Dietary Influence on Calcitropic Hormones and Adiposity in Caucasian and African American Postmenopausal Women Assessed by Structural Equation Modeling (SEM)

OBJECTIVES

To examine differences in hydroxycholecalciferol (25(OH)D) and parathyroid hormone (PTH) concentrations between Caucasian and African American (AA) postmenopausal women, as well as the effects of dietary calcium, protein and vitamin D intakes on 25(OH)D, PTH, and body adiposity using structural equation modeling (SEM).

DESIGN

Population-based prospective cohort study.

SETTING

Academic research using the baseline data from two longitudinal studies. Participants Included n=113 Caucasian and n=40 African American, postmenopausal women who completed the baseline data collection and met inclusion criteria (dietary calcium intake <900 mg/day and being generally healthy) between 2006 and 2010.

MAIN OUTCOME

Dietary intake of calcium and vitamin D, assessed by dietary records, were examined in relation to calcitropic hormones concentrations and adiposity markers. Independent t-tests, confirmatory factor analysis, SEM and multi-group analyses were conducted to examine the aforementioned relationships as well as group differences among hormones, dietary intake, anthropometrics, age and other factors.

RESULTS

Dietary calcium and protein intakes were significantly lower in AA women. Years since menopause were significantly higher in AA compared to Caucasian women. PTH and 25(OH)D levels were significantly lower in AA compared to Caucasian women. Dietary calcium and protein intakes did not influence body adiposity in either group of women. Dietary vitamin D had minimal indirect (via 25(OH)D levels) influence on adiposity.

CONCLUSION

The study confirmed the positive relationship of 25(OH)D with adiposity markers and both AA and Caucasian women. The study provides a unique example of the use of SEM in nutrition research within a clinical context. This model should be further tested in other populations.

Osteosarcopenic obesity is associated with reduced handgrip strength, walking abilities, and balance in postmenopausal women

We determined the prevalence of osteosarcopenic obesity (loss of bone and muscle coexistent with increased adiposity) in overweight/obese postmenopausal women and compared their functionality to obese-only women. Results showed that osteosarcopenic obese women were outperformed by obese-only women in handgrip strength and walking/balance abilities indicating their higher risk for mobility impairments.

INTRODUCTION

Osteosarcopenic obesity (OSO) is a recently defined triad of osteopenia/osteoporosis, sarcopenia, and adiposity. We identified women with OSO in overweight/obese postmenopausal women and evaluated their functionality comparing them with obese-only (OB) women. Additionally, women with osteopenic/osteoporotic obesity (OO), but no sarcopenia, and those with sarcopenic obesity (SO), but no osteopenia/osteoporosis, were identified and compared. We hypothesized that OSO women will have the lowest scores for each of the functionality measures.

METHODS

Participants (n = 258; % body fat ≥35) were assessed using a Lunar iDXA instrument for bone and body composition. Sarcopenia was determined from negative residuals of linear regression modeled on appendicular lean mass, height, and body fat, using 20th percentile as a cutoff. Participants with T-scores of L1-L4 vertebrae and/or total femur <-1, but without sarcopenia, were identified as OO (n = 99) and those with normal T-scores, but with sarcopenia, as SO (n = 28). OSO (n = 32) included women with both osteopenia/osteoporosis and sarcopenia, while those with normal bone and no sarcopenia were classified as OB (n = 99). Functionality measures such as handgrip strength, normal/brisk walking speed, and right/left leg stance were evaluated and compared among groups.

RESULTS

Women with OSO presented with the lowest handgrip scores, slowest normal and brisk walking speed, and shortest time for each leg stance, but these results were statistically significantly different only from the OB group.

CONCLUSION

These findings indicate a poorer functionality in women presenting with OSO, particularly compared to OB women, increasing the risk for bone fractures and immobility from the combined decline in bone and muscle mass, and increased fat mass.

Examining change in social support and fruit and vegetable consumption in African American adults

OBJECTIVE

To examine (a) inter-individual variation in African Americans' fruit and vegetable social support, behavior, and consumption trajectories by estimating latent growth curves (LGCs) and (b) the associations between these trajectories over time.

DESIGN

As part of a larger intervention study, data were collected from mid-life and older African Americans yearly for three years. The study incorporated a quasi-experimental design with random selection of participants, stratifying for age and gender.

SETTING

Six churches in North Florida.

PARTICIPANTS

Two hundred and thirty one (73% women; median age range of 57-63) older African Americans.

MEASUREMENTS

A structured questionnaire elicited personal data as well as information on dietary social support, eating-related behaviors, and fruit and vegetable dietary intake.

RESULTS

Age was positively associated with initial social support but negatively associated with the rate of change in social support. More important, the rate of change in dietary social support predicted eating-related behavior trajectories, which influenced the rate of change in fruit and vegetable consumption over time after controlling for the intervention.

CONCLUSION

These findings illustrate the mediating role of eating-related behaviors and the inter-locking nature of social support, behavior and consumption trajectories. This research has implications for future research as well as community interventions and programs.

Life dissatisfaction and eating behaviors among older African Americans: the protective role of social support

OBJECTIVES

To examine (a) the influences of life dissatisfaction and dietary social support on eating behaviors (a high-fat diet and fruit/vegetable consumption) of older African Americans and (b) the moderating role of perceived dietary social support on the association between their life dissatisfaction and unhealthy eating behaviors.

DESIGN

Baseline data from a larger intervention study of mid-life and older African Americans. The study incorporated a quasi-experimental design with random selection of participants, stratifying for age and gender.

SETTING

Six churches in North Florida.

PARTICIPANTS

One hundred and seventy-eight (132 females and 46 males with a median age of 60) older African Americans.

MEASUREMENTS

A structured questionnaire elicited personal data as well as information on eating behaviors, life dissatisfaction, and perceived dietary social support.

RESULTS

Older African Americans with more cumulative life adversity, as reflected by high life dissatisfaction, had significantly poorer eating behaviors including the consumption of a high-fat diet and low intake of fruits and vegetables. Older African Americans' dietary choices were also associated with their perceived social support. More importantly, perceived social support acted as a buffer to mitigate the influence of life dissatisfaction on older African Americans' eating behaviors.

CONCLUSION

Life dissatisfaction places older African Americans at risk for unhealthy eating behaviors. However, high levels of dietary social support can protect older African Americans from the influence of life dissatisfaction on unhealthy eating behaviors. There are practical implications of this research for health interventions and programming.

Lipid profile and bone paradox: higher serum lipids are associated with higher bone mineral density in postmenopausal women

BACKGROUND

Previous studies suggest a relationship between cardiovascular disease (CVD) and osteoporosis; however, the mechanism of the relationship and whether serum lipids are positively or negatively associated with bone mineral density (BMD) are unclear.

METHODS

We investigated the relationship among serum lipids, dietary saturated fat, BMD of various skeletal sites, and markers of bone turnover. This was a cross-sectional analysis in 136 Caucasian, healthy, postmenopausal women, who were not taking lipid-lowering medications or drugs affecting bone metabolism. BMD at multiple skeletal sites was assessed by DXA. Concentration of serum triglycerides, cholesterol, osteocalcin (OC), and undercarboxylated osteocalcin (UOC) and urinary cross-linked N-telopeptides were analyzed by routine methods. Saturated fat, total calcium (food and supplements), total vitamin K, alcohol, and energy intake were estimated using 3-day dietary records. Physical activity was assessed and used as a confounder with other anthropometric measurements.

RESULTS

Serum triglycerides were positively related to femoral shaft BMD and serum cholesterol to total body BMD (p < 0.05). Also, subjects with serum triglycerides above the median had significantly higher BMD in femoral Ward's triangle than those below the median (p = 0.037, by ANCOVA). Subjects with a serum cholesterol level of > or =240 mg/dL (cutoff for increased risk for CVD) had significantly higher BMD at the total body and at all sites of the femur (except neck). There was no relationship between serum lipids and markers of bone turnover. Saturated fat intake was not associated with BMD of any skeletal site.

CONCLUSIONS

These findings indicate that higher levels of serum triglycerides and cholesterol are positively associated with BMD of various skeletal sites. The mechanism of this association is not clear, and studies are needed to clarify this relationship.

Dual hip bone mineral density in postmenopausal women: geometry and effect of physical activity

The objectives of this study were to (1) obtain both femoral neck strength (FNS) and hip axis length (HAL) values from left and right femurs (regardless of hip dominance) measured by DXA and evaluate their relationship with BMD of all hip regions including total hip, (2) determine if there is a difference between dominant and nondominant hip BMD in any of the hip regions, and (3) determine how physical activity influences hip BMD. Participants were 136, generally healthy Caucasian women (57.4-88.6 years). BMD was measured by DPX-MD. Past and present activity was assessed by the Allied Dunbar National Fitness Survey for older adults and normal/brisk walking pace was measured in a straight hallway. FNS analysis uses femoral geometry to calculate stresses at the femoral neck for two loading conditions: Safety Factor Index (SF) indicates risk of fracture for forces generated during a one-legged stance, and Fall Index (FI) indicates risk of fracture for forces generated during a fall on the greater trochanter. Simple and multiple regression analyses were used to determine predictive ability of HAL, SF, and FI for respective hip BMD values. There was no statistical difference in BMD between two hips in any of the measured regions, however, the nondominant hip correlated better with other skeletal sites. Subjects with a faster normal walking speed had higher neck BMD in the nondominant hip, 0.832 +/- 0.12 vs. 0.791 +/- 0.10 g/cm2 (P < 0.05). Longer HAL of the left hip was negatively related to neck, trochanter, shaft, and total hip BMD. FI was significantly associated with all sites of the hip BMD, while SF was associated only with neck and wards BMC (P < 0.05). In summary: (1) a longer HAL is associated with lower BMD and a higher FI with higher BMD, (2) it might be sufficient to measure BMD in only the nondominant hip, and (3) walking at a faster pace may positively benefit femoral neck BMD. Therefore, it appears that HAL, SF, and FI all play important roles in estimating fracture risk and should be assessed along with BMD when using DXA.

Bone and nutrition in elderly women: protein, energy, and calcium as main determinants of bone mineral density

OBJECTIVE

Nutrition is an important factor in the prevention and treatment of osteoporosis. Our goal was to examine the relationship between various nutrients and bone mass of several skeletal sites in elderly women, taking into account possible confounding variables.

DESIGN/METHODS

A cross-sectional study in 136 healthy Caucasian, postmenopausal women, free of medications known to affect bone was carried out. Bone mineral density (BMD) and body composition (lean and fat tissue) were measured by dual X-ray absorptiometry using specialized software for different skeletal sites. Parathyroid hormone (PTH) and vitamin D, 25(OH)D, as possible confounders, were determined in serum samples. Dietary intake, including all supplements, was assessed by 3-day dietary record and analyzed using Food Processor. Past physical activity and present walking were examined as well and accounted for as potential confounders. Simple and multiple regression models were created to assess the relationships between nutrients and BMD. To examine the co-linear variables and their possible independent association with bone, subgroup analyses were performed.

RESULTS

: Showed independent influence of calcium, energy, and protein, examined separately and in multiple regression models on BMD of several skeletal sites. Magnesium, zinc and vitamin C were significantly related to BMD of several skeletal sites in multiple regression models (controlled for age, fat and lean tissue, physical activity and energy intake), each contributing more than 1% of variance. Serum PTH and 25(OH)D did not show significant association with bone mass.

CONCLUSIONS

Despite the cross-sectional nature of our study we were able to show a significant relationship between BMD and several critical nutrients: energy, protein, calcium, magnesium, zinc and vitamin C. The exact involvement of these nutrients and their clinical significance in bone health need to be further elucidated in humans and conclusions about the effects of a single nutrient on bone mass must be given cautiously, taking into account its interaction and co-linearity with others. Understanding relationships among nutrients, not just limited to calcium and vitamin D, but others that have not been investigated to such extent, is an important step toward identifying preventive measures for bone loss and prevention of osteoporosis.

Nutrition in bone health revisited: a story beyond calcium

Osteoporosis is a complex, multi-factorial condition characterized by reduced bone mass and impaired micro-architectural structure, leading to an increased susceptibility to fractures. Although most of the bone strength (including bone mass and quality) is genetically determined, many other factors (nutritional, environmental and life-style) also influence bone. Nutrition is important modifiable factor in the development and maintenance of bone mass and the prevention and treatment of osteoporosis. Approximately 80-90% of bone mineral content is comprised of calcium and phosphorus. Other dietary components, such as protein, magnesium, zinc, copper, iron, fluoride, vitamins D, A, C, and K are required for normal bone metabolism, while other ingested compounds not usually categorized as nutrients (e.g. caffeine, alcohol, phytoestrogens) may also impact bone health. Unraveling the interaction between different factors; nutritional, environmental, life style, and heredity help us to understand the complexity of the development of osteoporosis and subsequent fractures. This paper reviews the role of dietary components on bone health throughout different stages of life. Each nutrient is discussed separately, however the fact that many nutrients are co-dependent and simultaneously interact with genetic and environmental factors should not be neglected. The complexity of the interactions is probably the reason why there are controversial or inconsistent findings regarding the contribution of a single or a group of nutrients in bone health.

Change in bone mass after Colles' fracture: a case report on unique data collection and long-term implications

The cast immobilization of a fractured limb results in a loss of bone mass; however, the long-term implications of that effect with regard to bone mineral status, particularly in other skeletal sites, are less known. The purpose of this study was to describe changes in bone mass in different skeletal sites triggered by Colles' fracture. The case is unique regarding the existence of baseline measurements taken just a few days before the fracture on all measurable skeletal sites, including the fractured radius. Therefore, it was also possible to determine whether the injury caused long-term bone loss in the affected and unaffected skeletal sites. The patient was a healthy, premenopausal Caucasian woman, in her late forties, who fractured her nondominant wrist as a result of low-impact fall on ice. The arm and the metacarpals were immobilized to the elbow for 5 wk. Bone mass measurements were performed with DPX-MD densitometer (Lunar Corp. Madison, WI) at baseline and 5, 10, 13, 21, and 52 wk postinjury. At the 5-wk measurement (on plaster removal) there was a notable increase in bone mineral density (BMD) and bone mineral content (BMC) in all sites of ulna and radius of the injured forearm (from 10 to 73%), followed by the apparent decline to or below the baseline at 10, 13, 21 and 52 wk of follow-up. Other skeletal sites were measured at 10 wk when a substantial decrease in BMD and BMC in some of the hip regions and lumbar spine was noticed; most notably in L3-L4, Ward's triangle, and femoral neck (from 2 to 8%) and remained such after 1 yr. Although this patient had a normal bone mineral status and no osteopenia detected before fracture, the trauma of radial fracture caused long-standing bone loss in fracture-prone areas-hip and spine. Because about 70% of bone strength is explained by its mineral density, the patient might be at increased risk for fracture later in life. The changes in bone mass after injury should be monitored and interpreted carefully, and more elaborate treatment of patients presenting with wrist fractures are needed to prevent any potential risk for later osteoporotic fractures in spine and hip and possible refracture of the injured extremity.

Relation of nutrition, body composition and physical activity to skeletal development: a cross-sectional study in preadolescent females

OBJECTIVE

To examine the relation of anthropometric and growth parameters (weight, stature, body composition, age, and skeletal age), nutritional factors, and physical activity to the total body and radius bone mineral density and content and radiogrammetry parameters of the second metacarpal.

STUDY DESIGN

The study was a cross-sectional evaluation of 456 healthy, Caucasian girls, ages 8 to 13 years. Multiple regression models were created based on Cp statistics to determine the association between bone parameters and various independent variables.

RESULTS

Mean calcium intake was 956+/-381 mg/day, about 20% below the RDA of 1200 mg/day and about 36% below the threshold intake of approximately 1500 mg/day. The most significant predictors for total body and radius bone mineral density were corresponding bone areas, lean body mass, body fat, skeletal age, dietary calcium, and stature (only for total body) with corresponding R2(adjusted) of 48% and 36%. The total body and radius bone mineral content was positively associated with corresponding bone areas, lean body mass, body fat, calcium intake, and skeletal age with corresponding R2(adjusted) of 86% and 72%. Energy expenditure (corrected for BMI) was stratified into quartiles and bone mass parameters were distributed accordingly. A statistically significant difference in total body and radius bone mineral density and content was noted between the fourth and lower quartiles (ANOVA, p<0.05 to p<0.0001).

CONCLUSION

The most significant predictors of bone mass in preadolescent females evaluated in this study are bone area, lean body mass, body fat, skeletal age and dietary calcium.

Leptin is inversely related to age at menarche in human females

Over the last century there has been a trend toward an earlier onset of menarche attributed to better nutrition and body fatness. With the discovery of the obesity gene and its product, leptin, we reexamined this hypothesis from a new perspective. As delayed menarche and leanness are considered risk factors for osteoporosis, we also evaluated the relation between leptin and bone mass. Body composition and serum leptin levels were measured, and the timing of menarche was recorded in 343 pubertal females over 4 yr. Body composition was measured by dual x-ray absorptiometry, and leptin by a new RIA. All participants were premenarcheal at baseline (aged 8.3-13.1 yr). Leptin was strongly associated with body fat (r = 0.81; P < 0.0001) and change in body fat (r = 0.58; P < 0.0001). The rise in serum leptin concentration up to the level of 12.2 ng/mL (95% confidence interval, 7.2-16.7) was associated with the decline in age at menarche. An increase of 1 ng/mL in serum leptin lowered the age at menarche by 1 month. A serum leptin level of 12.2 ng/mL corresponded to a relative percent body fat of 29.7%, a body mass index of 22.3, and-body fat of 16.0 kg. A gain in body fat of 1 kg lowered the timing of menarche by 13 days. Leptin was positively related to bone area (r = 0.307; P < 0.0001) and change in bone area (r = 0.274; P < 0.0001). A critical blood leptin level is necessary to trigger reproductive ability in women, suggesting a threshold effect. Leptin is a mediator between adipose tissue and the gonads. Leptin may also mediate the effect of obesity on bone mass by influencing the periosteal envelope. This may have implications for the development of osteoporosis and osteoarthritis.

Calcitriol and bone mass accumulation in females during puberty

Adolescence is characterized by rapid skeletal development and high demands for bone minerals. Though the stimulative effect of calcitriol on intestinal calcium and phosphorus absorption is well understood, its effect on bone development is not completely clear. It may be directly involved in the facilitation of calcium economy during this critical phase of skeletal development. Therefore, we evaluated the serum concentrations of calcitriol in relation to skeletal development in a cross-sectional study of 178 healthy Caucasian females during different pubertal stages, extending from childhood to young adulthood. In addition, a subsample of 57 younger girls was followed for a 1-year period to evaluate the association among serum calcitriol, nutrition parameters (dietary calcium, phosphorus, and vitamin D), bone mass accumulation, and biochemical markers of bone turnover. The serum calcitriol concentration in a cross-sectional sample was the highest during pubertal growth spurt (sexual maturity index 3-4, age 11-13 years) (ANOVA; F = 2.4945; P = 0.0329). This correlated to the peak skeletal calcium accretion (g/year) and bone mass accumulation in total body and forearm. In a longitudinal sample, there was a positive association between annual change in TBBMC (P = 0.0255); TBBMD (P = 0.0168); proximal radius (1/3 distance from styloid process) BMC (P = 0.0096); BMD (P = 0.0541), and baseline calcitriol level in forward stepwise regression analyses. The results of the forward stepwise regression analyses with serum calcitriol as a dependent variable and different serum, urinary, and dietary parameters measured at baseline (age 11 years, n = 114) and after 1 year (age 12 years, n = 57) showed that osteocalcin was positively associated with calcitriol in both years; more so in a second year (P = 0.0514, P < 0.001, respectively). Dietary vitamin D and phosphorus showed negative association with serum calcitriol at age 11, and dietary Ca and P were selected at age 12. The results of this study show that calcitriol is a significant correlate of bone mass accumulation during pubertal growth, presumably in response to the high requirements for calcium during this critical phase of skeletal development.

Gain in body fat is inversely related to the nocturnal rise in serum leptin level in young females

Both genetic and environmental factors contribute to adolescent obesity. Evidence of a genetic basis for obesity development is substantial, although the exact mechanism of action has yet to be identified. The purpose of this study was to document the circadian rhythmicity of the serum leptin level in young females and to assess the impact of the change in body fat stores during growth on the nocturnal rise in the serum leptin level with implications for obesity traits. There was a significant rise in serum leptin at midnight and 0400 h, suggesting a diurnal variation in serum leptin concentrations (ANOVA F ratio = 6.2; P < 0.0001). There was also a strong association between relative total body fat and the average daytime serum leptin level (r = 0.78; P < 0.0001). The percent increase in the nocturnal leptin concentration was inversely related to the percent gain in total body fat (r = 0.45; P < 0.024). Forward stepwise regression analysis selected the change in total body fat over a 6-month interval as the most powerful determinant of the percent increase in the nocturnal leptin concentration (partial R2 = 0.203; beta = -0.450; SE of beta = 0.186; t = -2.418; P < 0.024). If the lack of a nocturnal rise in serum leptin persists over a longer period of time, it may have implications for the development of obesity, presumably by inadequate suppression of nighttime appetite.

Zinc balance in adolescent females consuming a low- or high-calcium diet

There is increasing evidence that calcium intake up to the threshold amount (1480 mg/d) increases bone mass during growth. However, there is concern that such a high calcium intake may interfere with the utilization of other nutrients such as zinc, which is also important for skeletal development. The purpose of our study was to investigate the effect of long-term calcium supplementation on zinc utilization in 26 adolescent females (mean +/- SD age 11.3 +/- 0.5 y) during a 14-d period. Each day subjects consumed a metabolic diet containing 722 mg Ca and 6.3 mg Zn. Participants were randomly assigned to receive either a placebo or a calcium supplement containing 1000 mg supplemental Ca/d as calcium citrate malate. Supplementation began 15 wk before the balance period to allow for adaptation to the greater calcium intake. Mean (+/-SD) zinc balance (0.8 +/- 0.8 compared with 0.3 +/- 1.1 mg/d, P = 0.23), fecal zinc (4.3 +/- 0.6 compared with 4.7 +/- 1.4 mg/d, P = 0.27), urinary zinc (0.4 +/- 0.2 compared with 0.5 +/- 0.1 mg/d, P = 0.55), and net zinc absorption (21% compared with 15%, P = 0.33) were not significantly different between the high- and low-calcium groups. Our results suggest that increasing the recommended dietary allowance of calcium to 1500 mg/d as recommended by the National Institutes of Health consensus panel will not have adverse effects on zinc utilization in adolescent females.

Skeletal age as a determinant of bone mass in preadolescent females

OBJECTIVE

To evaluate the association between chronological age, skeletal age, pubertal stage, and basic anthropometry with bone mass of the total body, forearm, and second metacarpal bone in 456 healthy Caucasian females, aged 8-13 years.

DESIGN

Total body and forearm bone measurements were performed by dual X-ray absorptiometry, while bone mass of the second metacarpal was assessed by radiogrammetry. Skeletal age (SA) was assessed by the FELS method and pubertal stage was self-determined by selecting corresponding illustrations of breast and public hair development. The Cp criterion was used to select the best multiple regression model containing the subset of independent variables with the least bias and best predictive ability for each of the measured bone mass variables.

RESULTS

Of all the independent variables, weight, stature, and SA emerged as the most significant predictors for almost all the bone mass variables. Multiple regression models were created based on the Cp criterion with the resulting R2 (adjusted) for bone mineral content of total body, proximal forearm, ultradistal forearm, length of second metacarpal, as well as of total, medullary, and cortical areas: 0.793, 0.523, 0.390, 0.602, 0.232, 0.073, and 0.264, respectively. The measured bone variables were also regressed on SA using either quadratic or linear equations, depending on the shape of the cubic splines used for the best curve fitting. Significant positive association (p < 0.0001) of SA and each of the bone variables was noted, the highest being with bone mineral density and content of total body (R2 = 0.176, 0.338) and proximal and ultradistal forearm (R2 = 0.216, 0.203, 0.106, 0.201), respectively, as well as with the length of the second metacarpal bone (R2 = 0.339). Chronological age and pubertal stage did not have statistically significant predictive abilities for bone mass variables in the multiple regression models.

CONCLUSIONS

We conclude that skeletal age is a powerful determinant of bone mass in children. It can be used as the criterion for the selection of a biologically homogeneous population with regard to bone mass. This may be important for the design of intervention studies targeting bone mass of children and adolescents.

Magnesium balance in adolescent females consuming a low- or high-calcium diet

Increasing emphasis is being placed on optimizing calcium intake during growth as a way to enhance peak bone mass. Although some studies in adults have shown that high calcium intake may negatively affect magnesium utilization, few data are available regarding the interaction of calcium and magnesium in healthy children. The purpose of our study was to measure the effect of calcium intake on magnesium balance in 26 adolescent girls (mean age 11.3 y) during a 14-d period. Subjects ate a controlled basal diet containing 667 mg Ca and 176 mg Mg. In addition to the basal diet, subjects were randomly assigned in a double-blind fashion to consume 1000 mg elemental Ca/d as calcium citrate malate or a placebo. Magnesium use did not differ between the low-calcium and high-calcium groups as measured by absorption (50% compared with 55%), urinary excretion (70 compared with 74 mg/d), and fecal excretion (88 compared with 79 mg/d). Accordingly, magnesium balance was not different in subjects consuming 667 or 1667 mg Ca/d and averaged 21 mg Mg/d for the whole study group. Magnesium balance was significantly correlated with magnesium intake (r = 0.511, P = 0.008) and magnesium absorption (r = 0.723, P < 0.001). Prediction intervals from the regression of magnesium balance on intake indicated that the current recommended dietary allowance of magnesium would result in magnesium balance > or = 8.5 mg/d in 95% of the girls. This value appears consistent with long-term accretion rates needed to account for the expansion of the total-body magnesium pool during growth. In summary, our observations support the adequacy of the current recommended dietary allowance for magnesium and indicate that alterations in magnesium utilization should not be anticipated in adolescent females consuming a high-calcium diet.

Urinary calcium, sodium, and bone mass of young females

Calcium is an important determinant of peak bone mass in young adults because of its influence on skeletal development during growth. Attainment of maximum peak bone mass requires optimal positive balance between calcium intake and obligatory losses of calcium, primarily in urine and feces. Urinary excretion is an important determinant of calcium retention in the body. Accordingly, the purpose of this study was to evaluate the influence of various nutrients on urinary calcium excretion, and to assess their impact on bone mass of young females, aged 8-13 y, during early puberty. The study was conducted in 381 healthy white females in pubertal stage 2. From each participant we collected basic anthropometric measurements, a 3-d food record, blood, a 24-h urine sample, and bone mass measurements of the total body and forearm by dual X-ray absorptiometry. Urinary sodium was found to be one of the most important determinants of urinary calcium excretion: [urinary calcium (mmol/d) = 0.01154 x urinary sodium (mmol/d) + 0.823], whereas calcium intake had relatively little impact: [urinary calcium (mmol/d) = 0.02252 x calcium intake (mmol/d) + 1.5261]. Urinary calcium was much higher at a calcium intake of approximately 37.5 mmol/d (1500 mg/d), supporting the notion that calcium is a threshold nutrient. Calcium intake had a significant positive influence on the bone mineral content and density of the whole body and radius shaft whereas urinary calcium had a negative influence, presumably by reducing calcium accretion into the skeleton.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of single photon and dual X-ray absorptiometry of the forearm in children and adults

We compared single photon absorptiometry (SPA) to dual x-ray absorptiometry (DXA) for determination of bone mineral content (BMC), bone mineral density (BMD), and bone width (BW) of the forearm. The SPA and DXA measurements were done on the same subjects, using Lunar densitometers. The measurements were performed over the proximal radius (1/3 shaft) of the nondominant arm in 285 healthy, Caucasian females and males, ages 9-53. Correlation, linear, and split regression analyses for all subjects, and for subgroups (adults and children), were performed to compare SPA and DXA measurements. Corresponding measurements performed on two densitometers were highly correlated: r = 0.987, 0.975, and 0.943 for BMC, BMD, and BW, respectively. The corresponding measurements were also very similar in value, ranging from 0.9% to 4.1% difference, although they were different statistically. Correlations dropped slightly when subjects were separated into adult and children subgroups, and therefore, split regression analysis was performed resulting in R2 (adjusted) values of 97.6%, 95.5%, and 89.0% for BMC, BMD, and BW, respectively. Because the group indicator was statistically significant (p < 0.001) only for the BMC measurements but not for BMD and BW, linear regression of the whole sample was done as well. The difference in fitted values between the two regression methods was insignificant; therefore, we concluded that linear regression was sufficient for description of the relationship between SPA and DXA measurements. The precision study showed that the DXA had better reproducibility than SPA. The DXA precision in vivo (CV%) for BMC, BMD, and BW was 1.06, 0.83, and 0.95, respectively; and the SPA precision for same variables was 2.08, 2.12, and 0.95, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Timing of peak bone mass in Caucasian females and its implication for the prevention of osteoporosis. Inference from a cross-sectional model

To determine the timing of peak bone mass and density, we conducted a cross-sectional study of bone mass measurements in 265 premenopausal Caucasian females, aged 8-50 yr. Bone mass and bone mineral density were measured using dual X-ray absorptiometry and single-photon absorptiometry at the spine (anteroposterior, lateral), proximal femur, radius shaft, distal forearm, and the whole body. Bone mass parameters were analyzed using a quadratic regression model and segmented regression models with quadratic-quadratic or quadratic-linear form. The results show that most of the bone mass at multiple skeletal locations will be accumulated by late adolescence. This is particularly notable for bone mineral density of the proximal femur and the vertebral body. Bone mass of the other regions of interest is either no different in women between the age of 18 yr and the menopause or it is maximal in 50-yr-old women, indicating slow but permanent bone accumulation continuing at some sites up to the time of menopause. This gain in bone mass in premenopausal adult women is probably the result of continuous periosteal expansion with age. Since rapid skeletal mineral acquisition at all sites occurs relatively early in life, the exogenous factors which might optimize peak bone mass need to be more precisely identified and characterized.

Calcium requirements for growth: are current recommendations adequate?

Peak adult bone mass is determined in each individual by a combination of endogenous and environmental factors. Insufficient accumulation of skeletal mass by the time young adulthood is reached appears to enhance the likelihood of fractures later in life. It is speculated that environment (nutrition and exercise) contributes to about 20% of the variance in bone mass. Although much is yet to be learned about how diet contributes to skeletal growth and development, it now appears that calcium intake may be an important factor in the attainment of peak bone mass. A review of the scientific literature suggests that the current recommended dietary allowance (RDA) for calcium may not be high enough to optimize the genetically programmed peak bone mass in a substantial number of growing individuals. New standards for dietary calcium intakes during growth may be indicated.

The oxidation of 3-hydroxybutyrate in developing rat jejunum

Ketone bodies and glutamine are the primary oxidative substrates in the intestine of fasting adult rats. Because suckling rats consume the majority of their calories in the form of lipid and as a result have elevated blood ketone concentrations similar to fasting adult rats, we examined the role of ketone oxidation as a source of energy in the intestine of suckling rats. In suckling rats the rate of [3-14C]DL-3-hydroxybutyrate oxidation in intestinal tissue slices, enterocytes, and intestinal mitochondria was 0.15 +/- 0.01 nmol/h/mg wet wt, 8.6 +/- 1.1 nmol/h/mg protein and 8.3 +/- 1.3 nmol/h/mg protein, respectively. In suckling rats the rate of intestinal oxidation of [3-14C]DL-3-hydroxybutyrate was 75% lower than the rate in weaned rats in intestinal tissue slices, 55% lower in enterocytes, and 50% lower in intestinal mitochondria. The activity of two enzymes involved in ketone oxidation, 3-hydroxybutyrate dehydrogenase, and acetoacetyl-CoA thiolase was 38% and 55% higher in intestinal homogenates of suckling rats compared with adult rats. In contrast, 3-oxoacid CoA-transferase activity, an enzyme also involved in 3-hydroxybutyrate oxidation, was one third that of adults. These studies indicate that intestinal oxidation of 3-HB occurs in suckling rat pups, but is lower than the rate of oxidation in the intestine of weaned rats.

The effect of fasting on rat portal venous and aortic blood glucose, lactate, alanine, and glutamine

Using a chronically catheterized rat model, the effect of fasting on portal venous, aortic, and venous blood concentration minus aortic blood concentration ([PV-A]) blood glucose, lactate, alanine, and glutamine concentrations was determined. It has been postulated that the intestine is a source of lactate and alanine, precursors for glycogen synthesis, in the fed state. After 48 h of fasting portal venous glucose, lactate, and alanine blood concentrations decreased by 31, 28, and 41%, respectively. Portal venous glutamine concentration was not affected by fasting. A glucose [PV-A] was not found in either fed or fasted states. Whereas the lactate [PV-A] was not present in fed rats, it was negative in fasted rats. Alanine [PV-A] was positive in fed and fasted rats. The glutamine [PV-A] was negative in fed and fasted rats. These data indicate that portal venous concentrations of the gluconeogenic precursors, lactate and alanine, decrease in fasted rats. In fasted rats intestinal utilization of lactate increases as reflected by a negative [PV-A]. Fasting did not affect alanine production by the intestine or glutamine utilization. Despite these changes with fasting, we conclude that the intestine does not appear to be able to maintain portal venous blood concentrations of gluconeogenic precursors.