Bone marrow fat unsaturation in young adults is not affected by present or childhood obesity, but increases with age: A pilot study

Thermal adaptation and fatty acid profiles of bone marrow and muscles in mammals: Implications of a study of caribou (Rangifer tarandus caribou)

Mammals have evolved several physiological mechanisms to cope with changes in ambient temperature. Particularly critical among them is the process of keeping the membrane of cells in a fluid phase to prevent metabolic dysfunction. In this paper, we examine variation in the fatty acid composition of bone marrow and muscle tissues in the cold-adapted caribou (Rangifer tarandus caribou) to determine whether there are systematic differences in fatty acid profiles between anatomical regions that could potentially be explained by thermal adaptation as influenced by cell function, including hematopoiesis. Our results indicate that the bone marrow and muscle tissues from the appendicular skeleton are more unsaturated than the same tissues in the axial skeleton, a finding that is consistent with physiological adaptation of the appendicular regions to thermal challenges. Because mechanisms of thermal adaptation appear to be widely shared among terrestrial mammals, we suggest that the same patterns may prevail in other species, possibly including humans.

Behavior during aging of bone-marrow fatty-acids profile in women's calcaneus to search for early potential osteoporotic biomarkers: a 1H-MR Spectroscopy study

AIM

Identify new potential biomarkers of osteoporosis at an early stage, by magnetic resonance spectroscopy (MRS), studying early changes in the metabolic profile of bone-marrow fatty acids in women's calcanei during healthy aging and osteoporosis status.

METHODS

Single voxel MRS was performed by using a point resolved spectroscopy (PRESS) sequence at 3T. Thirty-four Caucasian women (age range: 22-59 years) were recruited to investigate calcaneus bone marrow. The cohort was constituted of four groups according to age, menopausal status, and T-score evaluated after a DXA examination on the femoral neck. Women were classified in young control (n = 11, mean age = 26.5 ± 3.8 y, age range: 22-34 years), perimenopausal groups (n = 11, mean age = 42.0 ± 3.6 y, age range: 37-47 years), postmenopausal group (n = 9, mean age = 55.4 ± 2.9 y, age range: 50-59 years, mean T-score = -1.70 ± 0.50) and osteoporotic group (n = 6, mean age = 53.0 ± 2.8 y, age range: 50-58 years, mean T-score = -2.54 ± 0.10). The total lipid content (TL), the Unsaturation Index (UI), and the fraction of unsaturated/polyunsaturated fatty acid (f_UFA and f_PUFA) were calculated.

RESULTS

TL was significantly correlated with age (r = 0.73, p < 0.001). TL increases linearly with age in the young + perimenopausal population (r = 0.92, p < 0.001) but this trend is not significant in the postmenopausal subject (r = 0.48, p = 0.07). No significant correlation was found between T-Score and TL in postmenopausal and osteoporotic women, whereas a significant correlation was found between TL and time interval (t_p) between the age at menopause and the age of the subject at the MRS examination. Conversely, no correlation was found between T-score and t_p. The unsaturation index (UI) does not significantly discriminate between osteoporotic, peri- and postmenopausal women. On the other hand, f_UFA is significantly different in peri-menopausal and osteoporotic subjects (p = 0.02), while f_PUFA is significantly different both between peri- and postmenopausal women (p = 0.05) and postmenopausal and osteoporotic subjects (p = 0.03). Both f_UFA and f_PUFA did not correlate with subjects' age.

CONCLUSION

In the female calcaneus, f_UFA and f_PUFA are promising measurable quantities for the characterization of bone marrow's composition potentially correlated with the development of osteoporosis, whereas UI does not differentiate between subjects of varying osteoporotic status. The fact that the TL in the calcaneus is correlated with t_p, indicates that active metabolic changes are still occurring in these subjects, giving complementary information to the DXA about the changes in bone marrow's composition which may affect the whole bone health.

Abdominal adipose tissue and liver fat imaging in very low birth weight adults born preterm: birth cohort with sibling-controls

Preterm birth at very low birth weight (VLBW, < 1500 g) is associated with an accumulation of cardiovascular and metabolic risk factors from childhood at least to middle age. Small-scale studies suggest that this could partly be explained by increased visceral or ectopic fat. We performed magnetic resonance imaging on 78 adults born preterm at VLBW in Finland between 1978 and 1990 and 72 term same-sex siblings as controls, with a mean age of 29 years. We collected T1-weighted images from the abdomen, and magnetic resonance spectra from the liver, subcutaneous abdominal adipose tissue, and tibia. The adipose tissue volumes of VLBW adults did not differ from their term siblings when adjusting for age, sex, and maternal and perinatal factors. The mean differences were as follows: subcutaneous − 0.48% (95% CI − 14.8%, 16.3%), visceral 7.96% (95% CI − 10.4%, 30.1%), and total abdominal fat quantity 1.05% (95% CI − 13.7%, 18.4%). Hepatic triglyceride content was also similar. VLBW individuals displayed less unsaturation in subcutaneous adipose tissue (− 4.74%, 95% CI − 9.2%, − 0.1%) but not in tibial bone marrow (1.68%, 95% CI − 1.86%, 5.35%). VLBW adults displayed similar adipose tissue volumes and hepatic triglyceride content as their term siblings. Previously reported differences could thus partly be due to genetic or environmental characteristics shared between siblings. The VLBW group displayed less unsaturation in subcutaneous abdominal adipose tissue, suggesting differences in its metabolic activity and energy storage.

Fat unsaturation measures in tibial, subcutaneous and breast adipose tissue using short and long TE MRS at 3 T

Fat unsaturation and poly-unsaturation measures can be obtained in vivo with magnetic resonance spectroscopy (MRS) through the olefinic (≈5.4 ppm) and diallylic (≈2.8 ppm) resonances, respectively. Long echo time (TE) MRS sequences have been previously optimized for olefinic/methylene (≈1.3 ppm) or olefinic/methyl (≈0.9 ppm) measures. The objectives of this work, using a Point RESolved Spectroscopy (PRESS) sequence, are to: 1) Investigate olefinic, methyl and methylene resonance decay in subcutaneous, tibial, and breast adipose tissue to determine if a direct comparison of unsaturation measures can be made without correction for T₂ losses. 2) Assess intra-individual fat unsaturation and poly-unsaturation measures in the three adipose tissues. 3) Estimate correction factors for olefinic to methylene ratios to compensate for J-coupling and T₂ relaxation losses that take place when increasing PRESS TE from 40 ms to 200 ms (previously optimized long-TE). 4) Investigate the utility of an inversion recovery for resolving the olefinic resonance from water in adipose tissue. PRESS spectra were acquired from the three adipose regions (breast in female only) in healthy volunteers at 3 T. It was found that olefinic and methyl signal decays faster in breast tissue compared to in tibial bone marrow. Poly-unsaturation measures (diallylic/methylene) differ for tibial bone marrow compared to subcutaneous and breast adipose tissue, with average values of 1.7 ± 0.4, 2.2 ± 0.4, and 2.3 ± 0.8%, respectively. PRESS (TE = 40 ms) with an inversion recovery resolves the olefinic and water resonances in breast tissue with a signal to noise ratio approximately six times greater than that using PRESS with a TE of 200 ms. Stimulated Echo Acquisition Mode (STEAM) with a TE of 20 ms (mixing time of 20 ms) was also combined with IR to resolve the olefinic resonance from that of water is spinal bone marrow.

Quantitative evaluation of bone marrow fat content and unsaturated fatty index in young male soccer players using proton magnetic resonance spectroscopy (1H-MRS): a preliminary study

Background

Marrow fat exists as a distinct adipose tissue and plays a critical role in affecting both the quantity and quality of bone. However, the effect of soccer training on marrow fat has been rarely reported. This study aims to evaluate and characterize the marrow fat content and composition in different bone areas of soccer players and age-matched healthy subjects using proton magnetic resonance spectroscopy (¹H-MRS).

Methods

Between May 2020 and June 2020, 20 professional soccer players (20.7±0.9 years) and 20 age-matched healthy subjects (21.2±0.8 years) were enrolled in this cross-sectional study. The ¹H-MRS were acquired from the 3^rd lumbar vertebrae, bilateral femoral necks, and distal tibias of all subjects using a single-voxel point-resolved spatially localized spectroscopy (PRESS) sequence. Four soccer players underwent a second magnetic resonance (MR) examination within a 30-minute interval after the initial scan to evaluate test-retest reproducibility. Inter- and intra-observer measurement reliabilities were assessed using 10 randomly selected spectra from the soccer players group. All spectra were processed using the jMRUI software package (http://www.jmrui.eu/). Quantified water and lipid signals were used to calculate fat content (FC) and the unsaturated fatty index (UI).

Results

Compared with healthy subjects, we found that soccer players had a lower FC in L3 and bilateral femoral necks and higher UI in the left femoral neck (P<0.05). All FC and UI values of the bilateral distal tibias showed no significant differences between the two groups (P>0.05). The UI values of the right femoral neck or distal tibia were markedly higher than the left side in both inactive subjects and soccer players (P<0.05, except for the femoral neck in players), and there were notable ΔUI differences in the lower limbs between the soccer players and the healthy subjects (P<0.05).

Conclusions

Soccer practice can be considered a positive sport that contributes to decreasing FC in lumbar vertebrae and femoral necks and increasing the UI in femoral necks. Quantitative MRS provides an ideal modality to predict marrow fat metabolism caused by mechanical stimulation.

Fat and bone: the multiperspective analysis of a close relationship

The study of bone has for many years been focused on the study of its mineralized component, and one of the main objects of study as radiology developed as a medical specialty. The assessment has until recently been almost limited to its role as principal component of the scaffolding of the human body. Bone is a very active tissue, in continuous cross-talk with other organs and systems, with functions that are endocrine and paracrine and that have an important involvement in metabolism, ageing and health in general. Bone is also the continent for the bone marrow, in the form of "yellow marrow" (mainly adipocytes) or "red marrow" (hematopoietic cells and adipocytes). Recently, numerous studies have focused on these adipocytes contained in the bone marrow, often referred to as marrow adipose tissue (MAT). Bone marrow adipocytes do not only work as storage tissue, but are also endocrine and paracrine cells, with the potential to contribute to local bone homeostasis and systemic metabolism. Many metabolic disorders (osteoporosis, obesity, diabetes) have a complex and still not well-established relationship with MAT. The development of imaging methods, in particular the development of cross-sectional imaging has helped us to understand how much more laid beyond our classical way to look at bone. The impact on the mineralized component of bone in some cases (e.g., osteoporosis) is well-established, and has been extensively analyzed and quantified through different radiological methods. The application of advanced magnetic resonance techniques has unlocked the possibility to access the detailed study, characterization and quantification of the bone marrow components in a non-invasive way. In this review, we will address what is the evidence on the physiological role of MAT in normal skeletal health (interaction with the other bone components), during the process of normal aging and in the context of some metabolic disorders, highlighting the role that imaging methods play in helping with quantification and diagnosis.

Bone marrow fat fraction assessment in regard to physical activity: KORA FF4-3-T MR imaging in a population-based cohort

OBJECTIVES

To establish the effect of different degrees and kinds of physical activity on bone marrow fat (BMAT) content at different anatomical locations in a population-based cohort study undergoing whole-body MR imaging.

METHODS

Subjects of the KORA FF4 study without known cardiovascular disease underwent BMAT fat fraction (FF) quantification in L1 and L2 vertebrae and femoral heads/necks (hip) via a 2-point T1-weighted VIBE Dixon sequence. BMAT-FF was calculated as mean value (fat image) divided by mean value (fat + water image). Physical activity was determined by self-assessment questionnaire regarding time spent exercising, non-exercise walking, non-exercise cycling, and job-related physical activity.

RESULTS

A total of 385 subjects (96% of 400 available; 56 ± 9.1 years; 58% male) were included in the analysis. Exercise was distributed quite evenly (29% > 2 h/week; 31% ~ 1 h/week (regularly); 15% ~ 1 h/week (irregularly); 26% no physical activity). BMAT-FF was 52.6 ± 10.2% in L1, 56.2 ± 10.3% in L2, 87.4 ± 5.9% in the right hip, and 87.2 ± 5.9% in the left hip (all p < 0.001). Correlation of BMAT-FF between spine and hip was only moderate (r 0.42 to 0.46). Spinal BMAT-FF, but not hip BMAT-FF, was inversely associated with exercise > 2 h/week (p ≤ 0.02 vs. p ≥ 0.35, respectively). These associations remained significant after adjusting for age, gender, waist circumference, and glucose tolerance. No coherent association was found between BMAT-FF and physical activity in the less active groups.

CONCLUSIONS

In our study, exercise was inversely correlated with vertebral BMAT-FF, but not hip BMAT-FF, when exercising for more than 2 h per week. Physical activity seems to affect the spine at least preferentially compared to the hip.

KEY POINTS

• In our population-based cohort, at least 2 h of physical activity per week were required to show lower levels of bone marrow adipose tissue fat fraction in MRI. • Physical activity seems to affect bone marrow adipose tissue at least preferentially at the spine in contrast to the proximal femur.

MRI Assessment of Bone Marrow Composition in Osteoporosis

PURPOSE OF REVIEW

To provide an overview on recent technical development for quantifying marrow composition using magnetic resonance imaging (MRI) and spectroscopy (MRS) techniques, as well as a summary on recent findings of interrelationship between marrow adipose tissue (MAT) and skeletal health in the context of osteoporosis.

RECENT FINDINGS

There have been significant technical advances in reliable quantification of marrow composition using MR techniques. Cross-sectional studies have demonstrated a negative correlation between MAT and bone, with trabecular bone associating more strongly with MAT than cortical bone. However, longitudinal studies of MAT and bone are limited. MAT contents and composition have been associated with prevalent vertebral fracture. The evidence between MAT and clinical fracture is more limited, and, to date, no studies have reported on the relationship between MAT and incident fracture. Increasing evidence suggests a dynamic role of marrow fat in skeletal health. Reliable non-invasive quantification of marrow composition will facilitate developing novel treatment strategies for osteoporosis.

Standardised Nomenclature, Abbreviations, and Units for the Study of Bone Marrow Adiposity: Report of the Nomenclature Working Group of the International Bone Marrow Adiposity Society

Research into bone marrow adiposity (BMA) has expanded greatly since the late 1990s, leading to development of new methods for the study of bone marrow adipocytes. Simultaneously, research fields interested in BMA have diversified substantially. This increasing interest is revealing fundamental new knowledge of BMA; however, it has also led to a highly variable nomenclature that makes it difficult to interpret and compare results from different studies. A consensus on BMA nomenclature has therefore become indispensable. This article addresses this critical need for standardised terminology and consistent reporting of parameters related to BMA research. The International Bone Marrow Adiposity Society (BMAS) was formed in 2017 to consolidate the growing scientific community interested in BMA. To address the BMA nomenclature challenge, BMAS members from diverse fields established a working group (WG). Based on their broad expertise, the WG first reviewed the existing, unsystematic nomenclature and identified terms, and concepts requiring further discussion. They thereby identified and defined 8 broad concepts and methods central to BMA research. Notably, these had been described using 519 unique combinations of term, abbreviation and unit, many of which were overlapping or redundant. On this foundation a second consensus was reached, with each term classified as "to use" or "not to use." As a result, the WG reached a consensus to craft recommendations for 26 terms related to concepts and methods in BMA research. This was approved by the Scientific Board and Executive Board of BMAS and is the basis for the present recommendations for a formal BMA nomenclature. As an example, several terms or abbreviations have been used to represent "bone marrow adipocytes," including BMAds, BM-As, and BMAs. The WG decided that BMA should refer to "bone marrow adiposity"; that BM-A is too similar to BMA; and noted that "Ad" has previously been recommended to refer to adipocytes. Thus, it was recommended to use BMAds to represent bone marrow adipocytes. In conclusion, the standard nomenclature proposed in this article should be followed for all communications of results related to BMA. This will allow for better interactions both inside and outside of this emerging scientific community.

Marrow adipose tissue gradient is preserved through high protein diet and bed rest. A randomized crossover study

CONTEXT

Marrow adipose tissue (MAT) has a peripheral to central distribution in adults, higher in peripheral bones. Similarly, the spine has a caudal to cephalad MAT distribution, higher in lumbar vertebras. Diet and the level of physical activities are known modulators of MAT with significant impact on bone; however, whether these can modulate the MAT gradient is unknown.

OBJECTIVE

To measure the effect of high protein diet and bed rest interventions on the lumbar MAT gradient.

DESIGN PARTICIPANTS INTERVENTION

In a prospective randomized crossover trial, 10 healthy men participated in 2 consecutive campaigns of 21days head-down-tilt-bed-rest (HDTBR). They received either whey protein and potassium bicarbonate-supplemented or control diet separated by a 4-month washout period.

MAIN OUTCOME MEASURES

Ten serial MRI measures of lumbar vertebral fat fraction (VFF) were performed at baseline, 10days and 20days of HDTBR and 3 and 28days after HDTBR of each bed rest campaign.

RESULTS

The mean L5-L1 VFF difference of 4.2 ± 1.2 percentage point higher at L5 (p = 0.008) constituted a caudal to cephalad lumbar MAT gradient. High protein diet did not alter the lumbar VFF differences during both HDTBR campaigns (all time points p > 0.05). Similarly, 2 campaigns of 21days of HDTBR did not change the lumbar VFF differences (all time points p > 0.05).

CONCLUSIONS

This pilot study established that the lumbar vertebral MAT gradient was not altered by a high protein nor by 2 × 21days bed rest interventions. These findings demonstrated that this lack of mechanical stimulus was not an important modulator of the lumbar MAT gradient. The highly preserved MAT gradient needs to be measured in more situations of health and disease and may potentially serve to detect pathological situations.

Bone Marrow Adiposity: Basic and Clinical Implications

The presence of adipocytes in mammalian bone marrow (BM) has been recognized histologically for decades, yet, until recently, these cells have received little attention from the research community. Advancements in mouse transgenics and imaging methods, particularly in the last 10 years, have permitted more detailed examinations of marrow adipocytes than ever before and yielded data that show these cells are critical regulators of the BM microenvironment and whole-body metabolism. Indeed, marrow adipocytes are anatomically and functionally separate from brown, beige, and classic white adipocytes. Thus, areas of BM space populated by adipocytes can be considered distinct fat depots and are collectively referred to as marrow adipose tissue (MAT) in this review. In the proceeding text, we focus on the developmental origin and physiologic functions of MAT. We also discuss the signals that cause the accumulation and loss of marrow adipocytes and the ability of these cells to regulate other cell lineages in the BM. Last, we consider roles for MAT in human physiology and disease.

Is fatty acid composition of human bone marrow significant to bone health?

The bone marrow adipose tissue (BMAT) is a conserved component of the marrow microenvironment, providing storage and release of energy and stabilizing the marrow extent. Also, it is recognized both the amount and quality of BMAT are relevant to preserve the functional relationships between BMAT, bone, and blood cell production. In this article we ponder the information supporting the tenet that the quality of BMAT is relevant to bone health. In the human adult the distribution of BMAT is heterogeneous over the entire skeleton, and both BMAT accumulation and bone loss come about with aging in healthy populations. But some pathological conditions which increase BMAT formation lead to bone impairment and fragility. Analysis in vivo of the relative content of saturated and unsaturated fatty acids (FA) in BMAT indicates site-related bone marrow fat composition and an association between increased unsaturation index (UI) and bone health. With aging some impairment ensues in the regulation of bone marrow cells and systemic signals leading to local chronic inflammation. Most of the bone loss diseases which evolve altered BMAT composition have as common factors aging and/or chronic inflammation. Both saturated and unsaturated FAs originate lipid species which are active mediators in the inflammation process. Increased free saturated FAs may lead to lipotoxicity of bone marrow cells. The pro-inflammatory, anti-inflammatory or resolving actions of compounds derived from long chain poly unsaturated FAs (PUFA) on bone cells is varied, and depending on the metabolism of the parent n:3 or n:6 PUFAs series. Taking together the evidence substantiate that marrow adipocyte function is fundamental for an efficient link between systemic and marrow fatty acids to accomplish specific energy or regulatory needs of skeletal and marrow cells. Further, they reveal marrow requirements of PUFAs.

Bone marrow fat contributes to insulin sensitivity and adiponectin secretion in premenopausal women

PURPOSE

Bone marrow fat is a functionally distinct adipose tissue that may contribute to systemic metabolism. This study aimed at evaluating a possible association between bone marrow fat and insulin sensitivity indices.

METHODS

Fifty obese (n = 23) and non-obese (n = 27) premenopausal women underwent proton magnetic resonance spectroscopy to measure vertebral bone marrow fat content and unsaturation index at L4 level. Abdominal visceral, subcutaneous fat, and epicardial fat were also measured using magnetic resonance imaging. Bone mineral density was measured by dual-energy X-ray absorptiometry. Body composition was assessed by bioelectrical impedance analysis. Fasting serum glucose, insulin, lipids, adiponectin were measured; the insulin resistance index HOMA (HOMA-IR) was calculated.

RESULTS

Bone marrow fat content and unsaturation index were similar in obese and non-obese women (38.5 ± 0.1 vs. 38.6 ± 0.1%, p = 0.994; 0.162 ± 0.065 vs. 0.175 ± 0.048, p = 0.473, respectively). Bone marrow fat content negatively correlated with insulin and HOMA-IR (r = -0.342, r = -0.352, respectively, p = 0.01) and positively with high density lipoprotein cholesterol (r = 0.270, p = 0.043). From a multivariate regression model including lnHOMA-IR as a dependent variable and visceral, subcutaneous, epicardial fat, and bone marrow fat as independent variables, lnHOMA-IR was significantly associated with bone marrow fat (β = -0.008 ± 0.004, p = 0.04) and subcutaneous fat (β = 0.003 ± 0.001, p = 0.04). Bone marrow fat, among the other adipose depots, was a significant predictor of circulating adiponectin (β = 0.147 ± 0.060, p = 0.021). Bone marrow fat unsaturation index negatively correlated with visceral fat (r = -0.316, p = 0.026).

CONCLUSIONS

There is a relationship between bone marrow fat content and insulin sensitivity in obese and non-obese premenopausal women, possibly mediated by adiponectin secretion. Visceral fat does not seem to regulate bone marrow fat content while it may affect bone marrow fat composition.

Intra- and interindividual variability of fatty acid unsaturation in six different human adipose tissue compartments assessed by 1 H-MRS in vivo at 3 T

It is generally accepted that the amount and distribution of adipose tissue (AT) in the human body play an important role in the pathogenesis of metabolic diseases. In addition, metabolic effects of released saturated fatty acids (FAs) in blood are known to be more critical than those of unsaturated FAs. However, little is known about the variability in unsaturation of FAs in various AT compartments. The aim of this prospective study was the assessment of mono- and polyunsaturated FAs in various AT compartments by localized ¹ H-MRS in order to obtain insight into the intra- and interindividual variability. Associations of FA unsaturation with intrahepatic lipids (IHLs), insulin sensitivity and related AT volumes were analyzed. Fifty healthy Caucasians (36 male, 14 female) participated in this study. Spectroscopic examinations were performed in subcutaneous adipose tissue in the neck (SCAT_neck ), abdominal deep subcutaneous adipose tissue (DSCAT), abdominal superficial subcutaneous adipose tissue (SSCAT), visceral adipose tissue (VAT), tibial bone marrow (BM) and subcutaneous adipose tissue of the lower leg (SCAT_calf ) at 3 T. Unsaturated index (UI) was calculated by the ratio of olefinic and methyl resonances, polyunsaturated index (PUI) by the ratio of diallylic and methyl resonances. Volumes of AT compartments (by T₁ -weighted MRI) and IHL (single-voxel STEAM) were assessed at 1.5 T, insulin sensitivity by an oral glucose tolerance test. UI was highest for SCAT_calf (0.622) and lowest for BM (0.527). Highest PUI was observed for SSCAT (0.108), lowest for BM (0.093). Significant intraindividual differences between UIs-but not PUIs-are present for most compartments. There is a non-significant trend for higher UI in males but otherwise no correlation to anthropometric data (age, BMI). A significant negative correlation between UI and AT volume was observed for VAT but for none of the other compartments. Neither UIs nor PUIs show a relation with IHL; insulin sensitivity is significantly correlated to UI in BM (p < 0.01). Unsaturation indices in several distinct AT compartments are location dependent. Our cohort showed only moderate gender-related differences, with a trend towards less unsaturated FAs (mainly PUI) in females. In BM, insulin resistant subjects are characterized by a higher UI compared with the insulin sensitive ones. Further studies in larger cohorts are necessary to gain further insight into unsaturation of AT.

Exercise Regulation of Marrow Adipose Tissue

Despite association with low bone density and skeletal fractures, marrow adipose tissue (MAT) remains poorly understood. The marrow adipocyte originates from the mesenchymal stem cell (MSC) pool that also gives rise to osteoblasts, chondrocytes, and myocytes, among other cell types. To date, the presence of MAT has been attributed to preferential biasing of MSC into the adipocyte rather than osteoblast lineage, thus negatively impacting bone formation. Here, we focus on understanding the physiology of MAT in the setting of exercise, dietary interventions, and pharmacologic agents that alter fat metabolism. The beneficial effect of exercise on musculoskeletal strength is known: exercise induces bone formation, encourages growth of skeletally supportive tissues, inhibits bone resorption, and alters skeletal architecture through direct and indirect effects on a multiplicity of cells involved in skeletal adaptation. MAT is less well studied due to the lack of reproducible quantification techniques. In recent work, osmium-based 3D quantification shows a robust response of MAT to both dietary and exercise intervention in that MAT is elevated in response to high-fat diet and can be suppressed following daily exercise. Exercise-induced bone formation correlates with suppression of MAT, such that exercise effects might be due to either calorie expenditure from this depot or from mechanical biasing of MSC lineage away from fat and toward bone, or a combination thereof. Following treatment with the anti-diabetes drug rosiglitazone - a PPARγ-agonist known to increase MAT and fracture risk - mice demonstrate a fivefold higher femur MAT volume compared to the controls. In addition to preventing MAT accumulation in control mice, exercise intervention significantly lowers MAT accumulation in rosiglitazone-treated mice. Importantly, exercise induction of trabecular bone volume is unhindered by rosiglitazone. Thus, despite rosiglitazone augmentation of MAT, exercise significantly suppresses MAT volume and induces bone formation. That exercise can both suppress MAT volume and increase bone quantity, notwithstanding the skeletal harm induced by rosiglitazone, underscores exercise as a powerful regulator of bone remodeling, encouraging marrow stem cells toward the osteogenic lineage to fulfill an adaptive need for bone formation. Thus, exercise represents an effective strategy to mitigate the deleterious effects of overeating and iatrogenic etiologies on bone and fat.

MR-Based Assessment of Bone Marrow Fat in Osteoporosis, Diabetes, and Obesity

Bone consists of the mineralized component (i.e., cortex and trabeculae) and the non-mineralized component (i.e., bone marrow). Most of the routine clinical bone imaging uses X-ray-based techniques and focuses on the mineralized component. However, bone marrow adiposity has been also shown to have a strong linkage with bone health. Specifically, multiple previous studies have demonstrated a negative association between bone marrow fat fraction (BMFF) and bone mineral density. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) are ideal imaging techniques for non-invasively investigating the properties of bone marrow fat. In the present work, we first review the most important MRI and MRS methods for assessing properties of bone marrow fat, including methodologies for measuring BMFF and bone marrow fatty acid composition parameters. Previous MRI and MRS studies measuring BMFF and fat unsaturation in the context of osteoporosis are then reviewed. Finally, previous studies investigating the relationship between bone marrow fat, other fat depots, and bone health in patients with obesity and type 2 diabetes are presented. In summary, MRI and MRS are powerful non-invasive techniques for measuring properties of bone marrow fat in osteoporosis, obesity, and type 2 diabetes and can assist in future studies investigating the pathophysiology of bone changes in the above clinical scenarios.