Jason Boyd and Dr. Bruce H. Woolley, Food Science and Nutrition
Introduction
Peak bone mass of pre-menopausal women has been established as a major risk factor for postmenopausal osteoporosis and related bone fractures (4). Previous research evidence suggest that bone mass of both the peripheral and axial skeleton is stable for women before menopause (1,2). Teegarden et al.(3) established that peak Bone Mineral Content (BMC) is attained by age 26.2+/- 3.7 years. After peak BMC is obtained, mass of cortical and trabecular bone and BMC will decline at a rate of approximately 1.2% per year.
Several factors have been demonstrated to affect peak BMC. Body weight is positively associated with osteoporosis. Some studies (18) have shown very little or no incidence of osteoporotic fractures in women weighing more than 70 kg. Genetic background has been demonstrated (4,5) as a significant component. Physical activity, specifically weight-baring exercise, has been well established as a means of increasing peak BMC and reducing bone calcium losses (8-9,11-13). We will account for each of these factors in phase I of our study to identify the early markers of bone density concerns and their relative significance. Those factors which cannot be measured in the lab will be accounted for by a well constructed survey (attached).
The evidence regarding dietary considerations in maximizing peak BMD remains inconclusive. Calcium intake has been documented by some (6,7,10) as positively correlated with peak bone density. Several researchers (18-22) have conducted studies utilizing calcium supplementation of 500 to 1500 mg and found significant increases in BMC and BMD. Yet, others have found no significant change in bone mineral content with increased calcium intake. Friedlander, et al, (13) found no positive effect on bone parameters with calcium supplementation of up to 1500 mg per day. Mazess and Barden (4) also found no association of nutrient intake, calcium included, on BMD in healthy women aged 20-40.
Phosphorous is required to store calcium in the bone in the form of calcium phosphate. Lukert, et al, (lukert) found that dietary calcium to phosphorous ratio is important in maintaining bone density. Magnesium is also important in the PTH-vitamin D pathway of calcium storage and in maximizing BMC. Tranquilli, et al (17), have demonstrated that dietary calcium and magnesium levels are significantly reduced in osteoporotic women and were lower than the RDA in most women. Further, New et al. (1997) reported lower lumbar spine and trochanter BMD in middleaged pre-menopausal women with concomitant low intakes of magnesium in early adulthood. The results of these studies indicate a possible benefit for dietary supplementation of these minerals in pre-menopausal women as a prophylaxis to prevent osteoporosis.
Purpose
This data represents the first phase of a two-phase study. The purpose of Phase I was twofold: first, we wanted to identify several markers of bone mineral insufficiency in young, healthy women ages 18-24 years of age. While measuring BMD, we accounted for physical activity, calcium and magnesium intake, and family history. The second purpose of Phase I was to establish baseline data for the longitudinal Phase II and to confirm the justification for mineral supplementation in the pertinent population.
Phase II, which recently began, will attempt to establish the relationship between BMD improvement and calcium and/or magnesium supplements in young women. We will monitor the effects of each supplement group based on increased levels of calcium and magnesium.
Methods
Subjects
We recruited approximately 200 healthy volunteer female subjects (with ages ranging from 18 to 24) from Brigham Young University. Each subject completed a five page questionnaire which accounted for family history of BMC insufficiency, physical activity for the last five years, and a three day diet record.
Initial elimination criteria included pregnancy and recent ankle fractures. Phase two subjects will be pre-screened for those who will remain in the Provo area until at least May of 1999 so they may complete the study. We will eliminate those who are on birth control and those who plan to be pregnant. Any women with a critically low BMC will be eliminated from the study and will be counseled on improvement of bone density.
Bone Densiometry
We performed single energy X-ray absorptiometry of the Os Calcis (heel) on all of the subjects. We use a commercial instrument (Osteoanalyzer SXA 3000 by Dove) for all of the tests. The heel from the subject’s dominant leg is analyzed. The machine takes fourteen X-ray scans of the heel and generates a score to four significant figures of the Bone Mineral Content of the heel. The process requires about 195 seconds to complete for each subject.
Other Factors Studied
Each subject will fill out the questionnaire (attached) and an informed consent form (attached). The survey includes family history of osteoporosis or related problems, current and past physical activity levels, current and recent dietary intakes of calcium and magnesium, and a record of long-term amenorrhea. Calcium and magnesium intake levels will be assigned according to servings of various foods and supplements containing the minerals.
Physical activity levels include occupations, sports and exercise programs over the past five years. Each subject will complete a survey outlining their physical activity. The total activity will be measured based on Ainsworth et al’s Compendium of Physical Activity [21], an established comprehensive list of MET values for different activities. Each subject was given an average per day MET score with which to compare their BMC. Factors affecting the average per day MET score included intensity, frequency, and duration of the weight-bearing physical activities.
In the laboratory, we will also account for height and weight using a calibrated scale (Detecto). We will then estimate lean body mass by ascertaining body fat percentage using the electrical impedance method. This method involves sending a small electric current through the subject’s body via electrodes on the hands and feet. There is absolutely no dangerous exposure for the subject.
Data Analysis
Phase I: Analysis of the data was carried out using simple regression analysis to ascertain whether there is a significant correlation between bone mineral content and the following five factors: 1) physical activity, 2) intake of calcium, 3) intake of magnesium, 4) lean body mass, 5) body fat percentage and incidence of osteoporosis, supplement intake and bone density. The null hypothesis was to be rejected in each of the five analyses if p
Phase II: We will assign the sample to four cohorts of approximately 20 subjects each. Cohort I will be supplemented with 750 mg of a commonly marketed calcium salt. Cohort II will be supplemented with 750 mg of a commonly marketed magnesium salt. Cohort III will be supplemented with both the calcium salt and the magnesium salt. Cohort IV will be a control group and will be given a placebo (maltodextrin). Once the calcium and magnesium salts have been determined, they will be adjusted to assure equivalent doses of the elemental minerals.
A one-way ANOVA will be used to ascertain differences between the experimental groups and pre- and post-test values. When a significant F-ratio is detected, a Tukey HSD post hoc test will be used to analyze differences between means. The null hypothesis will be rejected when p<0.05.
Risks and Benefits
Risks: Risk due to x-ray during bone densiometry was minimal and was fully described in the consent form. Each scan has a radiation dose of 1.3 mRem, which is much less than that of a typical x-ray. Total x-ray exposure is less than the amount received in a year from background radiation. There will be no risk during phase II due to any of the supplements as all of the doses are at levels which have been established as safe by the FDA (RDA amounts).
Benefits: We believe that those in the supplemented groups will experience a significant gain in Boner Mineral Content over the course of the study. We will also use this study as an opportunity for individual health improvement. The subjects in phase I who are not chosen for the study will be offered recommendations for dietary and exercise improvements to minimize risk of osteoporosis. At the conclusion of the study, those in phase II will have the same opportunity. Until then, we will ask phase II subjects to maintain as constant a lifestyle as possible.
Data/Results of Phase 1
Multiple regression analyses were performed incorporating physical activity, calcium intake, magnesium intake, lean body mass, body weight in correlation to BMC (n=125).
Physical Activity: We found that a five year history of physical activity was a significant indicator of BMC for young adult women. (r=0.621, p<0.05). Subjects in the highest quartile of physical activity had an average BMC of 494.2 mg/cm2, whereas those in the lowest quartile had an average BMC of 396.5, a 20% decrease. (2nd quartile=471, 3rd=426). The most influential physical activities were those involving weight bearing exercise.
Mineral Intake: Regression analysis indicated that intakes of calcium and magnesium do not significantly correlate to BMC. BMCs in the lowest quartile of each mineral intake were not significantly lower than those of the highest quartile. However, the lowest quartile of calcium intakes did have the lowest average BMC (429 mg/cm2). Each quartile of magnesium intakes had a BMC very close to that of the overall average (448.6+/- 10 mg/cm2).
Body Weight and Body Fat: Body weight is a significant indicator of BMC (r=0.4261, p<0.05). Those in the highest quartile of body weight had an average BMC of 475.3 mg/cm2 while those in the lowest quartile were significantly lower (400.77 mg/cm2). Body fat percentage was not a significant indicator of BMC (r=0.1384, p=0.124) although the highest quartile of body fat percentage had by far a higher average BMC (472.2 mg/cm2) than the other three quartiles.
Discussion
Our study suggests that the most important factor in maximizing bone mineral content is weightbearing physical activity. Those with the greatest amounts of physical activity statistically tent to have higher bone mineral contents. The effect of diet on BMC remains inconclusive. Phase II, currently in operation, will test the effects of mineral supplementation on bone density over a two year period. While genetic predisposition is perhaps more important, we were unable to conduct complete examinations into family history of bone related illnesses in our subjects.
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