mogenates and subcellular fractions of cultured neuroblastoma cells. Effects of anesthetics. J Neurochem 1985;44:1551-8

16. Spence MW, Burgess JK. Acid and neutral sphin- gomyelinases of rat brain. Activity in developing brain and regional distribution in adult brain. J Neurochem 1978;30:917-9

17. Petkova DH, Momchilova-Pankova AB, Markovska

TT, Koumanov KS. Age-related changes in rat liver plasma membrane sphingomyelinase activity. Exp Gerontol 1988;23:19-24

18. Dudeja PK, Dahiya R, Brasitus TA. The role of sphingomyelin synthetase and sphingomyelinase in 1,2-dimethylhydrazine-induced lipid alteration of rat colonic plasma membranes. Biochim Bio- phys Acta 1986;863:309-12

Estimating Body Fat in Lean and Obese Women

The body fat content of lean and obese women was measured by six different methods: under- water weighing, the current “standard”; body water dilution; whole-body counting; skinfold measurements; bioelectrical impedance; and magnetic resonance imaging. Of these, mag- netic resonance imaging gave values most sim- ilar to those obtained by underwater weighing. All methods were subject to considerable varia- tion between and within subjects.

The estimation of in vivo body-fat content in human beings is of importance in a variety of clinical and research situations. The former include the assess- ment of normal and abnormal growth and the stan- dardization of a number of metabolic measure- ments. Until fairly recently, most of the body-fat estimates were based on underwater weighing (UWW), body-water dilution (BWD), whole-body counting (40K), or skinfold thickness (SFT) and its relationship to body density.’ All these methods are based on assumptions of interrelationships between the different compartments of the body and are likely to vary considerably between and possibly even within individuals. Among these methods UWW has usually been accepted as the method of choice, although the difficulties of achieving total immersion and the cooperation required toward that end sharply limit the suitability of the method. Recent studies2-’ comparing the ‘different methods have indicated considerable disagreement, both be- tween individuals and in group mean results.

McNeill et aL6 recently carried out an extensive, carefully conducted comparison of six methods of estimating body fat in a group of lean as well as overweight women. In addition to the methods al- ready mentioned above, body fat in the present study was also estimated by bioelectrical imped- ance (BEI) and magnetic resonance imaging (MRI) in both groups of women. (All six methods and their clinical applications have been detailed recently in this j~urnal.’.~)

Fourteen women volunteered for the study by

McNeill and colleagues.6 They were picked from a register of volunteers from the research institute where the study was conducted as well as from a local weight-loss group. The two groups of seven subjects each, one lean and one overweight, were of similar age, mean weight, and height. The mean body mass index (weight + height2) was 20.6 kg/m2 for the lean women and 31.1 kg/m2 for the obese women. When the women came for their first visit, they had been without food or drink for at least 12 hours. Body weight was measured to the nearest 0.1 kg and height was measured to the nearest 1 mm using a portable stadiometer. The six estimates were carried out by standardized procedures that may be assumed to have been as accurate as is cur- rently possible. In addition to the six estimates, body fat was further calculated by a combined BWD and 40K method (W + K), a procedure sug- gested by Bruce et aL9 to take into account various assumptions that are inherent in both methods. The results of all the estimates were statistically com- pared with the UWW result according to the method of Bland and Altman.” Their approach is based on calculating the difference between the es- timates of percentage body fat by the method in question and by UWW for each subject and on in- vestigating the relationship between this difference and the average of the estimates by the method and by UWW.

The results of the body fat estimations by the different methods showed large differences in both the lean and the overweight groups of women. The greatest difference occurred in the subject with the highest body fat by UWW and the measurement using SFT. The BWD and 40K and MRI measure- ments overestimated body fat in both lean and over- weight women in comparison with the UWW method. On the other hand, the BE1 method under- estimated body fat content in both groups of women in comparison with the UWW method. The findings for the SFT measurements were higher in the lean group in comparison with the UWW measurements, but lower than those of UWW in the obese group, particularly for those subjects with the higher body

80 Nutrition Reviews, Vol. 50, No. 3

fat contents. The BWD method resulted in the larg- est mean difference from the UWW method, with a variability similar to that of the SFT and BE1 meth- ods. The measurements from the 40K method gave a mean value closer to the mean UWW value, but showed the highest variability, whereas SFT mea- surements showed the smallest mean difference from UWW, but still with a high degree of variabil- ity. The combined W + K method gave a mean difference and a variability of differences from UWW between individuals that was less than that of either the BWD or the 40K method alone.

The large variation of percentage in body fat found for individual subjects in this study has also been observed by other workers. While the origin of these differences is not clear, they could arise from errors in the primary data or in the assumptions upon which the different methods are based. One example, concerning assumptions, was illustrated by the authors of the present study in relation to the BWD method. They indicated that the value of 73.2% of fat-free mass as water is used in that method and is based on the average of results that cover a range of at least 70-76% in most species; this range could result in considerable error in the calculation of body fat. Unfortunately, it is not pos- sible to ascertain from the present results which of the factors might have contributed to the discrep- ancies observed. By far the biggest discrepancy in the present data set occurred between the UWW and BWD values, particularly in the lean group of subjects. The authors consider the possibility that the conditions of the measurement, such as the tim- ing of the dilution studies in relation to eating and drinking, introduced some error into the BWD mea- surements. On the other hand, the difference be- tween BWD and UWW methods in this study is similar to the results of Webster et al.,4 who found the percentage of fat measured by the BWD method to be 6% higher than that estimated by UWW in 104 women with varying degrees of obesity. The large variability in the difference between the 40K mea-

surements and the UWW results suggests that, at least for the present set of data, the 40K method provided the least accurate body-fat estimate for individual subjects. Overall, the results suggest the need for caution in the intepretation of estimates of body fat content and fat-free weight in women.

1. Durnin JVGA, Womersley J. Body fat assessed from total body density, and its estimation from skinfold thickness. Br J Nutr 1974;32:77-97

2. Womersley J, Boddy K, King PL, Durnin JVGA. A comparison of the fat-free mass of young adults es- timated by anthropometry, body density and total body potassium content. Clin Sci 1972;43:469-75

3. Kryzwicki HJ, Ward GM, Rahman DP, Nelson RA, Consolazio CF. A comparison of methods for es- timating human body composition. Am J Clin Nutr 1974;27:1380-5

4. Webster JD, Hesp R , Garrow JS. The composition of excess weight in obese women, estimated by body density, total body water and total body po- tassium. Hum Nutr Clin Nutr 1984;38:29%306

5. Maughan RJ, Haggarty P, McGaw BA, Gvozdanovic D, Gvozdanovic S. Measurement of body compo- sition in lean athletic women. Proceedings of the Nutrition Society 1988;47:112A

6. McNeill G, Fowler PA, Maughan RJ, et al. Body fat in lean and overweight women estimated by six methods. Br J Nutr 1991;65:91-103

7. Heymsfield SB, Waki M. Body composition in hu- mans: advances in the development of multicom- partment chemical models. Nutr Res 1991 ;49:97- 108

8. Roubenoff R , Kehayias JJ. The meaning and mea- surement of lean body mass. Nutr Rev 1991;49: 163-75

9. Bruce A, Andersson M, Arvidson B, lsaksson B. Body composition. Prediction of normal body po- tassium, body water and body fat in adults on the basis of body height, body weight and age. Scand J Clin Lab Invest 1980;40:461-73

10. Bland JM, Altman DG. Statistical methods for as- sessing agreement between two methods of clin- ical measurement. Lancet 1986;1:307-10

Is Putrescine an Essential Nutrient for Avians?

Putrescine addition to a complete crystalline amino acid diet elicited a growth response in young chicks, suggesting that putrescine may be an essential nutrient in avians. However, high di- etary putrescine levels can depress weight gain.

Polyamines are essential for cell growth and are thought to be involved in regulating synaptic activ-

ity of nerve cells and in controlling synthesis of DNA, RNA, and pro te in . ' Putrescine (1,4- diaminobutane) is the simplest of the polyamines. Synthesized from ornithine via the ornithine decar- boxylase (ODC) reaction, putrescine is a precursor of the higher polyamines, spermidine and spermine. For years it has remained a mystery why crystalline amino acid chick diets will not sustain maximal

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