Nutritional aspects of bone health

Bone mass, geometry and microstructure, and bony tissue material level properties determine bone strength, hence the resistance to fracture. At a given age, all these variables are the consequence of the amount accumulated and of the structure developed during growth, up to the so-called peak bone mass, and of the bone loss and microstructure degradation occurring later in life. Genetic factors primarily contribute to the variance of the determinants of bone strength. Nutritional intakes are environmental factors that influence both processes, either directly by modifying modelling and remodelling, or indirectly through changes in calcitropic hormone secretion and action. Some effects of nutrition on the offspring bone could take place during foetal life. There are interplays between genetic factors, nutritional intakes and physical exercise. Among the nutrients, sufficient dietary intakes of calcium and protein are necessary for bone health in childhood and adolescence as well as later in life.

Introduction

Age-adjusted incidence of fractures starts to exponentially increase during the 7th decade of life [1]. Skeletal strength, hence resistance to fracture, depends on various quantitative traits (bone mineral content/density, bone turnover, bone geometry and microstructure, bony tissue material level properties), which evolve with time under the influence of genetic, hormonal, nutritional, physical and toxic factors ∗[2], [3]. The genetic determinants appear to be the most important ones, accounting for more than 70% of the variance of the various quantitative components of bone strength [3]. At a given age, bone strength variables are determined by the amount of bone accumulated and the structure developed by the end of skeletal growth, ie the so-called peak bone mass, and by the amount of bone lost and the microstructure degradation occurring thereafter [4], [5] (Fig 1). Thus, peak bone mass is a significant determinant of fracture risk later in life. In addition to genetics, the factors contributing to the large variance in bone mass and structure are race, gender, dietary intakes, endocrine factors, mechanical forces, or the exposure to deleterious influences [4], ∗[6]. The genetic influence is detectable well before puberty and bone growth is following a track throughout puberty [7]. Nutritional intakes are able to modulate this genetic potential, with effects starting as early as in utero [8], or even before conception [9]. Indeed, mother conditions seems to impact BMD in offsprings far later in life [8]. After birth early influence of environmental conditions is exemplified by the fact that prepubertal girls express benefits in bone mass long after the cessation of vitamin D supplements administered during the first year of life [10]. Thus, optimization of peak bone mass through a favourable influence of environmental factors, including nutrition, could be considered as an efficacious long-term prevention of osteoporosis in the oldest old (Fig. 2) [6].

Menopause is associated with a natural decline in oestrogen, that decreases bone mass, deteriorates bone structure and compromises bone strength through an elevation in the rate of bone turnover ∗[2], [11]. Menopause and loss of ovarian function lead to an increase in the prevalence of osteoporosis, which continues to increase through the oldest old period [1]. The rapid decline in BMD and alteration in bone structure after the menopause result in an increased risk of fractures at the sites of vertebrae, distal forearm, proximal humerus, pelvis, ribs, ankle and proximal femur [1]. Hormone replacement therapy (HRT) has been shown to reverse the loss in BMD associated with the menopause and to reduce the risk of fracture [12]. HRT is effective for the prevention of osteoporosis-related fractures in at-risk women and can be proposed within 10 years of menopause ∗[1], [13]. The same factors influencing bone growth are involved in the maintenance of bone health in adulthood and advancing age, in particular dietary intakes, physical exercise and lifestyle toxic factors, with again some interaction with genetics. In the following review, we will focus on the role of calcium and of protein on bone health, and their effects as a combination, in dairy products, will be summarized.