronic obstructive pulmonary disease. Am J Chn Nutr 2000 71 733-738. 

Zhu S, Heo M, Plankey M, et al. Associations of body mass index and anthropometric indicators of fat mass and fat free mass with all-cause mortality among women in the first and second National Health and Nutrition Examination Surveys follow-up studies. Ann Epidemiol 2003 13 286-293. 

Results are expressed as means (SD) NS, not significant FFM, fat-free mass BMI, body mass index. 

Lukaski, H.C.. Johnson, PF., Bolonchuk, W.W., and Lykken, G.I. (1985) Assessment of Fat-Free Mass Using Bioelectrical Impedance Measurements of the Human Body, Am. J. Clin. Nutr. 47,810-817. 

Complex impedance data can also be given as modulus and phase. Phase has been used as an index of nutrition. This is true only in comparison between vectors with the same modulus. For instance, short vectors with a small phase angle are associated with edema, whereas long vectors with an increased phase angle indicate dehydration. The prediction error of fat mass is too high for clinical use (standard error of the estimate in the order of 3-4 L for TBW and 3-4 kg for the fat-free mass) (Sun et al. 2003). 

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