Comparison of high-resolution peripheral quantitative computerized tomography with dual-energy X-ray absorptiometry for measuring bone mineral density.

BACKGROUND/OBJECTIVES: The objective of this study was to compare the measurement of areal bone mineral density (aBMD) by dual-energy X-ray absorptiometry (DXA) with the measurement of volumetric bone mineral density (vBMD) by high-resolution peripheral computerized tomography (HR-pQCT) in subjects with a wide range of body mass indices (BMI). SUBJECTS/METHODS: We scanned the arms and legs of 49 premenopausal women, aged 21-45 years, with BMI from 18.5 to 46.5, by HR-pQCT and found that there was a nonsignificant change in vBMD associated with increased BMI, whereas aBMD (DXA) was associated with a positive significant increase. HR-pQCT scans a slice at the extremity of the tibia and radius, whereas DXA scans the entire leg and arm. RESULTS: The correlation coefficients (r) of BMD (DXA) of the legs with BMI were 0.552, P<0.001, with %fat it was 0.378, P<0.01 and with W it was 0.633, P<0.001. The r of BMD (DXA) of the arms with BMI was 0.804, P<0.001, with %fat it was 0.599, P<0.001 and with W it was 0.831, P<0.001, whereas the r of the average bone density (D100) of legs and arms measured by HR-pQCT with BMI, W and %fat were not significant. CONCLUSIONS: Although HR-pQCT and DXA scan different parts of the bone, the high r of BMD with BMI and low r of bone density measured by HR-pQCT with BMI suggest that BMD measured by DXA is artifactually increased in the presence of obesity.

Effect of fat on measurement of bone mineral density.

OBJECTIVE: To determine if increasing fatness interferes with the measurement of fat and bone mineral density (BMD) by dual-energy X-ray absorptiometry (Lunar iDXA). METHODS: We performed measurements of BMD and fat on a section of a beef femur defatted by prolonged boiling in detergent, completely surrounded by increasing thicknesses of lard. Initially the bone was placed in the marked spine area, overlying a 6L plastic bottle which was placed in the marked trunk area of the iDXA. The plastic bottle was then removed and further measurements were carried out with increasing thicknesses of lard surrounding the bone. Measurements were repeated 4 times. RESULTS: The reported measurement of BMD progressively increased with each increased layer of lard surrounding the bone. All the iDXA BMD measurements were significantly (P<0.01) different from one another. When surrounded by 3 layers of lard the reported BMD was 20.5% greater than the reported BMD when the bone was not surrounded by any lard. The differences between the actual amount of fat measured by chemical analysis and weighing, and the reported measurement of fat by iDXA were significant with all 3 thicknesses of lard (P<0.01); the percentage difference between the fat measured by iDXA and that measured chemically decreased as the number of layers of lard increased. CONCLUSION: We found that iDXA overestimated fat by up to 11.1%. The percentage overestimation of fat diminished as the amount of fat increased. BMD was overestimated by 20.5% when surrounded by 3 layers of fat compared to when there was no surrounding fat. In contrast to fat, the percentage overestimation of BMD increased as increasing amounts of fat surrounding the bone Using earlier generation DXAs, others have reported that measurements were ± 20-50% inaccurate and differed according to the configuration of the phantoms. The measurement of BMD and fat is the main clinical purpose of iDXA; the present experiment has shown that there are substantial inaccuracies in the measurement of BMD and fat. It is not known how these inaccuracies compare with those of earlier generations of DXA machines.

Bone mineral density and mass in a cross-sectional study of white and Asian women.

The literature suggests that Asians have lower bone mineral density and mass than whites. It has been proposed that these differences may be due to differences in height, weight, and factors other than ethnicity, but no study has made the appropriate direct comparisons. We compared total-body bone mineral density and mass between Asian and white women while controlling for factors known to be associated with bone mineral density and mass. Measurements were made in 129 Asian (primarily of Chinese ancestry) and 274 white women. A subgroup was formed of women who did not have a history of alcoholism, premenopausal amenorrhea, kidney disease, estrogen use, birth control pill use, thyroid disease, steroid use, hysterectomy, or smoking. In both the main group and the subgroups, bone mineral mass was significantly lower in Asian than in white women, but after analysis of covariance with body weight, height, and age (or years since menopause) as covariates, the differences between ethnic groups disappeared, except in the large group of premenopausal women, in whom average bone mineral density in Asians actually exceeded (p < 0.04) that in whites. The data set was also searched for Asian-white pairs who matched on 17 characteristics related to bone mineral density and mass. In the resulting 16 matched pairs, bone mineral density and mass were not different between ethnic groups. Although Asian women have lower bone mineral mass than white women, when weight, height, and other factors are controlled, bone mineral density and mass do not differ between Asian and white women.

Bone mineral density and mass by total-body dual-photon absorptiometry in normal white and Asian men.

Total body bone mineral density, total body bone mineral mass (TBBM), and bone mineral densities (BMD) in seven different regions of the body were measured in 238 normal men (154 Whites and 84 Asians), age 22-94, using dual-photon absorptiometry. Although Asian men had lower TBBM and BMD in all regions (p less than 0.05) except the arms, when multiple regression was done with body weight, height, and age, no significant differences were found between Asians and Whites for bone measurements. Thus lower bone mineral densities and bone mineral mass in Asian males compared to White males appear to be due to differences in height and weight rather than to ethnic differences. The two groups were combined to derive multiple regression equations for TBBM, total-body BMD, and regional BMD. Age, weight, and height were significant in the multiple regression equations for TBBM, ribs BMD, and legs BMD. Age and weight, but not height, were significant for total-body BMD, trunk BMD, spine BMD, arm BMD, and pelvis BMD. Weight, but neither height nor age, was significant for head BMD. These reference normal bone mineral density and bone mineral mass standards are appropriate for both Asian and White males when adjusted for weight, height, and age.

Lack of sustained increase in VO2 following exercise in fit and unfit subjects.

Thermogenesis after exercise was studied by measuring oxygen consumption in 23 subjects who were classified into three groups according to their routine level of physical activity. VO2 was first measured after a 30-minute rest period 4-hours after breakfast. Then each subject either exercised for 20 minutes at approximately the anaerobic threshold or on a separate non-exercise day remained recumbent. The subject then returned to, or remained, at rest. There was no significant difference in VO2 from the resting level from 40 minutes to 3 hours after exercise, between exercise and non-exercise days in any fitness group. Seven subjects also exercised for a longer period or at a higher intensity. Again, there was no significant difference in the time course of VO2 from 40 minutes to 220 minutes after exercise, between exercise and non-exercise days. Because no sustained effect of moderate or intense exercise on VO2 was demonstrated, we conclude that no appreciable caloric loss beyond that generated by the exercise period itself and the early recovery phase is found in either fit or unfit subjects. These data do not support claims for sustained increases in metabolic rate after exercise in weight-control programs.

Body potassium by four-pi 40K counting: an anthropometric correction.

Four-pi whole body counting for the 1.46 meV photon of 40K has apparent advantages over single-crystal or two-pi counters in efficiency and in subject geometry independence. However, our studies of obese populations have disclosed a systematic undermeasurement of 40K, suggesting that nonhomogeneous K distribution results in systematic undercounting of 40K. In the current study 42K, emitting a 1.52 meV photon, was used in 109 volunteers ranging from 50 to 181 kg, and multiregression covariance analysis was applied to develop correction formulas based on anthropometrics. These corrections quantitatively account for the unappreciated loss of 40K and 42K photons in annular adipose tissue that surrounds the lean body, in which most K+ is concentrated. The correction ranges from 1 to 28% and is a linear (although different) function of weight in both sexes. Thus corrected, body potassium measurements, taken in conjunction with exchangeable sodium and water measurements, provide estimates for whole body osmolality that match measured serum values. Such a quantitative accounting for previously "lost" cation in 58 subjects provides independent evidence for the appropriateness and accuracy of the correction. With this correction, body potassium was recalculated in the 1,492 adult members of a previously reported group of 3,083 subjects.

Effect of lithium therapy on glomerular filtration rate.

Patients taking lithium had a slightly higher serum creatinine concentration than controls. Creatinine concentration was independent of lithium level or therapy length, suggesting that lithium decreases glomerular filtration but that this effect is small, noncumulative, and of marginal clinical significance.

Body composition in affective disorder before, during, and after lithium carbonate therapy.

The body composition of three unipolar and 17 bipolar patients, studied during the depressive state, was compared with that of 48 controls. The ratio of extracellular to intracellular water content was significantly less in male patients than in male controls: the difference between female patients and female controls was not statistically significant. In patients of both sexes, residual sodium concentration was greater in patients than in controls. The intracellular content of potassium was the same in patients as in controls. In six patients, body composition was studied prior to lithium carbonate therapy, after 17 days of treatment, and 17 days after discontinuing administration of the drug. The significant changes were a decrease in body weight during therapy, and a decrease in body water content and an increase in residual sodium concentration after drug treatment was discontinued.

Body composition measurements in normal man: the potassium, sodium, sulfate and tritium spaces in 58 adults.

The aqueous phase of total body composition varies with sex, and changes with age, necessitating age, sex and method-specific normal values. Consensus has developed for the methods most suited to clinical studies. 3H20 for water, 35SO4 for ECW, 24Na for NAc, and 40K for body potassium were used in 58 normal volunteers aged 19-80, 30 men and 28 women. This report shows significant age regressions in both sexes for total body water and intracellular water, and for both Na and K when referenced to body water. Combination of these findings with the highly age related decrease in total body potassium confirms previous studies that body cell mass decreases with age. Intracellular K+ and Na+ at the zero age intercept are 130 and 14.5 for males, and 124 and 24mEq/1 for females respectively. Secular decrease in K+ and increase in Na+ concentrations in the average cell are suggested although not established. Values for normal body composition, in subjects healthy by all available criteria are in some instances age and sex dependent, but in all parameters except fat show wide normal range (SD = 7% of mean) which derive more from biological variation than from limitations of measurement precision. However, use of body water rather than weight succeeds in narrowing this wide range, and is a more logical denominator which is readily subject to accurate measurement. In serial studies of the same subject, significant changes can be measured which are well within the normal range.

A possible low intracellular potassium in obesity.

In comparison with lean marathon runners whose fat content was 15%, obese subjects with a fat content of 55% showed a 36% reduction in intracellular potassium. It is hypothesized that as much as 60% of this decrease may have been due to the lower proportion of muscle (which has the highest intracellular potassium of any tissue) in the fat free mass of very obese subjects. The remainder of the decrease seems to have been a measurement artefact owing to self-absorption of gamma rays by adipose tissue, most of which is distributed over potassium rich lean tissue. The low intracellular potassium values found in obesity should not be interpreted as necessarily signifying potassium deficiency.