Introduction
Fractures are a major health concern in the ageing population and can result in disability and reduced quality of life [
1]. Whereas bone mineral density is the most extensively studied determinant of fracture risk, other factors including micro-damage, mineralization, bone turnover, macro-geometry of the cortical bone and micro-architecture of the trabecular bone are important determinants as well [
2]. A novel measure to assess micro-architecture is trabecular bone score (TBS). In brief, this measure combines information on connectivity density, trabecular separation and trabecular number in a single score [
2]. In Canadian postmenopausal women, spinal TBS was shown to predict osteoporotic fractures as did hip BMD, but their use in combination incrementally improved prediction [
3]. Moreover, spinal TBS was shown to be associated with prevalent and incident vertebral fractures independently of BMD in Dutch participants of the Rotterdam study [
4]. Therefore, TBS might be a relevant measure of trabecular bone integrity to study in relation to modifiable lifestyle factors, such as dietary intake.
High dietary acid load (DAL) reflects a diet which is rich in nutrients that are metabolized to non-carbonic acids (e.g. sulphuric acid from the metabolism of protein) in amounts that exceed the quantities of alkali bicarbonate produced from combustion of organic salts (such as potassium chloride in vegetables [
5]). Therefore, long-term consumption of such a diet might disturb the balance between CO
2 and HCO
3
−in blood and cause mild but chronic systemic acidosis [
6]. DAL has been suggested to affect bone because bone might serve as the primary buffering system for alkali components such as calcium and potassium in case of systemic acidosis [
7], but this hypothesis has been contradicted by others [
8]. Studies on the relation between DAL and vertebral fractures have shown inconsistent results, and potential associations between DAL and fracture risk are suggested to be mediated by differences in BMD [
9,
10]. That implies that DAL could influence fracture risk via influencing BMD. However, the role of TBS in this association is unclear.
On the one hand, by increasing DAL, dietary protein might have catabolic effects on bone. On the other hand, since the amino acids are important substrates for building bone matrix [
11], dietary protein has anabolic effects. It could therefore be hypothesised that associations between DAL and bone outcomes are non-linear.
Whereas the lungs are the primary organs used to neutralize acute metabolic acidosis, chronic disturbances of the acid-base balance are mainly regulated by the kidneys [
12]. Renal function is an essential determinant of the regulation of acid-base balance via bicarbonate resorption and acid secretion. Impaired renal function is associated with disturbances in mineral and bone metabolism [
13] and fracture risk [
7,
14]. For that reason, we hypothesise that participants with altered renal function are less able to maintain a proper acid-base balance when consuming a diet with high acid load and are therefore more likely to develop low BMD and TBS.
Different food groups are known contributors to DAL. Protein sources such as meat, dairy and grain products contribute to a high DAL, whereas sources of potassium such as vegetables contribute to a low DAL. It has been suggested that contrasting associations between DAL and bone outcomes might have been influenced by dietary fibre intake [
15]. More specifically, high intake of grains contribute to high DAL and high fibre intake, whereas high intake of vegetables contribute to low DAL and high fibre intake. As dietary fibre might reduce intestinal calcium absorption [
15], it could be argued that associations between DAL and bone outcomes might be more detrimental to bone in subjects with high intake of dietary fibre.
Therefore, our main aim was to study the associations of dietary acid load (DAL) with bone mineral density (BMD) and trabecular bone integrity (reflected by TBS) in middle-aged and elderly subjects of the Rotterdam study. Moreover, we explored potential non-linear associations. A secondary aim was to assess whether the magnitude of the associations differ according to renal function and intake of dietary fibres.
Conclusions
In our population of middle-aged and elderly, high NEAP was associated with low trabecular bone integrity. Associations of AnPro/K and VegPro/K and TBS were non-linear and differently shaped.
No significant associations with BMD were observed, nor was any interaction between DAL and renal function in relation to TBS or BMD. Only in participants with high intake of dietary fibre, NEAP might be detrimental to bone outcomes. These findings imply that nutrients that characterise a high DAL diet but are not incorporated in the DAL equation might influence associations of DAL with TBS and BMD.
Acknowledgements
Oscar H. Franco, Jessica C. Kiefte-de Jong and Ester A.L. de Jonge work in ErasmusAGE, a centre for ageing research across the life course funded by Nestlé Nutrition (Nestec Ltd.) and Metagenics Inc. Nestlé Nutrition (Nestec Ltd.) and Metagenics Inc. had no role in the design and conduct of the study; collection, management, analysis and interpretation of the data and preparation, review or approval of the manuscript. Dr. Fernando Rivadeneira received a grant from the Netherlands Organization for Scientific Research (NWO, VIDI 016.136.367). Ester A.L. de Jonge is supported by a grant from the NWO for the graduate program of 2010 (project number: 022.002.023).
The Rotterdam study is supported by the Erasmus MC University Medical Centre and Erasmus University Rotterdam; the Netherlands Organization for Scientific Research (NWO), the Netherlands Organization for Health Research and Development (ZonMw); the Research Institute for Diseases in the Elderly (RIDE); the Netherlands Genomics Initiative (NGI); the Netherlands Consortium of Healthy ageing (NCHA); the Ministry of Education, Culture and Science; the Ministry of Health, Welfare and Sports; the European Commission (DG XII) and the Municipality of Rotterdam. The contribution of inhabitants, general practitioners and pharmacists of the Ommoord district to the Rotterdam study is gratefully acknowledged.
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