Dietary intake, blood pressure and osteoporosis - Nature

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Journal of Human Hypertension (2009) 23, 451–455 & 2009 Macmillan Publishers Limited All rights reserved 0950-9240/09 $32.00 www.nature.com/jhh

ORIGINAL ARTICLE

Dietary intake, blood pressure and osteoporosis J Woo1, T Kwok1, J Leung2 and N Tang3 1

Department of Medicine and Therapeutics, The Chinese University of Hong Kong, Shatin, NT, Hong Kong; Jockey Club Centre for Osteoporosis Care and Control, The Chinese University of Hong Kong, Shatin, NT, Hong Kong and 3Department of Chemical Pathology, The Chinese University of Hong Kong, Shatin, NT, Hong Kong 2

Both hypertension and osteoporosis have common underlying nutritional aetiology, with regards to dietary cations intake. We tested the hypothesis that sodium intake reflected in urinary Na/Cr and blood pressure would be negatively associated with bone mineral density (BMD), whereas other cations may have opposite associations. Subjects were part of a study of bone health in 4000 men and women aged 65 years and over. A total of 1098 subjects who were not on antihypertensive drugs or calcium supplements and who provided urine samples were available for analysis. Logistic regression was used to examine associations between total hip and lumbar spine BMD, age, gender, body mass index (BMI), urinary Na/Cr, K/Cr, calcium and magnesium intake, systolic blood pressure and diastolic

blood pressure. Total hip BMD was inversely associated with age, being female and urinary Na/Cr, and positively associated with BMI, urine K/Cr and dietary calcium intake. Lumbar spine BMD was inversely associated with being female and urinary Na/Cr, and positively associated with BMI, dietary calcium intake and SBP. We conclude that sodium intake, reflected by urinary Na/Cr, is the major factor linking blood pressure and osteoporosis as shown by the inverse relationship with BMD. The findings lend further emphasis to the health benefits of salt reduction in our population both in terms of hypertension and osteoporosis. Journal of Human Hypertension (2009) 23, 451–455; doi:10.1038/jhh.2008.156; published online 18 December 2008

Keywords: blood pressure; osteoporosis; sodium; calcium; magnesium; potassium

Introduction The contribution of nutritional factors to the development of hypertension is well recognized, in particular with reference to the four cations sodium, potassium, calcium and magnesium,1–6 both in terms of nutrient intake quantities as well as frequency of consumption of food group.7,8 The link between hypertension and osteoporosis have not received as much attention, in terms of common underlying pathogenetic mechanism, perhaps as both conditions are common with ageing. One pathway would be the increased obligatory urinary calcium excretion with increased urinary sodium excretion, which occurs with increased dietary sodium intake. This has been confirmed in the Hong Kong Chinese population.9 Increased urinary calcium loss predisposes to increased bone loss and osteoporosis. A direct connection may also occur Correspondence: Professor J Woo, Department of Medicine and Therapeutics, The Chinese University of Hong Kong, 9/F, Clinical Sciences Building, Prince of Wales Hospital, Shatin, NT, Hong Kong. E-mail: [email protected] Received 2 September 2008; revised 5 November 2008; accepted 5 November 2008; published online 18 December 2008

through the calcium-dependent vasoactive properties of calcium-regulating hormones, which are linked to the activity of the rennin–angiotensin system.10 Salt and calcium intake exert reciprocal effects on these two hormone systems and on blood pressure. For example, a recent population study showed an inverse relationship between systolic blood pressure (SBP) and serum 25 hydroxyvitamin D concentrations.11 Both SBP and diastolic blood pressure (DBP) were higher in patients with low calcium intake and high serum parathyroid hormone concentration12–14 and age-related rise in SBP was inversely associated with calcium intake.15 However, a randomized controlled trial of calcium supplementation in older New Zealand women showed only a small and transient hypotensive effect in most women, the effect being more marked in those with a calcium intake of o600 mg day1,16 and no effect in normotensive or hypertensive people in another study.17 A recent Cochrane Database analysis concluded that the causal relationship between calcium supplementation on blood pressure is weak.18 Earlier studies in Chinese populations showed a positive association between sodium intake,19 but an inverse association with calcium intake was only

Diet, blood pressure and osteoporosis J Woo et al 452

observed in older vegetarian women, among whom the prevalence of osteoporosis is high.20 In a study of bone health, among 4000 men and women aged 65 years and over living in the community, we took the opportunity to examine the relationship between blood pressure, bone mineral density (BMD) and intake of calcium, sodium, potassium and magnesium. We hypothesize that sodium intake as reflected by urinary Na/Cr and blood pressure would be negatively associated with BMD, while other cations may have opposite associations.

Materials and methods A total of 2000 men and 2000 women aged 65 years and over living in the community were invited to attend a health check carried out in the School of Public Health of the Chinese University of Hong Kong, by placing recruitment notices in community centres for the elderly and housing estates. Several talks were also given at these centres explaining the purpose, procedures and investigations to be carried out. Subjects were volunteers, and the aim was to recruit a stratified sample so that approximately 33% were in each of these age groups: 65–69, 70–74, 75 þ years. The study was approved by the Clinical Research Ethics Committee of the Chinese University of Hong Kong, which requires informed consent to be obtained. A questionnaire containing information regarding drug history and dietary intake was administered by an interviewer. Dietary intake was assessed using a 7-day food frequency questionnaire, and man nutrient quantitation per day was calculated using food tables derived from McCance and Widdowson21 and the Chinese Medical Sciences Institute.22 The food frequency questionnaire consisted of items in the following seven categories: bread/pasta/rice (16 items); vegetables (63 items); fruits (26 items); meat (39 items)/fish (31 items)/eggs (5 items); beverages (37 items); dinsum/snacks (39 items); soups (10 items); and oil/salt/sauces. Items chosen were those most frequently consumed, based on previous local surveys. Each subject was asked to complete the questionnaire—the food item, the size of each portion, the number of times of consumption each day and each week. Portion size was explained to subjects using a catalogue of pictures of individual food portions. The amount of cooking oil was estimated according to the method of preparing different foods: 0.2 tablespoon for steaming fish or stir frying half a portion of vegetables, and 1 tablespoon for stir frying one portion vegetables or one portion of meat.23 Blood pressure was measured after 5 min rest in the sitting position using a standard mercury sphygmomanometer (WA Baum Co. Inc., Copiague, NY, USA). The first and fifth Korotkoff phases were recorded as SBP and DBP. One reading was taken. Body weight was measured, with subjects wearing a Journal of Human Hypertension

light gown, by the Physician Balance Beam Scale (Healthometer, Chicago, IL, USA). Height was measured by the Holtain Harpenden stadiometer (Holtain Ltd, Crosswell, UK). Body mass index (BMI) (weight in kg divided by the square of height in m) was calculated. BMD at the hip and lumbar spine region was measured using dual energy X-ray absorptiometry (Hologic 4500 W, software version 11.2). A fasting urine sample was collected for analysis of sodium (Na), potassium (K) and creatinine (Cr), and the ratios Na/Cr and K/Cr were used as indicators of dietary sodium and potassium intake. All urinary analytes were analysed in the routine hospital laboratory on a Roche service analyzer. Urine Na and K were measured by the principle of indirect Ion Selective Electrode and the analysis coefficient of variation (CVs) were 2.2 and 1.6% respectively. Urine creatinine was measured by kinetic colourimetric assay based on modified Jaffe´ Reaction, and CV of the assay was about 2.0%. Statistical analysis

Subjects who were on hypertensive medication or calcium supplements or who did not provide a urine sample were excluded from statistical analysis. Multiple logistic regression was carried out with total hip BMD as dependent variable. Independent variables were age, female gender, BMI, urine Na/Cr and K/Cr ratio, magnesium and calcium intake, and SBP and DBP. The analysis was repeated with lumbar spine BMD.

Results Of the 4000 subjects, after excluding those using antihypertensive medication and calcium supplements (n ¼ 2061) and those without urine samples (n ¼ 963), data from 1098 subjects were available for analysis (809 women, 289 men). Total hip BMD was inversely associated with age, being female and urinary Na/Cr, and positively associated with BMI, urine K/Cr and dietary calcium intake. Together the variables account for 37% of the variance (Table 1). Urine Na/Cr, K/Cr and calcium intake have similar magnitude of association with BMD, being less than age, gender or BMI. No association was observed between SBP or DBP and BMD. Lumbar spine BMD was inversely associated with being female and urinary Na/Cr, and positively associated with BMI, dietary calcium intake and SBP (Table 2). These variables account for 27% of the variance. Dietary magnesium intake was not associated with BMD at either site.

Discussion In our Chinese population, an earlier population survey showed a high sodium and low calcium

Diet, blood pressure and osteoporosis J Woo et al 453

Table 1 Regression of total hip BMD (g cm2) (n ¼ 1098) Variable Age Female BMI (kg m2) Urine Na/Cr (mmol mmol1) Urine K/Cr (mmol mmol1) Dietary magnesium per day (mg) Dietary calcium per day (mg) SBP DBP

Mean (s.d.)/prevalence

Unit

72.3 (5.3) 73.7% 23.2 (3.6) 17.3 (10.4) 7.8 (3.3) 379.9 (195.9) 579.6 (282.2) 141.7 (18.9) 77.7 (8.9)

5.3 No/yes 3.6 10.4 3.3 195.9 282.2 18.9 8.9

Percent difference per unit (95% CI) 3.5 16.1 8.6 1.2 1.2 0.1 1.7 0.2 0.1

(4.4, 2.6) (18.2, 14) (7.8, 9.5) (2.1, 0.3) (0.2, 2.1) (0.9, 1) (0.7, 2.7) (1.2, 0.9) (1, 1.1)

R2 ¼ 0.3741 Abbreviations: BMD, bone mineral density; BMI, body mass index; CI, confidence interval; DBP, diastolic blood pressure; SBP, systolic blood pressure. Bold values (Po0.05).

Table 2 Regression of lumbar spine BMD (g cm2) (n ¼ 1098) Variable Age Female BMI (kg m2) Urine Na/Cr (mmol mmol1) Urine K/Cr (mmol mmol1) Dietary magnesium per day (mg) Dietary calcium per day (mg) SBP DBP

Mean (s.d.)/prevalence

Unit

72.3 (5.3) 73.7% 23.2 (3.6) 17.3 (10.4) 7.8 (3.3) 379.9 (195.9) 579.6 (282.2) 141.7 (18.9) 77.7 (8.9)

5.3 No/yes 3.6 10.4 3.3 195.9 282.2 18.9 8.9

Percent difference per unit (95% CI) 0.8 16.9 9.1 1.4 0.8 0.5 2.9 1.5 1.2

(0.4, 1.9) (19.6, 14.2) (8, 10.3) (2.6, -0.2) (0.5, 2.1) (0.8, 1.7) (1.6, 4.1) (0.1, 2.9) (2.6, 0.2)

R2 ¼ 0.2728 Abbreviations: BMD, bone mineral density; BMI, body mass index; CI, confidence interval; DBP, diastolic blood pressure; SBP, systolic blood pressure. Bold values (Po0.05).

intake.24 Only 22% of the population had an intake of sodium p2300 mg day1, while the average calcium intake was 570 mg day1. The finding of this study would be indicative of associations for a population with high sodium and low calcium intake. Both low hip and spine BMD were associated with higher sodium intake, as indicated by urinary Na/Cr ratio. However, this adverse effect may be ameliorated by higher calcium intake and also higher intake for potassium (as indicated by higher urinary K/Cr) for hip BMD. This finding supports the pathway of increased obligatory calcium excretion with increased sodium excretion as a result of increased dietary sodium intake, as the common underlying pathogenetic mechanism for both hypertension and osteoporosis. Furthermore, BMD appeared to be more correlated with Na/Cr and K/Cr compared with BP, as one would expect the same associations to be present between BP and BMD, but no significant association was observed between BP and hip BMD. Although an association between SBP and lumbar spine BMD was observed, the direction of the association was positive instead of negative. Although the explanation for this observation is unclear, salt intake

reflected by urinary Na/Cr would not be a factor as it is negatively associated with BMD. Therefore it is possible that in this population, the adverse effect of high salt intake on BMD could be greater, if not at least as great as that on BP. In this setting, increasing calcium intake would be all the more important. This study also highlights the beneficial role of potassium and magnesium in relation to hypertension. Increasing potassium intake would also be beneficial for bone health. The major food source of magnesium and potassium are vegetables, fruits and dairy products, suggesting that the finding from the DASH study using a dietary approach to hypertension with fruits and vegetables among the major components may also be relevant for bone health. There are several limitations in this study. There are inherent inaccuracies in nutrient quantitation from food frequency questionnaires, and casual urine collection (rather than 24 h urine collection) as indicators of sodium and potassium intake. Estimation of sodium intake is particularly difficult in this population owing to the liberal use of added salt, soya sauces and monosodium glutamate in preparing food. The 24 h urine collection quantifying sodium excretion is the accepted method in Journal of Human Hypertension

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many epidemiological studies examining the relationship between salt intake and blood pressure.25,26 However, collection of 24 h urine in elderly people is more prone to inaccuracies as the procedure is dependent on adequate manual dexterity and cognitive function. The volume collected tends to be less than the true 24 h urine output. Therefore, a casual urine sample would be more feasible in a large-scale epidemiological study of the elderly. Furthermore, studies using casual urine specimens have yielded similar results to studies using 24 h urine collections in examining the relationship between dietary cation intake and BP.27–29 We did not carry out urine Ca/Cr measurements to show that Na/Cr is associated with Ca/Cr. Blood pressure was only measured once. However, we were able to study a large number of subjects with DEXA measurements, dietary intake and urinary measurements. In spite of the limitations, we conclude that sodium intake, reflected by urinary Na/Cr, is the major factor linking blood pressure and osteoporosis as shown by the inverse relationship with BMD. The findings lend further emphasis to the health benefits of salt reduction in our population both in terms of hypertension and osteoporosis.

What is known about topic K The prevalence of hypertension and osteoporosis increases with age. K Sodium intake is a well-established risk factor for both conditions. K Relative contributions of other cations such as calcium, magnesium and potassium to both conditions are not as clear. What this study adds K Sodium is the major factor linking blood pressure and osteoporosis. K Calcium and potassium may ameliorate the adverse effect of sodium on bone health.

Acknowledgements This study received support from the Jockey Club Charities Trust, the Research Grants Council of Hong Kong CUHK 4101/02M.

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