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Difference in Bone Mineral Density Between Young Versus Midlife Women Sonya Sanderson,a* Pamela S. Anderson,b and Melissa J. Bentonc

aValdosta State University, Kinesiology and Physical Education; bGeorgia Gwinnett College, School of Science & Technology; cUniversity of Colorado at Colorado Springs, Helen and Arthur E. Johnson Beth-El College of Nursing and Health Sciences

ABSTRACT Background: Older age is a risk factor for low bone mineral density (BMD). Older women have been found to have lower BMD than younger women. Recent trends for decreased calcium consumption and physical activity may place younger women at greater risk than previously anticipated. Purpose: The purpose of this study was to evaluate the effect of age on BMD in young (19.1 ^ 0.4 years) versus midlife (46.3 ^ 0.4 years) women. Methods: BMD was assessed with the Alara MetriScan. Results: Young women had lower BMD compared to midlife women (T score: 21.18 ^ 0.78 versus 0.25 ^ 0.12; P , .05), lower body weight (65.6 ^ 1.6 kg versus 79.6 ^ 1.9 kg, P , .001), and lower body mass index (BMI; 23.7 ^ 0.6 versus 29.3 ^ 0.7 kg/m2, P , .001). Discussion: Based on these data, younger women are at greater risk for loss of BMD and early onset osteoporosis than their older counterparts. Translation to Health Education Practice: Health Educators should teach that calcium and vitamin D supplementations are vital in maintaining bone mineral density throughout an individual’s life span.

ARTICLE HISTORY Received 1 September 2015 Accepted 11 November 2015

Background

Population studies demonstrate that bone mineral density (BMD) is lost with age,1 resulting in decreased bone strength and increased fracture risk.2 Women are considered at greater risk than men, primarily due to lower absolute (peak) bone mass.1 Peak bone mass is the point at which bone density is strongest during the life span.3 Women develop peak bone mass in late adolescence and as much as 70% should be deposited by age 20.4 After the age of 25, BMD is lost at a rate of approximately 0.5% per year until menopause, when losses increase at an even greater rate.5 Nutrition and physical activity during childhood and early adolescence exert a strong influence on accretion of BMD.6-8

Nutritional risks include inadequate daily calcium intake. Current research demonstrates that females are not consuming sufficient amounts of calcium to maximize bone density.9 This results in continual reabsorption of calcium from the bones to maintain metabolic function, weakening the bones over time and increasing the risk for osteoporosis.10 As found in multiple research studies, calcium malnutrition can lead to other chronic diseases beyond osteoporosis. Calcium and vitamin D deficits increase the risk of malignancies,

particularly of colon, breast, and prostate gland; chronic inflammatory and autoimmune diseases; as well as metabolic disorders.11 Although the recommended calcium intake for adolescent females is 1300 mg a day,8 it is estimated that more than 75% of young women in this age group are not consuming the recommended amount.9 The lifetime risk of any fracture of the hip, spine, or distal forearm is almost 40% in white women from age 50 years onward. Overall health costs associated with fractures are expensive, increased societal attention must be given to the design and implementation measures to decrease these chronic issues.12

In addition to inadequate nutrition, lack of physical activity has a negative effect on accrual of peak bone density. Although strength-based and high-impact sports in particular are associated with higher BMD, any weight-bearing exercises including walking, hiking, jogging, stair climbing, weight training, tennis, and dancing can be beneficial.13 Unfortunately, over the past 15 years there has been a decline in physical activity among adolescent girls compared to boys.14 Current data from the Youth Behavior Risk Factor Surveillance Survey indicate that as much as 30% of adolescent females are essentially sedentary and never achieve the recommended 60 minutes a day of physical activity.15

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CONTACT Sonya Sanderson slsanderson@valdosta.edu Valdosta State University, Kinesiology and Physical Education, 1500 N. Patterson St., Valdosta, GA 31698.

AMERICAN JOURNAL OF HEALTH EDUCATION 2016, VOL. 47, NO. 3, 149–154 http://dx.doi.org/10.1080/19325037.2016.1157533

Historically, age has been considered a primary risk factor for osteoporosis. After 40 years of age, bone density losses increase dramatically, and by age 50 women are found to have significantly lower BMD than women who are 20 years of age.16 However, recent trends for decreased dairy consumption and physical activity may place younger women at greater risk than previously anticipated. Parental encouragement to obtain adequate nutrition and exercise during adolescence has decreased over time,17 and since 1970, the consumption of dairy products, especially milk, has decreased by approximately 30%.18

Purpose

Current trends for inadequate consumption of calcium- rich foods and inadequate participation in physical activity may place younger women at greater risk for low BMD than women who are older and may not have had similar lifestyle risks. Therefore, the purpose of this study was to compare BMD and lifestyle characteristics in young versus midlife women.

Methods

Sample and recruitment

Young women between 18 and 20 years old were recruited from a college campus in the southeastern United States, and midlife women between 40 and 50 years old were recruited from the surrounding community. Exclusion criteria were current pregnancy or diagnosis of any type of cancer. Approval for this study was granted from the Valdosta State University Institutional Review Board. After informed consent, all participants completed a one- time assessment that included a self-reported demo- graphic and lifestyle questionnaire regarding calcium intake and exercise behaviors based on the Behavioral Risk Factor Surveillance System,19 followed by bone density measurement. The principal investigator directly super- vised all data collection.

Calcium intake

Two questions were used to assess calcium intake. Participants were asked to report regular consumption of dietary calcium (calcium containing foods such as milk, yogurt, or cheese) as number of servings per day and per week. Use of calcium supplements was reported as yes or no.

Exercise behaviors

Two questions were used to assess exercise behaviors. Participants were asked to report frequency of exercise as

hours per week. The second question asked them to categorize the intensity of exercise as recreational, moderate intensity, or high intensity.

Bone mineral density assessment

A detailed account of the BMD assessment procedure has previously been published.20 Briefly, the Alara MetriScan (Alara, Inc., Fremont, CA) was used for BMD assessment of the nondominant hand using predeter- mined procedures provided by the manufacturer.21,22

The Alara MetriScan is a peripheral x-ray absorptio- metry device that has been found to have good correlation with axial BMD measurement.22,23 It has previously been used to assess BMD in healthy adult women over the age of 18.22,24,25 For purposes of analysis, BMD was reported as T scores, which are commonly used for classification of BMD and diagnosis of osteoporosis and osteopenia. Specifically, T scores less than 21.0 and greater than 22.5 are indicative of osteopenia, and T scores equal to or less than 22.5 are indicative of osteoporosis.26

Data analysis

Statistical analysis was performed using SPSS version 21.0 (IBM Inc., Chicago, IL). Statistical significance was set at P , .05 and all data are reported as mean and standard error (SE). Descriptive statistics were used to assess participant characteristics. Analysis of variance was used to compare between-group differences based on age and lifestyle factors related to exercise and nutrition. Variables for age, T score, servings of calcium (dairy foods) per week, and hours of exercise per week were analyzed as continuous data, and use of calcium supplements (yes/no) and exercise intensity (rec- reational/moderate–high) were dichotomized. Pearson correlations were used to identify relationships between continuous variables and Spearman correlations were used to identify relationships with dichotomous variables.

Results

Seventy-seven young (19.1 ^ 0.4 years) women and 101 midlife (46.3 ^ 0.4 years) women completed the study (Table 1). Young women had lower body mass than midlife women (65.6 ^ 1.6 versus 79.6 ^ 1.9 kg; P , .001) and met the criteria for normal weight (body mass index [BMI] 23.7 ^ 0.6 kg/m2), whereas midlife women were overweight (BMI 29.3 ^ 0.7 kg/m2; P , .001). Midlife women had significantly higher BMD than young women (T score 0.25 ^ 0.12 versus

150 S. SANDERSON ET AL.

21.18 ^ 0.78; P , .05; Figure 1), and young women reported significantly more exercise than midlife women (9.1 ^ 0.7 versus 4.2 ^ 0.4 hours per week; P , .001). Both groups reported similar intake of servings of calcium (dairy foods) per week. When dichotomized by calcium supplementation (yes/no), participants reporting regular use of supplements were significantly older (P , .001). By comparison, when dichotomized by exercise level (recreational/moderate–high intensity), participants engaged in moderate- to high-intensity exercise were significantly younger (P , .001). Pearson correlations demonstrated a significant inverse relation- ship between age and hours of exercise per week (r ¼ 20.42; P , .001). Spearman correlations demon- strated a significant direct relationship between age and use of calcium supplements (r ¼ 0.34; P , .001) and a significant inverse relationship between age and exercise intensity (r ¼ 20.38; P , .001).

Discussion

Our finding that BMD was greater in midlife women compared to young women is a novel one and differs from previous research. Over 30 years ago, Wall et al.27

reported loss of bone density with age. Ten years ago, Riggs et al.1 also reported lower bone density in older women compared to younger women. Although it is possible that the young women in our study may not have fully achieved peak bone density, we do not believe that this adequately explains the significant difference in T scores we observed. Furthermore, the majority of bone mass should be accrued by age 204 and the average age of our young group was 19 years. We believe that our findings most likely reflect current dietary and activity trends in adolescent females in the United States that have changed dramatically over the 30-year period since Wall and colleagues27 initially reported their findings.

The current study builds on previous research by Kalkwarf et al.,28 who reported that young women in the United States consumed inadequate dietary calcium to support normal bone density. Although both young and midlife women reported similar intake of calcium (dairy foods), they reported an average of only 7 servings a week, which does not meet the recommended intake of 3 servings per day needed for adequate bone health.29

Furthermore, we found that young women were more likely to report no use of calcium supplements. Although current inadequate calcium intake may not have had a profound effect on midlife women who had already achieved peak bone density, it is possible that it has had a significant effect on the young women in our study by impeding bone deposition to the extent that normal peak values have not been achieved. In addition, poor calcium intake in young women who are in the process of

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Criteria for osteopenia: T-score less than –1.0

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Figure 1. Comparison of mean T scores between young (age 18–20) and midlife (age 40–50) women. Young women had significantly decreased bone mineral density compared to midlife women (P , .001).

Table 1. Participant characteristics.a

Young (n ¼ 77) Midlife (n ¼ 101) Age (years) 19.1 ^ 0.4 46.3 ^ 0.4** Body mass (kg) 65.6 ^ 1.6 79.6 ^ 1.9** BMI (kg/m2) 23.7 ^ 0.6 29.3 ^ 0.7** T score 21.18 ^ 0.78 0.25 ^ 0.12* Calcium (servings/week) 6.9 ^ 0.3 6.6 ^ 0.3 Exercise (hours/week) 9.1 ^ 0.7 4.2 ^ 0.4** Calcium supplements (%) Yes 13 37 No 87 63 Exercise intensity (%) Recreational 16 58 Moderate–high 84 42

BMI indicates body mass index. Data presented as mean ^ standard error or percentage of group participants.

*Significant difference between groups (P , .05). **Significant difference between groups (P , .001).

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accruing peak bone density may be compounded by lack of vitamin D. Although not assessed as part of this study, inadequate levels of vitamin D have been reported specifically for children in the southeast United States where our data were collected.30 Without adequate vitamin D and calcium, younger females may be depositing even less bone density than previously reported.

Amount and intensity of exercise were also found to be greater in young women who had lower bone density. This would appear counterintuitive, because exercise, especially at higher intensities, is believed to be protective of bone density. However, bone serves as a calcium repository from which calcium may be resorbed during exercise if not available from dietary sources. In fact, an acute bout of exercise has been observed to result in bone resorption for more than 24 hours, without evidence of bone formation.31 In contrast, oral calcium supplemen- tation prior to exercise can attenuate this effect and prevent bone demineralization.32,33 It is possible that the young women in our study who engaged in high levels of exercise may have actually depleted their bones of calcium as a result of insufficient dietary intake.

It is of clinical concern that the young women in our study fit the World Health Organization criteria for osteopenia based on T scores34 and, hence, we believe that our findings have significance for clinical manage- ment of women in the United States. Based on our findings, education targeting younger females may be needed, especially to promote maximal deposition of peak bone mass. Furthermore, routine BMD assessment for younger females may be clinically appropriate to capture the losses reflected by our data. For example, the Alara MetriScan used for the current study represents a noninvasive, relatively low-cost assessment strategy that involves little patient or clinician burden. Measurements take less than one minute, require no clothing adjustment, and avoid unnecessary exposure to radiation that would occur with dual x-ray absorptiometry. Routine assessment of BMD in community settings could provide valuable objective data to reinforce educational strategies.

We recognize that there are limitations to this study. Our grouping of participants by age was an a priori decision based on evidence demonstrating achievement of 60%–70% peak bone density between 18 and 20 years of age35 and significant losses between 40 and 50 years of age.16 Furthermore, this was an observational study that measured BMD at one time only, so causality cannot be inferred. Although both Pearson and Spearman corre- lations were significant, they were of moderate strength only. However, we believe that our results have clinical significance based on the potential long-term health

consequences if younger women continue the downward trend we observed in bone mass and nutritional intake to promote bone health. Based on the results of this study, younger women are at greater risk for loss of BMD and early onset osteoporosis than their older counterparts. If indeed the majority of BMD is accrued prior to the age of 20, then the young women in this study are unlikely to achieve the peak bone density values that must have been achieved by their midlife counterparts. Another limi- tation of the study was that the questionnaire used had not been previously validated, though we did use questions dealing with calcium and exercise that were similar to the Behavioral Risk Factor Surveillance System and other validated questionnaires dealing with calcium and exercise assessments.36

In this group of women, BMD was significantly lower in young women who were found to be osteopenic with insufficient calcium intake to support adequate depo- sition of peak bone density. Calcium intake and exercise intensity were potential moderators of this outcome. Although more work in this area is undoubtedly needed, our findings support the need for greater education to promote bone deposition and clinical attention to BMD assessment of younger women in order to identify risk prior to significant bone loss.

Translation to Health Education Practice

The significant findings in our current study should be confirmed by additional research. If the data are confirmed, health educators should include in Health Education and health promotion interventions that calcium and vitamin D supplementation are vital not only to peak bone density acquisition in childhood, adolescence, and young adulthood but critical to maintaining bone mineral density across the life span. Young women and girls are at particular risk of compromising their bone density and should be aware of the nutritional and exercise conditions that might impact their bones as they age. Weight-bearing exercise can improve bone density, but certain types of exercise can instigate bone resorption, which can exceed bone formation. In the past 5 years there has been increased popularity of fitness trends that are based upon high- intensity exercise (e.g., CrossFit, High-Intensity Interval Training, Boot Camp, Zumba, and Spinning). Thomp- son37-43 has documented this movement among exercisers worldwide. High-intensity exercise requires the body to increase its’ metabolic output, which escalates the need to liberate calcium from “storage” with the intention of meeting elevated muscular demand. Health Educators should draw the connection between current fitness trends that tend to be made up of high-intensity

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and generally high-impact activities and the physiology of bone remodeling. Correspondingly, it is important to reinforce the concept that bones are repositories of calcium and when muscular demand for calcium exceeds the amount of calcium and vitamin D ingested and readily available, the body will source the necessary calcium from its repository—the bones. Therefore, the processes that support the acquisition of peak bone mineral density should be reflected prominently across the health education curriculum of school-aged children, adolescents, and young adults.

Acknowledgments

This study was supported by Dr. Ryan C. Moorman, DC, Care Medical in Valdosta, Georgia, and by Donna K. Sledge, Abraham Baldwin Agricultural College.

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