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* Department of Medicine, Sarah W. Stedman Center for Nutritional Studies, Duke University Medical Center, Durham, NC;
MRC Human Nutrition Research, Cambridge, UK;
** Welch Center for Prevention, Epidemiology, and Clinical Research, Johns Hopkins University, Baltimore, MD;
Center for Health Research, Kaiser Permanente Northwest Region, Portland, OR;
Department of Medicine, University of Cincinnati, Cincinnati, OH;
INSERM research Unit 403 and Synarc, Lyon, France;
# Nestle Research Centre, Lausanne, Switzerland;
Duke Hypertension Center, Duke University Medical Center, Durham, NC
3To whom correspondence should be addressed. E-mail: Lin00004{at}mc.duke.edu.
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ABSTRACT |
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 TOP ABSTRACT SUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • bone metabolism • calcium metabolism • DASH diet • sodium • osteoporosis
Osteoporosis is a major public health problem because of the large health care costs associated with its common clinical consequence, skeletal fractures. Worldwide projections suggest dramatic increases in the prevalence of osteoporosis by 2050 (1), likely due not only to the aging population but also to adverse changes in lifestyle and diet (2).
Research on diet and bone metabolism or bone mineral status has focused primarily on the benefits of calcium and vitamin D (3). Other nutrients such as potassium and magnesium may also benefit bone metabolism, but a clear relationship has not been established (4). Further, cross-sectional studies have shown that a diet rich in fruits and vegetables is associated with higher bone mineral density (5–7). Plausible mechanisms for the effects include a lower dietary acid load and the promotion of a positive calcium balance from high potassium and magnesium intakes (8).
Dietary factors that may have a negative effect on bone health have also been identified, including a high sodium intake and high dietary acid load. Numerous studies have shown that a high sodium intake is associated with increased urinary calcium excretion (9,10) which, in combination with an inadequate calcium intake, may result in disturbances in calcium metabolism and bone loss mediated by a rise in parathyroid hormone (PTH)3 (11,12). Short-term intervention studies have shown that increased sodium intake increases bone resorption in postmenopausal women (13,14) but not in younger adults (13,15). The long-term effects of dietary sodium on bone mineral density are unknown (16,17). A longitudinal study documented a significant association between greater sodium excretion and greater bone loss in postmenopausal women (16).
A high dietary acid load may also have a negative effect on bone health by increasing calcium excretion and bone resorption (8). A plant-based diet has been suggested to have a lower acid load, whereas animal products increase acidity (18). Urinary calcium excretion has been reported to be lower when urine was more alkaline, and higher when urine was more acidic (19). Acidosis may also inhibit osteoblast function and increase osteoclast activity, thus limiting bone formation and increasing bone loss (20).
The Dietary Approaches to Stop Hypertension (DASH) diet is a calcium-rich diet that emphasizes fruits, vegetables and low fat dairy products; it has been shown to lower blood pressure effectively compared with a control diet that is typical of what many Americans eat (21). In addition to the rich calcium content, several aspects of this dietary pattern may potentially benefit bone including the lower acid load (18) and the higher fruit, vegetable, potassium and magnesium contents. Thus, the primary aim of this study was to determine the effects of the DASH diet, compared with a control diet, and three levels of sodium intake on serum markers of bone formation [osteocalcin (OC)], and bone resorption [C-terminal telopeptide of type I collagen (CTX)]. Secondary aims were to examine the effects of the diets and sodium levels on fasting serum PTH, 24-h urinary calcium and urinary cAMP levels.
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SUBJECTS AND METHODS |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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This study was ancillary to the DASH-Sodium trial and was conducted among all participants at two of the four clinical sites (Duke University Medical Center and Johns Hopkins University). The DASH-Sodium study was a multicenter, randomized feeding trial that determined the effects on blood pressure of three levels of sodium intake and two dietary patterns among adults with higher than optimal blood pressure (>120/80 mm Hg), including those with Stage 1 hypertension (140–160/90–95 mm Hg). Detailed descriptions of the trial design and its main results were published (22,23). Measurements of bone and calcium metabolism were included retrospectively for this ancillary study only.
The two dietary patterns included a control diet typical of what many Americans eat, and the DASH diet, which emphasizes fruits, vegetables and low fat dairy foods, includes whole grains, poultry, fish and nuts, and is reduced in fats, red meat, sweets and sugar-containing beverages. The DASH diet contains reduced amounts of total fat, saturated fat and cholesterol, and increased amounts of potassium, calcium, magnesium, dietary fiber and protein (Table 1). The three sodium levels were defined as "higher," "intermediate" and "lower" (Table 1). Within each sodium level, the daily sodium intake was proportionate to the total energy requirements of individual participants, so that larger or very active persons would receive more food and therefore more sodium than smaller or less active persons. The control and the DASH dietary patterns were composed with the higher, intermediate, and lower sodium levels. The nutrient composition of the diets was validated and monitored by chemical analysis.
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Participant eligibility.
Volunteers were eligible if they were 
22 y old and had a blood pressure of 120–159 mm Hg systolic and 80–95 mm Hg diastolic averaged over three screening visits. The trial targeted 50% enrollment of African-Americans and women. Exclusion criteria were heart disease, renal insufficiency, poorly controlled hyperlipidemia or diabetes mellitus, insulin-requiring diabetes, special dietary requirements, intake of >14 alcoholic drinks per week, or use of antihypertensive drugs or other medications that would affect blood pressure or nutrient metabolism. Participants were not allowed to take any mineral supplements during the entire study and were asked to discontinue any supplement use 1 mo before the beginning of the study.
Measurements.
During screening and the last week of each sodium period, a 24-h urine collection was obtained and a blood sample was collected after an overnight fast for 12–14 h. During the 24-h period of urine collection, specimens were refrigerated or kept in a cool place, then divided into aliquots within 24 h and frozen without additives at -70°C for later analyses. Blood specimens were centrifuged at 1500 x g for 15 min, processed and frozen within 2 h of collection. The serum and urine specimens were shipped on dry ice to the laboratories for all analyses and were still frozen upon receipt. The time interval between baseline and the beginning of the run-in period varied from a few days to a few weeks, depending on when the participants started the screening process.
Participants and dietary staff were not aware of the outcome data during the study; personnel involved in collection of outcome data were not aware of the diet assignment. Dietary adherence was assessed by review of a daily diet checklist, observation of on-site meal consumption and measurement of 24-h urinary excretion of sodium, potassium, phosphorus, and urea nitrogen.
Bone turnover was assessed using two markers, i.e., the serum marker of bone formation, OC, which is released by the osteoblast into circulation during the mineralization of newly synthesized collagen (24), and the serum marker of bone resorption, CTX, which is released from bone collagen into circulation following its degradation by osteoclasts. Although markers of bone resorption have been measured traditionally in urine, measurement of serum CTX offers advantages including greater precision and lower variability (25). Urinary calcium, cAMP and fasting serum PTH were measured to assess the effects of the two dietary patterns and sodium intake on calcium metabolism. Fasting serum PTH changes rapidly and may not represent dietary intakes closely; the cAMP level, measured over 24 h, may be a better indicator of PTH response.
Serum intact PTH was analyzed by a two-site immunoradiometric assay (Diagnostic Systems Laboratories, Webster, TX), which had an intra-assay CV of 
3.6% and interassay CV of 5%. Serum CTX was measured by an automatic analyzer (b Crosslaps/serum; Roche Diagnostics, Meylan, France) by a two-site assay using monoclonal antibodies raised against an 8 amino-acid sequence from the C-telopeptide of human type I collagen (26). The intra-assay CV was <3% and interassay CV was <8%. Serum total OC was measured on an automatic analyzer (N-Mid OC, Roche Diagnostics) by a two-site immunoassay recognizing the intact and N-terminal Mid fragment. Intra- and interassay CV were <6%. Serum CTX and OC were measured centrally in Synarc Laboratories (P. Garnero, Lyon, France). Urinary calcium was measured by colorimetry utilizing o-cresolphthalein complexone (Hitachi 917, Roche Diagnostics), with inter- and intra-assay CV of 1.2 and 1.5%, respectively. Urinary sodium was measured by ion selective electrode (Hitachi 917), with inter- and intra-assay variations of 1.0 and 1.1%, respectively. Urinary cAMP was measured by RIA (cAMP RIA 100; Immuno-Biological Laboratories, Hamburg, Germany). The intra-assay CV was <10% and the interassay CV was < 9%. Both urinary calcium and cAMP were expressed relative to urinary creatinine.
Statistical analysis.
A unified generalized estimating equation model with an exchangeable covariance matrix was used for all primary analyses. Baseline differences were examined by simple t test. Changes in serum OC and CTX over time were evaluated by ANOVA and Tukey’s test. All analyses were performed using the SAS software (version 8.2, SAS Institute, Cary, NC). The primary outcome variables were serum OC and CTX. In each model, body weight, clinical center, cohort and the baseline value of the outcome variable were represented as fixed effects; intervention feeding periods were included as random effects to allow for within-person correlation. The model included indicators of cohort, clinical center and one period carry-over. Specifically, a comparison of the diet effects (DASH vs. Control) was made within each sodium level and sodium effects (Higher vs. Lower; Higher vs. Intermediate; Lower vs. Intermediate) were compared within each diet. In addition, interaction tests were conducted to examine the hypothesis that all diet effects within each pair of sodium levels were the same. The linearity of the sodium effects within the control or DASH diet was assessed by comparing the change in the outcome variable going from higher to intermediate sodium with the corresponding change from intermediate to lower sodium levels. Subgroup data, as defined by hypertension status, race, gender and age, were presented mainly for descriptive purposes and to support interpretation of the primary results. Before any analysis of the data was conducted, we removed outliers using a conservative multivariate criterion. Data from five individuals with multiple extreme (outlying) values, likely nonphysiologic, were removed from analyses. We also performed the analyses including these outliers and the results did not differ. The following results do not include the outliers. Statistical significance was defined as P < 0.05.
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RESULTS |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Compared with the control diet, the DASH diet reduced serum OC by 8 to 11% at each sodium level (Table 4; P < 0.0001). The DASH diet also reduced serum CTX by 16–18% (P < 0.0001) compared with the control group. These findings appeared to be consistent among subgroups defined by hypertension status, race, gender and age (Table 5) but further confirmation is required in future studies. Serum PTH and urinary calcium did not differ between the DASH and control groups. Similarly, urinary cAMP (Table 4) did not differ between the DASH and control groups.
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There was no effect of sodium intake on serum OC (Table 4) in the control diet group. However, reducing sodium from the higher to lower level in participants consuming the DASH diet decreased serum OC by 3% (P < 0.01). Sodium intake did not affect serum CTX in either diet group. Fasting serum PTH was related to sodium intake in the control group only, and was reduced significantly when sodium was reduced from the higher to the intermediate or lower level (Table 4).
Urinary calcium excretion decreased dose dependently as sodium intake decreased in both diet groups (Table 4). When sodium intake was reduced from the higher to the lower level, urinary calcium decreased by 12 mg/g creatinine in subjects consuming the DASH diet (P < 0.05) and by 22 mg/g creatinine in the control group (P < 0.001). The decrease was greater when sodium was reduced from higher to either intermediate or lower than from intermediate to lower (P = 0.35 for linearity within the control diet and P = 0.52 for DASH diet). The ratios of urinary calcium excretion per 100 mmol increase in sodium excretion were 0.83 and 0.66 for the control and the DASH diet groups, respectively. Sodium intake and urinary cAMP were not correlated in either diet group.
Sequential changes in serum OC and CTX.
Regardless of the order of sodium intakes, we also examined the percentage change in OC and CTX from baseline as a function of time in response to the diets (Fig. 1). There was a significant diet group effect for both variables (P < 0.01). In response to the DASH diet, serum CTX declined rapidly by 12.7% from baseline at the end of the first 30-d feeding period, and by 16.8 and 20.2% at the end of the second and third feeding period, respectively. On the other hand, serum OC declined by only 5.5% at the end of the first feeding period and by 11.0 and 10.8% at the end of the second and third period, respectively. At every time point, each variable differed between the DASH and the control groups (P < 0.0001). There was a significant time effect for OC (P = 0.05) and it was almost significant for CTX (P = 0.06). There was a significant diet by time interaction (P < 0.0001) for OC only; periods 1 and 2 and 1 and 3 differed significantly.
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DISCUSSION |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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It is not clear whether the reduction in bone turnover is due to the higher calcium content of the DASH diet alone or to an additive effect of potassium, magnesium or other dietary factors. The mechanism of calcium’s effect on bone was elucidated by data from small, short-term studies in adults (29–31) and large, long-term studies in pre- and postmenopausal women (32–35). In those studies, calcium supplementation was associated with a significant reduction in bone resorption alone or both resorption and formation markers, depending on the duration of the intervention.
Our results are presented in terms of sodium levels, which may make it seem as if the decrease in bone resorption and formation markers happened concurrently. This is not the case. Examining OC and CTX changes sequentially shows that CTX levels decreased rapidly by the end of the first feeding period, whereas the greatest reduction in OC levels occurred at the end of the second period. On the contrary, both markers increased when participants were consuming the control diet. Our results support the current theory that change in the formation marker usually lags 6–8 wk behind change in the resorption marker.
The combined effects of higher potassium and low acid load of the DASH dietary pattern may have contributed to the calcium retention and subsequent reduced bone turnover. Despite the much higher calcium content of the DASH diet, urinary calcium levels did not differ between the controls and those consuming the DASH diets. This suggests that a certain amount of calcium was conserved, possibly due in part to the high potassium intake. Our finding is consistent with the finding of a recent study (36) that a 90 mmol/d potassium supplement prevented sodium-induced increases in urinary calcium excretion. Urinary calcium retention and positive calcium balance in healthy adults (37–39) and significant reduction in bone turnover and improvement in calcium balance in postmenopausal women in response to potassium have also been shown (40). Cross-sectional studies have shown both positive associations between potassium intake from fruits and vegetables (5,6) or urinary potassium excretion (4) and bone mineral density. Apart from the independent role of potassium, the low acid load of the DASH diet may also have increased calcium conservation. A high acid load diet has been shown to increase urinary calcium excretion by 74% and bone resorption by 19% in healthy men (8).
Fruit and vegetable-derived phytochemicals may also affect bone metabolism. Studies by Muhlbauer and Li (41) showed that a large variety of commonly consumed vegetables significantly reduced bone loss in rats. This result is unlikely to be due to an alkali effect (42) but more likely results from an unidentified plant-derived compound (43). Other nutrients such as vitamin C may also have contributed to the positive effect observed in our study (44).
Total protein intake was higher in the DASH diet but we cannot confirm whether the higher protein content exerted a calciuric effect. It is possible that such an effect may have been offset by the greater alkalinity of the DASH diet. There is increasing evidence that a higher protein intake may have beneficial effects on bone (45,46) and may reduce bone loss (47,48), particularly in the elderly and those consuming adequate calcium levels (49–51). The higher protein content in the calcium-rich DASH diet may benefit bone health.
Our second study objective was to determine the effects of sodium intake, alone or combined with the DASH diet, on bone and calcium metabolism. In agreement with numerous studies (9,10), a higher sodium intake dose dependently increased urinary calcium in both diet groups. When sodium intake was increased from the lower to the higher level, calcium excretion increased significantly in both diet groups. The increase was greatest among those who were hypertensive and consuming the control diet (31 mg/g creatinine) and the least among the nonhypertensive consuming the DASH diet (11 mg/g creatinine). The ratio of urinary calcium/100 mmol increase in sodium excretion was higher in subjects consuming the control diet (0.83) than in those consuming the DASH diet (0.66), but both were comparable to those reported by others (10,14–16). These urinary losses did not affect PTH levels in the DASH diet group. However, in the control group, PTH was significantly lower at the lower sodium level, compared with the higher sodium, indicating that compensatory mechanisms may have been initiated to correct for the urinary calcium losses. Reducing sodium from the higher to lower level with the DASH diet reduced serum OC (0.6 µg/L) significantly, but to a lesser degree than the effect from the DASH diet itself (4.7 µg/L). The reduction was significant in the entire group. Our results are consistent with previous findings that hypertensive subjects may be particularly susceptible to the effects of a high sodium intake with greater calcium losses, elevated PTH level (52,53) and greater risk for osteoporosis (54–57). A disturbance in calcium metabolism, perhaps secondary to a genetic defect in sodium excretion by the kidney, might be the link between blood pressure and bone density (58). Surprisingly, sodium intake did not affect bone turnover in the control diet group, despite a difference in PTH and urinary calcium levels when subjects consumed the lower and higher sodium diets. These findings conflict with previous reports that increased sodium intake increases bone resorption in postmenopausal women (13,14) but agree with studies in younger adults that showed no effect of sodium intake on bone turnover (13,15). It is possible that elevated bone resorption may have been evident earlier in the intervention, but by the end of the 30-d period, the up-regulation of the compensatory mechanisms [PTH, cAMP, and probably 1,25-dihydroxy vitamin D3 (1,25(OH)2D3)] may have restored basal bone turnover. Measurement of 1,25(OH)2D3 would have shed further light on the adaptive mechanisms that may have been initiated in response to urinary calcium losses.
Despite the consistent and significant effect of sodium intake on urinary calcium excretion, the long-term effects of sodium on bone health are unclear. A limited number of studies have investigated the relationship between dietary sodium and bone mineral density and have produced conflicting results (16,17). When calcium and potassium intakes are limited, a high sodium intake may adversely affect bone health due to uncompensated urinary calcium losses.
In summary, we showed that the DASH diet significantly reduces bone turnover, which if sustained, may improve bone mineral status and ultimately reduce the risk of osteoporosis. The positive effect of the DASH diet on bone turnover might have resulted from a variety of factors including higher calcium, potassium and magnesium intakes, its lower acidity and possibly fruit and vegetable-derived antioxidants and phytochemicals. Even though sodium intake was consistently associated with urinary calcium, it did not consistently affect bone turnover. In view of the many potential benefits of the DASH dietary pattern, recommending this dietary pattern as an effective strategy to prevent and treat osteoporosis would be a prudent public health strategy. Sodium reduction should be encouraged as well, but a reduction to the lower level tested here could be a challenge without major modification to the current food environment. For the elderly who have a higher risk of osteoporosis and often a change in taste acuity, the DASH diet may be an effective alternative. Ultimately, long-term trials with clinical outcomes such as bone mineral status are required to verify the long-term beneficial effect of the DASH diet and reduced sodium intake on bone health.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Supported by grants HL50968, HL50972, HL50977, HL50981, and HL50982 from National Heart, Lung, and Blood Institute, grant RR-00722 from the National Center for Research Resources and a grant from the Nestec Ltd.
4 Abbreviations used: CTX, C-terminal telopeptide of type I collagen; DASH, Dietary Approaches to Stop Hypertension; OC, osteocalcin; 1,25(OH)2D3; 1,25-dihydroxy vitamin D3; PTH, parathyroid hormone.
Manuscript received 11 June 2003. Initial review completed 27 June 2003. Revision accepted 20 July 2003.
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