Acute continuous moderate-intensity exercise, but not low-volume high-intensity interval exercise, attenuates postprandial suppression of circulating osteocalcin in young overweight and obese adults

The role of bone in energy metabolism: A focus on osteocalcin

Understanding the mechanisms involved in whole body glucose regulation is key for the discovery of new treatments for type 2 diabetes (T2D). Historically, glucose regulation was largely focused on responses to insulin and glucagon. Impacts of incretin-based therapies, and importance of muscle mass, are also highly relevant. Recently, bone was recognized as an endocrine organ, with several bone proteins, known as osteokines, implicated in glucose metabolism through their effects on the liver, skeletal muscle, and adipose tissue. Research efforts mostly focused on osteocalcin (OC) as a leading example. This review will provide an overview on this role of bone by discussing bone turnover markers (BTMs), the receptor activator of nuclear factor kB ligand (RANKL), osteoprotegerin (OPG), sclerostin (SCL) and lipocalin 2 (LCN2), with a focus on OC. Since 2007, some, but not all, research using mostly OC genetically modified animal models suggested undercarboxylated (uc) OC acts as a hormone involved in energy metabolism. Most data generated from in vivo, ex vivo and in vitro models, indicate that exogenous ucOC administration improves whole-body and skeletal muscle glucose metabolism. Although data in humans are generally supportive, findings are often discordant likely due to methodological differences and observational nature of that research. Overall, evidence supports the concept that bone-derived factors are involved in energy metabolism, some having beneficial effects (ucOC, OPG) others negative (RANKL, SCL), with the role of some (LCN2, other BTMs) remaining unclear. Whether the effect of osteokines on glucose regulation is clinically significant and of therapeutic value for people with insulin resistance and T2D remains to be confirmed.

The Impact of Acute Calcium Intake on Bone Turnover Markers during a Training Day in Elite Male Rowers

INTRODUCTION

Although an acute exercise session typically increases bone turnover markers (BTM), the impact of subsequent sessions and the interaction with preexercise calcium intake remain unclear despite the application to the "real-life" training of many competitive athletes.

METHODS

Using a randomized crossover design, elite male rowers ( n = 16) completed two trials, a week apart, consisting of two 90-min rowing ergometer sessions (EX1, EX2) separated by 150 min. Before each trial, participants consumed a high (CAL; ~1000 mg) or isocaloric low (CON; <10 mg) calcium meal. Biochemical markers including parathyroid hormone (PTH), serum ionized calcium (iCa) and BTMs (C-terminal telopeptide of type I collagen, osteocalcin) were monitored from baseline to 3 h after EX2.

RESULTS

Although each session caused perturbances of serum iCa, CAL maintained calcium concentrations above those of CON for most time points, 4.5% and 2.4% higher after EX1 and EX2, respectively. The decrease in iCa in CON was associated with an elevation of blood PTH ( P < 0.05) and C-terminal telopeptide of type I collagen ( P < 0.0001) over this period of repeated training sessions and their recovery, particularly during and after EX2. Preexercise intake of calcium-rich foods lowered BTM over the course of a day with several training sessions.

CONCLUSIONS

Preexercise intake of a calcium-rich meal before training sessions undertaken within the same day had a cumulative and prolonged effect on the stabilization of blood iCa during exercise. In turn, this reduced the postexercise PTH response, potentially attenuating the increase in markers of bone resorption. Such practical strategies may be integrated into the athlete's overall sports nutrition plan, with the potential to safeguard long-term bone health and reduce the risk of bone stress injuries.

Prior aerobic exercise mitigates the decrease in serum osteoglycin and lipocalin-2 following high-glucose mixed-nutrient meal ingestion in young men

Osteoglycin (OGN) and lipocalin-2 (LCN2) are hormones that can be secreted by bone and have been linked to glucose homeostasis in rodents. However, the endocrine role of these hormones in humans is contradictory and unclear. We examined the effects of exercise and meal ingestion on circulating serum OGN and LCN2 levels in eight healthy males {age: 28 [25, 30] years [median ± interquartile range (IQR)] and body mass index [BMI]: 24.3 [23.6, 25.5] kg/m²}. In a randomized crossover design, participants ingested a high-glucose (1.1 g glucose/kg body wt) mixed-nutrient meal (45% carbohydrate, 20% protein, and 35% fat) on a rest-control day and 3 and 24 h after aerobic cycling exercise (1 h at 70%-75% V̇o_2peak). Acute aerobic exercise increased serum LCN2 levels immediately after exercise (∼61%), which remained elevated 3-h postexercise (∼55%). In contrast, serum OGN remained similar to baseline levels throughout the 3-h postexercise recovery period. The ingestion of a high-glucose mixed-nutrient meal led to a decrease in serum OGN at 90-min (approximately -17%) and 120-min postprandial (approximately -44%), and a decrease in LCN2 at 120-min postprandial (approximately -26%). Compared with the control meal, prior exercise elevated serum OGN and LCN2 levels at 120-min postprandial when the meal was ingested 3-h (OGN: ∼74% and LCN2: ∼68%) and 24-h postexercise (OGN: ∼56% and LCN2: ∼16%). Acute exercise increases serum LCN2 and attenuates the postprandial decrease in OGN and LCN2 following high-glucose mixed-nutrient meal ingestion. The potential endocrine role of circulating OGN and LCN2 in humans warrants further investigation.NEW & NOTEWORTHY We provide novel evidence that OGN and LCN2 decrease 120 min after ingesting a high-glucose mixed-nutrient meal in healthy adults. Acute aerobic exercise increases circulating LCN2 for up to 3-h postexercise, whereas circulating OGN remains similar to baseline. Despite differing postexercise responses, postprandial LCN2 and OGN are elevated when the high-glucose meal is ingested 3-h and 24-h postexercise. Findings support that OGN and LCN2 are dynamically linked to energy homeostasis in humans.

Osteocalcin and its forms respond similarly to exercise in males and females

INTRODUCTION

Acute exercise increases osteocalcin (OC), a marker of bone turnover, and in particular the undercarboxylated form (ucOC). Males and females differ in baseline levels of total OC and it is thought the hormonal milieu may be driving these differences. Males and females adapt differently to the same exercise intervention, however it is unclear whether the exercise effects on OC are also sex-specific. We tested whether the responses of OC and its forms to acute High Intensity Interval Exercise (HIIE) and High Intensity Interval Training (HIIT) differed between males and females. Secondly, we examined whether sex hormones vary with OC forms within sexes to understand if these are driving factor in any potential sex differences.

METHODS

Total OC (tOC), undercarboxylated OC (ucOC), and carboxylated OC (cOC) were measured in serum of 96 healthy participants from the Gene SMART cohort (74 males and 22 females) at rest, immediately after, and 3 h after a single bout of HIIE, and at rest, 48 h after completing a four week HIIT intervention. Baseline testosterone and estradiol were also measured for a subset of the cohort (Males = 38, Females = 20). Linear mixed models were used to a) uncover the sex-specific effects of acute exercise and short-term training on OC forms and b) to examine whether the sex hormones were associated with OC levels.

RESULTS

At baseline, males had higher levels of tOC, cOC, and ucOC than females (q < 0.01). In both sexes tOC, and ucOC increased to the same extent after acute HIIE. At baseline, in males only, higher testosterone was associated with higher ucOC (β = 3.37; q < 0.046). Finally, tOC and ucOC did not change following 4 weeks of HIIT.

CONCLUSION/DISCUSSION

While there were no long-term changes in OC and its forms. tOC and ucOC were transiently enhanced after a bout of HIIE similarly in both sexes. This may be important in metabolic signalling in skeletal muscle and bone suggesting that regular exercise is needed to maintain these benefits. Overall, these data suggest that the sex differences in exercise adaptations do not extend to the bone turnover marker, OC.

The effect of exercise training on osteocalcin, adipocytokines, and insulin resistance: a systematic review and meta-analysis of randomized controlled trials

Recently, it has been reported that osteocalcin (OC), in particular its undercarboxylated (ucOC) form, is not only a bone remodeling marker but also an active hormone that intercedes glucose metabolism in humans. This study aimed to determine the impact of an exercise intervention on ucOC, adiponectin, leptin, and insulin resistance (measured by HOMA-IR). PubMed, CINAHL, Medline, Google Scholar, and Scopus databases and reference lists of included studies were searched. Twenty-two randomized controlled trials (RCTs) of exercise training impact in adults were included in the analysis. Results showed an overall significant increase in serum ucOC (MD: 0.15 ng/ml; 95% CI: 0.05 to 0.25) and adiponectin (MD: 2.83 mg/ml; 95% CI: 1.67 to 3.98), a significant decline in leptin (MD: - 4.89 pg/ml; 95% CI: - 6.94 to - 2.84), fasting glucose (MD: - 2.29 mg/dl; 95% CI: - 4.04 to - 0.54), fasting insulin (MD, - 8.90 μIU/ml; 95% CI: - 13.81 to - 3.98), and HOMA-IR (MD: - 1.96; 95% CI: - 3.11 to - 0.80). However, after removal of studies that had prescribed a balanced diet along with exercise intervention, total OC (TOC) levels also increased in the exercise group compared with the control group (MD: 0.36 ng/ml; 95% CI: 0.07 to 0.65). Our findings demonstrate that exercise-induced increases in ucOC are the probable cause of increased adiponectin. Additionally, increases in ucOC itself are probably due to changes in leptin levels and other factors, rather than its direct impact on bone and its osteoblastic activity. Further studies are required to clarify the mechanisms underlying the impact of exercise training on ucOC, adipocytokines, and insulin resistance.

High-glucose mixed-nutrient meal ingestion impairs skeletal muscle microvascular blood flow in healthy young men

Oral glucose ingestion leads to impaired muscle microvascular blood flow (MBF), which may contribute to acute hyperglycemia-induced insulin resistance. We investigated whether incorporating lipids and protein into a high-glucose load would prevent postprandial MBF dysfunction. Ten healthy young men (age, 27 yr [24, 30], mean with lower and upper bounds of the 95% confidence interval; height, 180 cm [174, 185]; weight, 77 kg [70, 84]) ingested a high-glucose (1.1 g/kg glucose) mixed-nutrient meal (10 kcal/kg; 45% carbohydrate, 20% protein, and 35% fat) in the morning after an overnight fast. Femoral arterial blood flow was measured via Doppler ultrasound, and thigh MBF was measured via contrast-enhanced ultrasound, before meal ingestion and 1 h and 2 h postprandially. Blood glucose and plasma insulin were measured at baseline and every 15 min throughout the 2-h postprandial period. Compared with baseline, thigh muscle microvascular blood volume, velocity, and flow were significantly impaired at 60 min postprandial (-25%, -27%, and -46%, respectively; all P < 0.05) and to a greater extent at 120 min postprandial (-37%, -46%, and -64%; all P < 0.01). Heart rate and femoral arterial diameter, blood velocity, and blood flow were significantly increased at 60 min and 120 min postprandial (all P < 0.05). Higher blood glucose area under the curve was correlated with greater MBF dysfunction (R² = 0.742; P < 0.001). Ingestion of a high-glucose mixed-nutrient meal impairs MBF in healthy individuals for up to 2 h postprandial.

Osteocalcin and its forms across the lifespan in adult men

PURPOSE

Osteocalcin (OC), an osteoblast-specific secreted protein expressed by mature osteoblasts, is used in clinical practice and in research as a marker of bone turnover. The carboxylated (cOC) and undercarboxylated (ucOC) forms may have a different biological function but age-specific reference ranges for these components are not established. Given the different physiological roles, development of reference ranges may help to identify people at risk for bone disease.

METHODS

Blood was collected in the morning after an overnight fast from 236 adult men (18 to 92 years old) free of diabetes, antiresorptive, warfarin or glucocorticoid use. Serum was analyzed for total osteocalcin (tOC) and the ucOC fraction using the hydroxyapatite binding method. cOC, ucOC/tOC and cOC/tOC ratios were calculated. Reference intervals were established by polynomial quantile regression analysis.

RESULTS

The normal ranges for young men (≤30 years) were: tOC 17.9-56.8 ng/mL, ucOC 7.1-22.0 ng/mL, cOC 8.51-40.3 ng/mL (2.5th to 97.5th quantiles). Aging was associated with a "U" shaped pattern for tOC, cOC and ucOC levels. ucOC/tOC ratio was higher, while cOC/tOC ratio was lower in men of advanced age. Age explained ∼31%, while body mass index explained ∼4%, of the variance in the ratios.

CONCLUSIONS

We have defined normal reference ranges for the OC forms in Australian men and demonstrated that the OC ratios may be better measures, than the absolute values, to identify the age-related changes on OC in men. These ratios may be incorporated into future research and clinical trials, and their associations with prediction of events, such as fracture or diabetes risk, should be determined.