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1 From the Department of Medicine and Therapeutics, University of Aberdeen, Medical School Buildings, Aberdeen, United Kingdom (HMM and DMR); the Centre for Nutrition and Food Safety, School of Biomedical and Molecular Sciences, University of Surrey, Guildford, United Kingdom (SAN); the Department of Clinical Chemistry, Royal Liverpool University Hospital, Liverpool, United Kingdom (WDF); and the Health Services Research Unit, University of Aberdeen Medical School, Aberdeen, United Kingdom (MKC).
2 Presented in part at the annual Bone and Tooth Society Meeting, Cardiff, United Kingdom, 2002. 3 The views expressed herein are those of the authors. 4 Supported by the UK Food Standards Agency and the UK Arthritis Research Campaign (DMR). The Health Services Research Unit is funded by the Chief Scientist Office of the Scottish Executive Health Department. 5 Reprints not available. Address correspondence to HM Macdonald, Osteoporosis Research Unit, Victoria Pavilion, Woolmanhill Hospital, Aberdeen, AB25 1LD. E-mail: h.macdonald{at}abdn.ac.uk.
ABSTRACT
Background: The Western diet may be a risk factor for osteoporosis. Excess acid generated from high protein intakes increases calcium excretion and bone resorption. Fruit and vegetable intake could balance this excess acidity by providing alkaline salts of potassium. Algorithms based on dietary intakes of key nutrients can be used to approximate net endogenous acid production (NEAP) and to explore the association between dietary acidity and bone health.
Objective: We investigated the relation between dietary potassium and protein, NEAP (with an algorithm including the ratio of protein to potassium intake), and potential renal acid load (with an algorithm including dietary protein, phosphorous, potassium, magnesium, and calcium) and markers of bone health.
Design: Measurements of bone mineral density (BMD) (n = 3226) and urinary bone resorption markers (n = 2929) at the lumbar spine and femoral neck were performed in perimenopausal and early postmenopausal women aged 54.9 ± 2.2 y (
Conclusions: Dietary potassium, an indicator of NEAP and fruit and vegetable intake, may exert a modest influence on markers of bone health, which over a lifetime may contribute to a decreased risk of osteoporosis.
Key Words: Fruit • vegetables • net endogenous (noncarbonic) acid production • NEAP • potential renal acid load • PRAL • acid base balance • dietary potassium • bone resorption markers • bone mineral density • menopause
INTRODUCTION
Osteoporosis is a major public health problem in the Western World and is increasing in the developing world (1). Although diet is likely to play a role, most research has concentrated on calcium and vitamin D at the expense of other nutrients that could plausibly influence bone health. Recent population-based studies have shown beneficial effects of dietary potassium and fruit and vegetables on indexes of bone health (2–4). Also, an ancillary study to the Dietary Approaches to Stopping Hypertension (DASH) trial showed that a diet rich in fruit and vegetables and in low-fat dairy products reduced bone turnover compared with a control diet (5). It was suggested that osteoporosis may, in part, be caused by the continual release of alkaline salts from bone for acid-base balance (6). The Western diet, in particular, generates a large amount of acid. Without sufficient alkaline-forming foods in the diet, bone health may be compromised (7). Short-term supplementation of postmenopausal women with potassium bicarbonate for 18 d (8) or potassium citrate for 3 mo (9) was shown to reduce bone turnover, and other studies have shown that calcium balance is improved with the administration of organic salts of potassium (10–12). Compared with a base-forming diet, an acid-forming diet was shown to increase urinary calcium excretion by 74% and C-telopeptide excretion by 19% (13). At the cellular level, metabolic acidosis causes calcium efflux from bone (14), stimulates osteoclastic action, and inhibits osteoblastic action (15), whereas the opposite is true of metabolic alkalosis (16). In vitro studies using cell cultures of rat osteoclasts have shown that even a small decrease in pH from 7.25 to 7.15 resulted in a 6-fold increase in the number of resorption pits (17) as a result of osteoclast stimulation; there was little or no resorption at pH values >7.3 (18).
In large population studies, the measurement of acid-base balance is not practical, but algorithms based on the ratio of protein to potassium have been used to estimate net endogenous (noncarbonic) acid production (NEAP) (19). Reanalysis of baseline data from our group collected in 1993 from a subset of 1065 women showed lower BMD with higher dietary NEAP and a trend for higher bone resorption in a group of 62 women (20). The aim of this study was to determine whether there is an association between NEAP or potential renal acid load (PRAL) and indexes of bone health (bone resorption markers and BMD) in the complete population of >3000 women (now perimenopausal and early postmenopausal) 5–7 y from the first visit, and whether there is an independent association between dietary potassium or protein and markers of bone health.
SUBJECTS AND METHODS
Subjects
The subjects were taken from a population-based screening program for osteoporotic fracture risk, involving 5119 women aged 45-54 y, that took place between 1990 and 1994. The women were drawn at random from a 25-mile (40-km) radius of Aberdeen, a city with a population of 250000 in the northeast of Scotland, with the use of Community Health Index records (21, 22). All participants underwent bone densitometry and risk factor assessment by questionnaire, and the women were invited to undergo further assessment between 1997 and 1999. A total of 3883 women underwent a second assessment. There were no significant differences in age, height, weight, or baseline BMD between the women who returned for the second assessment and those who did not. However, women who were still menstruating were more likely to return for the second assessment (53.9% compared with 49.4%), and there were slightly fewer postmenopausal women (46.1% compared with 50.6%). At the second visit, 3510 women provided a second early morning, fasting urine sample for the measurement of bone resorption markers. Most of the women (n = 3239) completed a diet and physical activity questionnaire at the time or shortly after the follow-up visit. So as not to bias the data excessively, only 6 women were excluded from the analysis, because they had excessive intakes of dietary potassium (>8000 mg/d). From this group, there were 3226 measurements of BMD and 2929 measurements of bone resorption markers. Information on health, menopausal status, hormone replacement therapy (HRT), and other medication use was also collected. At the follow-up visit, for the women who had BMD measurements, a total of 336 women were still menstruating, 2877 women were postmenopausal (of whom 1180 women were receiving HRT at the time of the second visit), and 13 women were of unknown menopausal status. For the women who had bone marker measurements, 304 women were still menstruating, 2614 women were postmenopausal (of whom 1073 women were currently receiving HRT), and 11 women were of unknown menopausal status. A summary of the subject numbers is given in Figure 1. Menopausal status and use of HRT were considered independent variables in linear regression models. Self-reported osteoarthritis (n = 597) and thiazide use (n = 262) were also considered independent variables in the analysis. Subjects with other diseases and conditions, reported at the second visit, that could affect bone metabolism were included. However, the analysis was also repeated with the exclusion of women who self-reported having a disease known to affect bone metabolism (eg, breast cancer, thyroid disease, malabsorption; n = 334) and recent users of bisphosphonates (n = 22), selective estrogen receptor modulators (n = 2), tamoxifen (n = 39), or oral glucocorticoids (n = 92).
FIGURE 1.. Summary of the numbers of subjects who had bone mineral density (BMD) measurements (n = 3226) and bone resorption markers analyzed (n = 2929) at the follow-up visit.
The women were weighed at both visits with a set of balance scales (Seca, Hamburg, Germany) calibrated to 0.05 kg while wearing light clothing and no shoes. Height was measured with a stadiometer (Holtain Ltd, Crymych, United Kingdom). Written informed consent was obtained from all of the women, and the study was approved by the Grampian Research Ethics Committee.
Bone mineral density measurements
Bone mineral density (BMD) of the left femoral neck (FN) and lumbar spine (LS) (L2-L4) was measured by dual-energy X-ray absorptiometry (Norland XR26 and XR36; Cooper Surgical Inc, Trumbull, CT) as described previously (23). Most of the women were scanned by using the XR26, but 388 (14%) women were scanned by using the XR36. A comparison of 50 phantom measurements with both machines showed a small difference (1.258%) in mean BMD between the machines; therefore, a correction factor was used to convert the XR36 values to XR26-equivalent values. The same trends in results were seen whether or not this correction factor was used.
Urinary bone resorption markers
A second early morning fasting urine sample was used for the analysis of free pyridinoline (fPYD) and free deoxypyridinoline (fDPD) by using a modification of the HPLC method described by Black et al (24). Acidified urine was applied to microgranular cellulose (CC31) in butanol (1/4) and washed before elution with heptafluorobutyric acid (0.1%). The eluent was then analyzed by ion-pair reversed-phase HPLC using fluorescence detection. Acetylated PYD (Quidel/Metra Biosystems, Oxford, United Kingdom) was used as an internal standard. Creatinine was measured in urine with standard automated techniques (Roche, Lewes, United Kingdom), and the results were expressed as fPYD/Cr and fDPD/Cr (nmol/mmol). The interassay CV for both marker methods was <5.5% across the working concentration range for the assay, which was established by performing repeated analysis of a range of patient and quality-assurance samples (25).
Diet and physical activity
Dietary assessment was made at the follow-up visit with a food-frequency questionnaire (FFQ) that had been validated with the use of 7-d weighed intakes (26) and serum concentrations of antioxidants (27). A subgroup of women (n = 898) also completed the same FFQ at baseline. For most of the women there was little change in nutrient intakes, although mean energy had decreased from a mean (±SD) of 8.1 ± 1.2 to 7.9 ± 1.1 MJ/d (P < 0.01, paired t test). Protein had decreased by a mean of 2 to 79.4 ± 21.4 g/d (P < 0.01), but there was no significant change in mean potassium intake (3329 ± 790 mg/d). The database used for the nutrient analysis is based on McCance and Widdowson's Composition of Foods version 5 (28), which does not include the newer estimates of vitamin D contribution from meat (based on assumptions regarding the greater potency of vitamin D metabolites from meat sources). Physical activity levels (PALs) were obtained by using the same questions as used for the Scottish Heart Health Study (29). The PAL was calculated from the numbers of hours in a 24-h period doing heavy, moderate, or light activities and how many hours were spent sleeping or resting in bed. These questions were asked separately for working and nonworking days. PAL is normally defined as the ratio of energy expenditure divided by the basal metabolic rate (BMR), which is calculated from Schofield equations (30). These equations were derived from data collected from European women. For women aged 30–50 y, the equation is BMR (MJ/d) = 0.034 x weight (kg) + 3.538 (SEE = 0.47.)
Estimation of the acid-generating potential of the diet or NEAP was calculated according to the equations of Frassetto et al (19):
RESULTS
Subject characteristics in relation to dietary acidity
Division of the women into 4 groups according to quartiles (Q) of estimated NEAP showed that those in the bottom quartile (Q1) produced a mean of 30 mEq acid/d, whereas those in the top quartile (Q4) produced a mean of 52 mEq/d (Table 1). Characteristics of the women according to each NEAP group showed no significant differences with the exception of height, BMI, FN BMD change, vegetarianism, and smoking. Women in the lowest quartile of NEAP were marginally taller on average and had a lower mean BMI than the women in Q4 (P < 0.05, ANOVA with Tukey's test). Q1 contained significantly fewer smokers (P = 0.03) and more women consuming vegetarian diets (P < 0.001), although there were only 28 vegetarians and 2 vegans in this study. There appeared to be less loss of FN BMD in Q4 than in Q1 (P = 0.05), but this difference was not significant after adjustment for confounders (age, weight, height, socioeconomic status, PAL, menopausal status, HRT use, and baseline FN BMD). Similar trends were seen across quartiles of energy adjusted NEAP (data not shown), with no difference observed in the distribution of vegetarians; however, the difference in FN BMD change between Q1 and Q4 of energy-adjusted NEAP was not significant (P = 0.10).
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TABLE 1. Subject characteristics according to quartile (Q) of estimated net endogenous acid production (NEAP)1
In terms of the other algorithm for estimating dietary NEAP, PRAL also increased from –9.3 mEq/d for Q1 of NEAP to 15.5 mEq/d for Q4 of NEAP (Table 2). The correlation between PRAL and NEAP was 0.93. There was little difference in the quartile categorization of NEAP and PRAL; 76% women were classified in the same quartile and <0.3% women were grossly misclassified (a greater than one quartile difference). The difference between PRAL calculated by using calcium from the diet only and PRAL calculated by using total calcium including supplements was small and only the former was used for subsequent analyses. Adding an estimate for organic acids to give an estimate of overall net acid excretion showed good agreement with NEAP; the correlation coefficient was 0.86.
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TABLE 2. Selected nutrient and food intakes according to quartiles of estimated net endogenous acid production (NEAP)1
Potassium, fruit, and vegetable intakes decreased with increasing quartile of NEAP (P < 0.001, ANOVA). Conversely, there was a progressive increase in the intakes of protein, calcium, vitamin D, and meat (including processed meat, red meat, poultry, and offal) (P < 0.001, ANOVA). Quartiles were significantly different from one another by Tukey's test (P < 0.05), and tests for linearity were significant (P < 0.001) (Table 2). Energy intake was lower in the lower dietary acidity groups (P < 0.001 between quartiles, ANOVA with Tukey's test). There were also more women who were low energy reporters [the ratio of energy intake to calculated basal metabolic rate (EIBMR) was <1.1]; 38% in Q1 compared with 18% in Q4. Similar trends were seen across quartiles of energy-adjusted NEAP (data not shown), with the exception of energy intake (8.1 MJ/d for all 4 groups; P = 0.79), EIBMR (1.38–1.40; P = 0.75), and calcium intake, for which the mean intake was greater in Q1 (1097 ± 388 mg/d) than in Q4 (1011 ± 328 mg/d) (P < 0.001). Also, there was no significant difference in the numbers of low energy reporters in each quartile (26%).
Bone resorption
Concentrations of bone resorption markers were significantly greater in the highest quartile of estimated NEAP than in the lowest quartile [P < 0.01 (ANOVA) and P < 0.01 (Tukey's test) for both fDPD/Cr and fPYD/Cr). Concentrations of the bone resorption markers fDPD/Cr and fPYD/Cr were significantly greater in the highest quartile of estimated PRAL than in the lowest 2 quartiles [Q1 compared with Q4: P < 0.01 (ANOVA) and P < 0.01 (Tukey's test); Q2 compared with Q4: P < 0.02 (ANOVA with Tukey's test)]. The differences in NEAP and PRAL remained significant after adjustment for the confounding variables age, weight, height, socioeconomic status, PAL, menopausal status, and HRT use (P < 0.01 for fDPD/Cr and P = 0.01 for fPYD/Cr, ANCOVA), as shown in Figure 2 for fDPD/Cr. According to categories of energy-adjusted potassium intake, bone resorption markers were significantly greater in the lowest quartile than in the other quartiles [Q1 compared with Q4: P = 0.001 and P < 0.01 (Tukey's test) for both markers; Q1 compared with Q2 and Q3: P < 0.05 (ANOVA with Tukey's test) for fDPD/Cr; Q1 compared with Q3: P < 0.05 (Tukey's test) for fPYD/Cr]. For both bone resorption markers, the associations with potassium intake were still significant after adjustment for confounders (P < 0.01, ANCOVA; Figure 2). Bone resorption marker concentrations were greater for Q1 of energy-adjusted protein intake than for Q3 (P = 0.01 for fPYD/Cr and P = 0.09 for fDPD/Cr, before adjustment for confounders; Tukey's test). The association between protein intake and bone resorption markers was significant after adjustment for confounders (P < 0.01 for fPYD/Cr and P = 0.02 for fDPD/Cr; Figure 2). Tests for linearity were significant for NEAP, PRAL, and potassium with both bone resorption markers (P < 0.01) and for protein with fPYD/Cr (P = 0.02) but not with fDPD/Cr. Similarly, statistically significant results were obtained when these analyses were repeated with the exclusion of women who had diseases or who were taking medication that could affect bone metabolism.
FIGURE 2.. Mean (±2 SEM) concentrations of the bone resorption marker free deoxypyridinoline expressed relative to creatinine (fDPD/Cr) with increasing quartiles (Q) of energy-adjusted potassium intake, estimates of dietary acidity (net endogenous acid production, NEAP; potential renal acid load, PRAL), and energy-adjusted protein intake for all women who had bone resorption markers measured (n = 2929). Means were adjusted for age, weight, height, socioeconomic status, physical activity level, menopausal status, and hormone replacement therapy status. Analysis of covariance: P < 0.01 for potassium, P < 0.01 for NEAP, P < 0.01 for PRAL, and P = 0.02 for protein. Quartiles within a variable with different lowercase letters are significantly different, P < 0.05 (ANOVA with Tukey's test). The test for linearity across quartiles was significant (P < 0.01) for potassium, NEAP, and PRAL but not for protein (P = 0.06). Mean (±SD) values from Q1 to Q4 for dietary potassium were 2771 ± 609, 3275 ± 600, 3578 ± 712, and 4151 ± 811 mg/d. Mean (±SD) values from Q1 to Q4 for dietary protein were 69.0 ± 18.0, 76.4 ± 17.5, 84.3 ± 19.7, and 99.3 ± 27.0 g/d. Similar trends were seen for the bone resorption marker free pyridonoline; however, for this marker, Q3 was significantly different from Q1 (P = 0.01, ANOVA with Tukey's test) for protein, and Q1 was significantly different from Q3 and Q4 (P = 0.03 and P < 0.01, respectively; ANOVA with Tukey’s test), but not from Q2, for potassium.
Using stepwise multiple linear regression, it was found that the continuous variables of estimated NEAP, PRAL, and dietary potassium were significant independent predictors of bone resorption (Table 3). We found no significant interaction between postmenopausal women (including past and present HRT users) and women who were still menstruating (and not receiving HRT) with dietary potassium or estimates of dietary acidity (NEAP, PRAL) on bone resorption markers.
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TABLE 3. Results of multiple regression analyses to identify independent predictors of bone resorption for all women1
Bone mineral density
For all women (n = 3228), who were of mixed menopausal status, there was a small (2%) difference in BMD between the lowest and highest quartiles of potassium intake that was significant at the FN (P = 0.02 ANOVA with Tukey's test) but not at the LS (P = 0.10), although both sites were significant for linearity (FN: P < 0.01; LS: P = 0.02). There was a significant interaction for BMD with menopausal status (between those women still menstruating and those who were postmenopausal) and with dietary potassium (FN: P < 0.01; LS: P = 0.03) and a trend for interaction with NEAP (P < 0.10) and energy-adjusted NEAP (FN: P = 0.05 FN; LS: P < 0.05).
For the subgroup of women who were still menstruating (n = 336), the difference in FN BMD between Q1 and Q4 of potassium intake was 8%. This difference was statistically significant by one-way ANOVA with Tukey's test (P < 0.01), was significant for linearity (P = 0.01), and remained significant after adjustment for confounders (P < 0.01) (Figure 3). A similar trend was seen at the LS; there was a difference of 6% in BMD between the top and bottom quartiles of potassium intake, which was not statistically significant by ANOVA (P = 0.06) or ANCOVA (P = 0.20). There were no significant differences in BMD at either site between quartiles of estimated NEAP or PRAL or between quartiles of dietary protein intake.
FIGURE 3.. Mean (±2 SEM) bone mineral density (BMD) at the femoral neck (FN) and lumbar spine (LS) with increasing quartiles (Q) of energy-adjusted potassium intake for premenopausal women (n = 336); the values were adjusted for age, weight, height, socioeconomic status, physical activity level, and regularity of menstruation. Analysis of covariance: P < 0.01 for FN BMD and P = 0.20 for LS BMD. There was an interaction for BMD between premenopausal and postmenopausal women (including hormone replacement therapy users) with potassium intake (P < 0.01 for FN BMD and P = 0.03 for LS BMD). Quartiles within a variable with different lowercase letters are significantly different, P < 0.01 (ANOVA with Tukey's test); P = 0.06 for LS BMD (ANOVA with Tukey's test). The test for linearity across quartiles (ANOVA) was significant for FN BMD (P < 0.01) and LS BMD (P = 0.02). Mean (±SD) values from Q1 to Q4 for dietary potassium were 2680 ± 599, 3229 ± 610, 3477 ± 721, and 4289 ± 979 mg/d.
For BMD change, there were no differences at either site between quartiles of dietary protein or potassium intake. BMD loss at the highest quartile of NEAP and PRAL was lower than at the lowest quartile (P = 0.05 for FN BMD and NEAP and P = 0.03 for LS BMD and PRAL, ANOVA with Tukey's test), but these differences were not significant after adjustment for confounders nor were there differences between quartiles of energy-adjusted NEAP and PRAL.
Because we found no significant differences in dietary potassium intake at each visit in a subset of 898 women who had completed dietary questionnaires on both occasions, the association between potassium intake at follow-up and BMD at baseline was tested in women who were menstruating regularly at baseline (n = 1541). There was a significant association between quartile of potassium intake and LS BMD [P = 0.03 (ANOVA); BMD was significantly greater in Q4 than in Q1, P < 0.05 (Tukey's test) and a significant trend for linearity, P < 0.01)] but not between quartile of potassium intake and FN BMD (P = 0.06, ANOVA).
When regression analysis with BMD was used as the dependent variable, potassium intake was found to be a weak predictor of BMD in the full group of women, but it was not significantly different after adjustment for confounding variables.
For the subgroup of women who were still menstruating, NEAP was inversely associated with FN BMD and LS BMD, and dietary potassium was a positive independent predictor of FN BMD and LS BMD after adjustment for confounders (Table 4) , but neither protein intake nor PRAL added significantly to the model at either site. Similar statistically significant associations were found for BMD at the baseline visit (data not shown). None of the dietary variables was found to be a predictor of BMD change at either site.
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TABLE 4. Results of multiple regression analyses to identify independent predictors of bone mineral density (BMD) at follow-up for the subsets of premenopausal and perimenopausal women1
The exclusion of women who had conditions or who were taking medication likely to affect bone metabolism showed both potassium and dietary acidity (NEAP) to be significant predictors of BMD at both visits and both sites and accounted for a greater percentage (1–3%) of the variation in BMD.
DISCUSSION
Bone resorption
This study showed that, for women around the time of menopause, NEAP was associated with increased concentrations of bone resorption markers. At a 6–8% increase in bone resorption markers between the lowest and highest quartiles of NEAP or potassium, the difference in mean potassium intake between these groups was only 17.1 mmol (for quartiles of NEAP) or 35 mmol (for quartiles of energy-adjusted potassium intake). In comparison, Buclin et al (13) found 19% less C-telopeptide excretion in men consuming a base-producing diet than in those consuming an acid-producing diet, in which the difference in potassium intake between the 2 diets was 89 mmol. Modest increases in bone resorption with increased NEAP are consistent with diet playing a long-term role in contributing a mild, low-grade acidosis over a lifetime that is within physiologically normal limits and yet may still have a chronic effect on bone health. It is known that osteoclasts are stimulated to resorb bone at an acidic pH, being most sensitive to changes at pH values of 7.1 (34, 35). Models of metabolic acidosis in which bone was incubated in reduced bicarbonate medium showed stimulation of bone resorption by osteoclasts and inhibition of bone formation by osteoblasts (36). It would appear that even subtle chronic acidosis could be sufficient to cause considerable bone loss over time (37).
Bone mineral density
For women who were still menstruating, we found a difference of 8% BMD between those in Q1 and Q4 of dietary potassium intake, which can be considered to be biologically significant because it is equal to one-half the SD for this population. If maintained into old age, this could reflect a 30% decrease in fracture risk for those with higher intakes of potassium. Weight and height accounted for most of the variation in BMD. BMD change in the early postmenopausal period is dominated by hormonal influences (estrogen, progesterone), which may explain why we did not observe an association between estimated NEAP and BMD for the immediate postmenopausal group and why protein and potassium intakes were not associated with BMD change. The difference in BMD change observed for quartiles of NEAP and PRAL may have been a result of the women in the lowest group of NEAP being slightly taller and having a lower BMI, because this finding was not significant after adjustment for energy or after adjustment for confounders.
Dietary protein
Diets that contain less animal protein and more fruit and vegetables have been suggested to be beneficial to bone health by virtue of their high potassium content (38). However, if foods are scored according to their PRAL (39), cereals, grains, and cheeses have high PRAL values; the PRAL for high-protein cheeses is more than twice that of meats or fish (39, 40). This may explain why some studies have not found a link between vegetarianism and increased BMD (41), although other factors (eg, weight, exercise) likely differentiate vegetarians from nonvegetarians (42). We found more vegetarians in the lowest quartile of NEAP, although the number of reported vegetarians in our study was very low (0.1%). The overall prevalence reported in the general UK population is 5%, but the proportion of vegetarian women decreases with age [from 11% of 19–34-y-olds to 4% of 35–64-y-olds (43)], and the mean age of our population was older.
Our data suggest that a low protein intake may be detrimental to bone health because women in the lowest protein group had significantly greater concentrations of bone resorption markers. The Framingham Study found that a higher animal protein intake was associated with lower bone loss and that nonanimal protein sources were not related to BMD (44). The influence of protein may depend on whether the overall diet is balanced in terms of its acid-generating potential. A beneficial role for protein was noted in elderly subjects, provided that they were replete in calcium (45). Most of the women in our study had sufficient calcium in their diet. However, we cannot be certain that the higher concentration of bone resorption markers in the lowest quartile of protein intake was a result of low intakes of protein or to some other nutrient that was significantly lower in the lowest quartile (such as zinc or vitamin D). In the United Kingdom, meat and meat products provide 40% of the zinc in the diet (46), and zinc is associated with increased bone mass in premenopausal women (2). Because the bone resorption markers were expressed relative to creatinine, and creatinine concentrations increase with increasing protein intakes, this could be another reason why concentrations of the bone resorption markers were greater in the lowest quartile of protein intake. Similarly, if there is confounding by creatinine, we could be underestimating the association of NEAP with markers of bone resorption.
Dietary potassium
Potassium salts may benefit bone health by providing an anion that can be metabolized completely to carbon dioxide, or they may influence calcium excretion directly (10–12, 47). Potassium citrate has been shown to prevent bone resorption induced by dietary salt (sodium chloride) (48), and the DASH diet reduced bone turnover at low, medium, and high sodium intakes, although there was no difference in sodium excretion between the DASH and the control diet groups at each level of salt intake (5). Barzel (49) suggests that the role of the anion chloride on salt-induced hypercalcuria is a special aspect of the general effect of acid-base imbalance on bone. We were unable to explore the role of sodium chloride in this study because the FFQ is not a reliable tool for estimating salt intake.
Dietary potassium could simply be a marker of fruit and vegetable intake, and it may be other components in fruit and vegetables that have an influence on bone metabolism, eg, vitamin K (50), vitamin C (51), folate (52), and phytoestrogens (53). It has also been argued that the healthy kidney is able to cope with the demands of an acidic diet (54) and that the acid-base hypothesis is only relevant for persons with impaired renal function. However, in disputing this, it was claimed that small increases in blood pH that are within physiologically normal values can still virtually eradicate net renal acid excretion and affect bone metabolism (17, 55). Vegetables and herbs had a beneficial effect on bone resorption in young rats, over and above that of providing alkaline metabolites (56), but it is not known whether these diets are directly comparable with a normal human diet and, if so, whether similar effects on bone resorption would be observed, especially in an older population.
This study involved a large population-based investigation of diet and bone health in women who were randomly selected from the community and for whom many important confounding variables were adjusted for. Some limitations of the study were that the data on diet and PAL were obtained by self-reported questionnaire (which could lead to possible overreporting on specific aspects of the diet, such as fruit and vegetable consumption), the data for the complete population were collected at the follow-up visit and may not fully represent the diet over the longer term, and the bone resorption data would reflect only recent bone turnover. Also, a limitation of dual-energy X-ray absorptiometry is that it only provides a measurement of areal BMD. To unravel the subtle changes of dietary acidity on bone health, a more detailed evaluation of bone status, which includes bone geometry and bone quality, may be required. This could be attained by using other techniques, such as computed tomography, quantitative ultrasound, and trabecular structural analysis. However, each of these methods has its own problems of interpretation. All women were included so as not to bias the sample. When women who had a condition or were taking medication that could affect bone metabolism were excluded, the magnitude of the associations was found to be stronger.
These data suggest that it would be worthwhile to explore further the link between dietary potassium, dietary protein, and markers of bone health. A longer-term intervention study is required to fully evaluate whether fruit and vegetable intakes affect human bone metabolism through the provision of organic salts of potassium or other components.
ACKNOWLEDGMENTS
We are grateful to David Grubb of the Rowett Research Unit for running the ORACLE program to analyze our FFQ and physical activity questionnaire data and to the following persons, who assisted with the FFQ data: Fidelma Moore (data entry), Fiona Downie (estimation of fruit and vegetable intakes), and Yinka Mackay (estimation of meat intake). We are also extremely grateful for the hard work of the radiographers and research nurses at the Osteoporosis Research Unit and to all of the women who kindly participated in the study.
HMM carried out the study and was responsible for the data analysis and for writing the manuscript. SAN was involved in the design of the baseline dietary study and reviewed the manuscript. WDF provided the bone marker results and critically reviewed the manuscript. MKC gave statistical advice and reviewed the manuscript. DMR was responsible for the study design of APOSS and reviewed the manuscript. None of the authors had a financial or commercial interest in any company or organization sponsoring the research for this study.
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