Introduction

Obesity is a severe public health problem with an increasing prevalence and high impact on mortality and morbidity worldwide. Obesity leads to many adverse health consequences. In fact, it induces an insulin-resistant state in adipose tissue, liver, and muscle and represents a strong risk factor for the development of type 2 diabetes mellitus, cardiovascular diseases, and cancer [1].

The World Health Organization defines obesity as an abnormal or excessive fat accumulation that presents a risk to health, characterized by a body mass index (BMI) of ≥30 kg/m2. However, this definition, based on the BMI, does not consider the body composition of the subjects and the relative contributions of fat-free mass and fat mass to body weight and their implication in determining the chronic metabolic alterations induced by obesity. While it is well recognized the detrimental effect of abdominal adipose tissue on chronic metabolic alterations, less is known about the role of lean mass in obese subjects and the potential interactions between adipose and lean tissues. Since recent studies have demonstrated that obesity should not be considered only as an increase in body weight, it has become clear how it should be stressed the importance of body composition and the role that the various components of fat and lean mass might play in metabolic control, more than BMI itself.

Indeed, several hormonal and environmental factors can be involved in influencing body weight and fat accumulation and it is known that increased production of adipokines by visceral fat results in adverse effects on metabolic status, cardiovascular homeostasis, and skeletal health [2].

Moreover, obesity has been associated with sarcopenia, a condition characterized by progressive decline of muscle mass, quality, and strength likely due to the reduced physical activity over the time in obese individuals, leading to a new pathological condition defined as sarcopenic obesity [3].

Osteoporosis is a metabolic bone disease characterized by an increased risk of developing spontaneous or traumatic fracture due to skeletal fragility [4]. The relationship between fat and bone is a rapidly growing area in bone research, and a number of findings are relevant to the pathophysiology of increased bone fragility in obese individuals [5, 6].

In fact, a cross talk between fat, muscle, and bone has been speculated, and it might constitute a homeostatic feedback system in which adipokines, cytokines, hormones, and growth factors produced by adipose, muscle, and skeletal tissues represent the link of an active bone–muscle-adipose axis which probably represents a homeostatic system to maintain health.

Interestingly, the growth hormone (GH)/Insulin-like grow factor-1 (IGF-1) axis is involved in a large number of functions, from childhood to senescence, regulating growing, aging, and lifespan [7]. In particular, a declined function of GH/IGF-1 axis has been linked to age-related changes in body composition, metabolic functions, and to an increased risk of cardiovascular disease [8].

Thus, reduced GH levels emerge as a potential important biomarker of aging, and GH decline can accelerate aging processes and it might be associated with significant “costs” in terms of longevity.

Circulating levels of GH and IGF-1, the key mediator of the most GH actions, decline with age starting soon after sexual and physical maturation attempts [9].

In addition, the chronic low-grade inflammation associated with aging-related changes in fat distribution and function is considered, as well as the ‘inflammatory load’, related to the life-time history of infectious diseases, and has thought of being among the major determinants of human aging and lifespan [10, 11]. Increase of adipose tissue in obese humans and animals involves adipocyte hypertrophy as well as hyperplasia, which promote inflammation by changing the secretory cell profile, including production and secretion of proinflammatory adipokines as TNF-α and IL-6 [12, 13].

Thus, aim of this study was to evaluate the potential correlation among IGF-1, BMD, and inflammation status in adult subjects affected by obesity.

Materials and methods

Subjects

This cross-sectional study included 426 obese subjects (86 men and 340 female; mean age: 44.8 ± 14 years; BMI 34.9 ± 6.1 kg/m2), selected from a cohort of patients admitted to the day hospital of the Department of Experimental Medicine, Section of Medical Pathophysiology, Endocrinology and Nutrition, Policlinico Umberto I, Sapienza University of Rome, for clinical evaluation and treatment of obesity.

Exclusion criteria were: chronic medical conditions or use of medications affecting bone metabolism, hormonal, and nutritional status, 25-OH vitamin D (vitamin D) supplementation, recent weight loss, and prior bariatric surgery. All subjects provided informed consent before taking part in the study, and the local ethical committee approved the research protocol.

Patients underwent complete medical history and clinical examination. Blood samples were collected in the morning (07.00–08.00 am) after an overnight fast. Sera were then kept at −80 °C until analysis. Anthropometric measurements included weight and height; body weight was evaluated as the subjects were fasting overnight and wearing underwear. BMI was calculated as weight (kg)/height2 (m2). Patients underwent measurements of bone mineral density at lumbar and hip site (BMD g/cm2) and of body composition [lean mass (LM), total and trunk fat mass (TFM)] by dual X-ray absorptiometry (DEXA), and were evaluated for hormonal and metabolic profile and inflammatory markers.

Assays

Osteocalcin (OSCA) was measured with solid-phase commercial RIAs (provided by Diagnostic Products, Los Angeles, CA, and Diagnostics Systems Laboratories, Inc.Webster, TX). Measurements of glucose concentrations were assessed by the standard immuno-enzymatic methods, while insulin was measured by radioimmunoassay. IGF-1 was assayed by an immunoradiometric assay (Diagnostic System Laboratories Inc., Webster, TX, USA). Insulin levels were measured by radioimmunoassay, while fibrinogen levels were measured by an automated clinical chemistry analyzer (Modular P and Modular E for PSA, Roche Diagnostics GmbH, Mannheim, Germany). The intra- and inter-assay coefficients of variations for all hormonal assays ranged 3.4–6.2 and 3.6–8.4%, respectively. All measurements were performed in duplicate.

Dual-energy-X-ray absorptiometry measurement

Body fat mass, fat-free mass (kg), and both lumbar and femoral BMD were measured by DEXA (Dual-energy-X-ray absorptiometry, Hologic 4500 RDR), with coefficient of variation of <1% for bone density and <1.5% for fat mass. Amount of trunk fat mass was distinguished from peripheral and appendicular fat mass as a measure of abdominal adiposity. In particular, trunk fat was defined as the adipose tissue localized within the region below the chin, delineated by vertical lines within the left and right glenoid fossae bordering laterally to the ribs, and by the oblique lines that cross the femoral necks and converge below the pubic symphysis.

Statistical analysis

Data are depicted as the mean ± SD of absolute value except for skewed variables, which were instead presented as median (interquartile range 25–75%). Continuous variables were normally distributed (Shapiro–Wilk test) and were analyzed using Student’s t test for paired or unpaired data. Pearson’s 2 test, Wilcoxon’s signed-rank test, and Spearman’s correlation analysis as appropriate. Statistical analysis was performed using the computer statistical package SPSS/10.0 (SPSS, Chicago, IL, USA) and SAS/6.4 (SAS Institute Cary, NC, USA).

Results

A total of 426 obese subjects (86 men and 340 women), mean age 44.8 years, were included in the final analysis. Clinical and biochemical characteristics of this population have already been described elsewhere [14]. Briefly, BMI of the female population was 35.2 ± 5.2, while BMI of male population was 35.4 ± 6.1. As previously described [14], no differences were found between male and female gender in any of the parameters considered, as depicted in Table 1. Interestingly, in both female and male subjects, we found an inverse correlation between IGF-1 and fat mass (Fig. 1, p < 0.001, r 2 = 0.12), accordingly with the observation that higher levels of IGF-1 were directly associated with higher degree of lean mass, as we previously demonstrated elsewhere [14].

Table 1 Mean biochemical values in both male and female adult subjects
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Fig. 1
figure 1

Correlation between IGF-1 (ng/mL) and fat mass (FM%)

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Moreover, a positive correlation between IGF-1 and the non-collagenic matrix protein osteocalcin, marker of bone formation (Fig. 2, p < 0.002, r 2 = 0.14), as well as an inverse correlation between IGF-1 and nonspecific inflammatory markers, such as fibrinogen (Fig. 3a, p < 0.009, r 2 = 0.04) and erythrocyte sedimentation rate (Fig. 3b, p < 0.0001, r 2 = 0.03), were found.

Fig. 2
figure 2

Correlation between IGF-1 (ng/mL) and osteocalcin (OSCA-µg/L)

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Fig. 3
figure 3

Correlation of IGF-1 levels and inflammation markers. a Correlation between IGF-1 and fibrinogen (FBR—mg/dL); b correlation between IGF-1 (ng/mL) and eritrosedimentation rate (ESR—mm/h)

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Finally, IGF-1, as expected being this factor anabolic for the bone, was directly correlated with higher BMD, at both lumbar (Fig. 4a, p < 0.02, r 2 = 0.03) and femoral site (Fig. 4b, p < 0.04, r 2 = 0.03). Proinflammatory cytokines were not significantly modified in this population of young obese subjects (dat not shown). Thus, no correlations were found among IGF-1, BMD, and fat issue (data not shown).

Fig. 4
figure 4

Correlation of IGF-1 levels with bone mineral density in different skeletal sites. a Correlation between IGF-1 (ng/mL) and Lumbar BMD; b correlation between IGF-1 (ng/mL) and Hip BMD

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Discussion

The results presented in the manuscript demonstrate that higher levels of serum IGF-1 in adult obese patients are associated with a lower inflammatory pattern and that a direct association between both lumbar and femoral BMD is present for higher IGF-1 serum levels.

Aging correlates with increased risk of cancer, diabetes, atherosclerosis, metabolic syndrome, sarcopenia, and osteoporosis [15]. Furthermore, an important change associated with aging is a decline in immune system function and many aspects of aging involve inflammatory processes [16]. It is well known that a strong association between aging and chronic low-grade inflammatory activity might progress to long-term tissue damage and systemic chronic inflammation [17]. This latter could cause organ specific illness and, at the end, might increase the risk of mortality. With growing elderly populations and extended longevity in developed countries, there is increasing interest in elucidating the age-related inflammation processes. In addition, increasing incidence of obesity in industrialized countries can intensify this problem, due to known proinflammatory action induced by excess of abdominal adipose tissue [18]. On the opposite, calorie restriction diet, known to extend longevity, was recurrently shown to induce a low body weight, enhance the immune function, and ameliorate inflammation [19].

The most recent literature reports that GH is able to exert both anti- and proinflammatory activities in humans [20] and that inflammation can influence GH release and actions [21]. Studies in mice have demonstrated that blood levels of the proinflammatory cytokines, interleukin 6 (IL-6), and tumor necrosis factor alpha (TNF-α) are reduced in long-lived GH-deficient and GH-resistant mice [22]. Moreover, circulating levels of adiponectin, an anti-inflammatory adipokine, are consistently elevated. This latter observation was unexpected, since the absence of GH signals in these mutants leads to increased adiposity, and because, in both animal models and humans, plasma levels of adiponectin were repeatedly shown to be inversely rather than directly related to adiposity [23]. According to this theory, hereditary GH resistance in an extensively evaluated population of individuals with Laron dwarfism in Ecuador has been associated with striking, nearly complete protection from cancer and diabetes [24]. Moreover, a cohort of GH-deficient dwarfs studied in Brazil was shown to be unexpectedly protected from atherosclerosis in spite of obesity and unfavorable serum lipid profiles [25]. In contrast, experiments in mice fed with high-fat diet (HFD)-induced obesity reported hypothalamic inflammation and gliosis [26]. The hypothalamus plays a fundamental role for the coordination of food intake and energy expenditure [27], with specialized neurons relaying information provided by leptin and insulin regarding fat depot sites to central mechanisms that regulate hunger and energy expenditure. Hypothalamic inflammation due to HFD-induced obesity has been associated with the development of secondary complications [28], and GH and IGF-1 axis have been described to play anti-inflammatory actions [29].

Since GH and IGF-1 can modulate inflammatory processes in both manners, no definitive conclusions can be drawn. Our data show the link between nonspecific inflammatory markers and IGF-1 levels, in a large adult population affected by obesity.

Obesity is associated with abnormalities of the GH/IGF-1 axis and the role of serum IGF-1 measurement for recognition of hypothalamic-pituitary disorders in obesity is still a matter of debate. Interestingly, skeletal maturation can be influenced by multiple factors and almost 25% of children affected by obesity show advanced bone age, associated with increased BMI Z-score, and one or more of the following altered values: insulin, leptin, DHEAS, IGF-1, and rate of weight gain [30]. No clear relationship or defined mechanisms of actions have been established, as yet, regarding the link that occurs among IGF-1, BMD, and obesity in adult population.

Our data demonstrate that higher levels of IGF-1 are associated with a lower inflammatory pattern and a higher BMD in adult obese individuals. IGF-1 seems to play a role in this homeostatic model of inflammation, aging, and bone health. Higher levels of IGF-1 were also associated with a better quality of life and lower degree of frailty in elderly subjects. IGF-1 may affect both muscle and bone through systemic and tissue specific signaling pathways. A trial by Terracciano et al. revealed that osteoporosis is related to muscle atrophy, which is linked with BMD and a reduced level of Akt, a component of the IGF-1/phosphatidylinositol 3-kinase (PI3-K)/Akt pathway. In addition, abnormalities in GH/IGF-1 signaling have a negative effect on muscle and bone in elder subjects [31]. IGF-1 action is modulated by IGF-binding proteins (IGFBPs) which are secreted by muscle and might act on bone tissue.

In a cohort study, appendicular skeletal muscle mass correlated with cortical thickness and trabecular BMD. In that study, serum IGFBP-2 concentration was the most robust negative predictor of appendicular skeletal muscle [32]. The results of that trial led us to hypothesize that IGFBP-2 may bind to IGF-1 in the blood circulation and inhibit the beneficial effects of somatomedin on muscle and bone tissues. Moreover, in another trial, high IGFBP-2 circulating concentrations were associated with lower BMD in men and women [33].

Interestingly, IGF-1 has also been associated with inflammatory pattern in specific group of patients. For instance, an inverse relationship between inflammation and IGF-1 availability was observed in a clinical trial on HIV patients. In the study, inflammatory mediators showed positive correlations with IGFBP1/IGFBP2 and negative correlations with IGF-1/IGF-2, indicating that inflammation suppresses IGF-1/IGF-2 production [34]. IGF-1 has also showed an active immune regulatory capability versus monocytes in inflammatory bowel disease murine models. This hormone can induce IL-10 expression in monocytes, suppressing proinflammatory pattern in colitis [35].

Our data suggest that this aspect of signaling status could also be evident in obese adults, where a proinflammatory status might be triggered by adipose tissue mediators. Finally, we observed a positive correlation between IGF-1 and osteocalcin, an important non-collagenic protein of the bone matrix, produced by osteoblasts and considered a marker of bone formation. As we previously published, in the same population, lean body mass strongly correlates with osteocalcin and IGF-1 levels in a positive manner [14]. In addition, other authors demonstrated that in obese premenopausal women, affected by abdominal obesity, 6-month treatment with injections of GH increases the levels of IGF-1 and P1NP, the last one being an important marker of bone formation, as well as OSCA [36]. In their elegant study, these authors observed that GH treatment reduced the amount of abdominal fat and increased preadipocyte factor 1 (Pref 1), a preadipocyte secreted factor that inhibits adipogenesis, modulates mesenchymal stem cells commitment to chondrocytes and osteoblasts, and inhibits their differentiation into adipocytes [36].

A link between bone, fat, and muscle has been previously described by our group [2, 15] as well by others. Several potential mechanisms have been proposed to explain the complex relationship among these mesenchymal-derived tissues. In fact, adipose tissue secretes various inflammatory cytokines (such as IL-6, TNF-α), which are thought to have adverse metabolic and skeletal consequences. Moreover, other mediators, such as leptin, resistin, and adiponectin, are also involved in bone metabolism as we have recently described in vitro model system elsewhere [37, 38].

We realize that there are some study limitations linked to the lack of data regarding correlation between IGF-1 levels and other inflammatory markers, such as proinflammatory cytokines in our population of obese individuals, however, recent data show correlation between GH/IGF-1 status and inflammation in healthy volunteers [29].

In conclusion, our results show that higher levels of serum IGF-1 in adult obese patients are associated with lower inflammatory pattern and better skeletal homeostasis, as demonstrated by higher lumbar and femoral BMD and higher OSCA levels, leading to speculate the existence of a bone-adipose-muscle interplay in the modulation/regulation of energy/glucose homeostasis, skeletal metabolism, and chronic inflammation in adult subjects affected by abdominal obesity.

New research aimed to better clarify the link that occurs between body composition, GH/IGF-1 axis, inflammation status, and bone metabolism in obese subjects which are necessary, and they might lead to discover new lines of treatment of this pathological status.