Our previous study of the distribution of fragility fractures and
osteoporosis in Portugal raised our interest in the upstream determinants of osteoporosis.

Modifiable factors for non-optimal bone accrual during adolescence, such as body composition parameters, are interesting targets for the prevention of future fragility fractures. Partly due to methodological issues, research on the association between adiposity and bone quality in adolescence has resulted in conflicting findings. For a long time, fat mass was believed to have a positive effect on bone accrual. More recently, a deleterious role for adiposity on peak bone mass acquisition was suggested, mediated by systemic inflammation.

In order to understand the potential inflammatory mechanism linking
adiposity and sub-optimal bone mineral density (BMD) it is essential that we understand to which extent this relation is mediated by increased bone resorption and translated by bone remodelling markers.

A major advance in the comprehension of the regulation of osteoclastogenesis was the discovery of the receptor activator of nuclear factor kB (RANK) signalling pathway. The cytokines involved may be useful indicators of metabolic bone disease and as markers of non-optimal bone accrual, resulting from inflammation.

We intend to understand whether adiposity is associated with the process of osteoclastogenesis and overall bone turnover during adolescence, and whether this might reflect a systemic inflammatory process. Ultimately, we intend to assess if this potential mechanism has an effect on our main outcomes: BMD in late adolescence and the increase in bone mass observed up to that point.

Objectives

By using prospective data from a cohort of adolescents born in 1990, our main objectives are:

1. To assess whether systemic low grade inflammation is an intermediate step between obesity and non-optimal bone mineral accrual, using both BMD and bone turnover markers as outcomes

2. To understand which factors are responsible for the difference in the
direction of the association between adiposity and BMD obtained with
cross-sectional and longitudinal approaches

Participants and measurements

We will use data collected from Portuguese adolescents born in 1990
assembled and followed as part of the Epidemiological Health Investigation of Teenagers in Porto (EPITeen).
Baseline and follow-up evaluations included extensive data collection,
comprising two self-administered questionnaires and a physical examination, using the same protocol in both evaluations.
Bone mass was estimated through bone mineral density (BMD, g/cm2) measured at the distal forearm. Anthropometric evaluation comprised weight, height, and fat distribution: waist, hip and arm circumferences, and tricipital and bicipital skinfold thickness. Fat and lean mass were obtained by bioimpedance. Pubertal maturity was estimated using menarche age in girls and serum testosterone in boys.
In both evaluations, 12-hour overnight fasting blood samples were drawn. Serum calcium, total alkaline phosphatase, and high sensitivity C-reactive protein (hs-CRP) were determined in both evaluations.

For the present objectives, we will select 600 participants (300 overweight and 300 normoponderal) and we will determine serum concentrations of leptin, osteoprotegerin, receptor activator of nuclear factor kB ligand, N-terminal propeptide of type I collagen and the CTX domain of the C-terminal telopeptide region of the α1 chain of type I collagen.

We have planned the following analytical steps:

1. To quantify the associations between fat mass and
1.1. bone bioactive molecules secreted by adipocytes (leptin and estradiol)
1.2. a generic marker of inflammation (high sensitivity C-reactive protein)
1.3. cytokine markers of osteoclastogenesis (OPG, RANKL, and OPG/RANKL ratio)

2. To estimate the magnitude of the associations between a generic marker of inflammation (high sensitivity C-reactive protein) and
2.1. markers of bone formation and resorption and the balance between these
2.2. cytokine osteoclastogenesis markers, as a possible causal intermediate between systemic inflammation and increased resorption

3. To quantify the associations between bone mass (BMD at 13 and at 17 and variation in BMD between 13 and 17) and
3.1. a generic marker of inflammation
3.2. cytokine markers of osteoclastogenesis

4. We will conduct a comparative analysis between cross-sectional and
longitudinal approaches to estimate the impact of study design.

By using a large population-based cohort of adolescents with the same age, this investigation will provide an insight into the improvement of bone quality towards peak bone mass, namely on whether populational interventions designed to tackle obesity may be expected to have a further protective effect against fragility fractures.