Reduced fibre size, capillary supply and mitochondrial activity in constitutional thinness' skeletal muscle

Similar maximal aerobic capacity but lower energy efficiency during low-to-moderate exercise in women with constitutional thinness: new results from the NUTRILEAN study

PURPOSE

Individuals with constitutional thinness have been presented with a lower muscular energy metabolism at the cellular level but their effective aerobic capacities and exercise-related energy efficiency remains unexplored. The present study compares maximal and sub-maximal aerobic capacities between subjects with constitutional thinness and age-matched normal-weight ones.

METHODS

Anthropometric measures, body composition (Dual-X-ray absorptiometry), physical activity and sedentary time (GT3x actigraphs), and maximal aerobic capacities (cyclingV ˙ O 2peak test) were assessed in 18 constitutionally thin (CT-body mass index < 17.5 kg m-2) and 17 normal-weight (NW-body mass index between 20 and 25 kg m-2) women. Energy efficiency was assessed during a submaximal cycling test and a walking exercise.

RESULTS

CT had a lower body mass and body mass index compared to NW. Absolute peak oxygen uptake and maximal aerobic power were lower in CT subjects compared to NW (ES: - 1.63 [- 2.40; - 0.86] and - 1.32 [- 2.05; - 0.58], p < 0.001).V ˙ O 2peak related to body mass was not different between groups. Gross and net efficiency (ES: - 0.78 [- 1.48; - 0.06], p = 0.03 and ES: - 0.73 [- 1.43; - 0.01], p = 0.05) were lower in CT compared to NW during the submaximal cycling exercise. The gross energy cost of walking related to body mass was lower in subjects with CT (ES: - 1.80 [- 2.60; - 0.97, p = 0.05), with no difference for the net one. Perceived exertion was similar between groups in responses to both submaximal exercises.

CONCLUSION

Constitutionally thin women do not show impaired aerobic capacities at moderate to maximal intensities despite lower energy efficiency while cycling and walking at low-to-moderate intensities.

Constitutional thinness might be characterized by physiologically adapted and not impaired muscle function and architecture: new results from the NUTRILEAN study

PURPOSE

While muscle mass and skeletal muscle fibers phenotype have been shown atypical in constitutional thinness (CT), force production capacities and its architectural determinants have never been explored. The present study compared muscle functionality and architecture between participants with CT and their normal-weight (NW) counterparts.

METHODS

Anthropometry, body composition (Dual-X-ray Absorptiometry), physical activity/sedentary behavior (ActiGraph wGT3X-BT), ultrasound recording of the Vastus Lateralis (2D-ultrasound system), and functional capacities at maximal isometric and isokinetic voluntary contractions (MVCISO and MVCCON) during knee extension (isokinetic dynamometer chair Biodex) have been measured in 18 women with CT (body mass index < 17.5 kg/m2) and 17 NW women.

RESULTS

A lower fat-free mass (ES: -1.94, 95%CI: -2.76 to -1.11, p < 0.001), a higher sedentary time, and a trend for a lower time spent at low-intensity physical activity, were observed in CT vs NW participants. While absolute MVCISO, MVCCON, rate of torque development (RTD), and torque work were all markedly lower in CT, these differences disappeared when normalized to body or muscle mass. Muscle thickness and fascicle length were found lower in CT (ES: -1.29, 95%CI: -2.03 to -0.52, p < 0.001; and ES: -0.87, 95%CI: -1.58 to -0.15, p = 0.02, respectively), while pennation angle was found similar.

CONCLUSION

Despite lower absolute strength capacities observed in CT, present findings support the hypothesis of physiological adaptations to the low body and muscle mass than to some intrinsic contractile impairments. These results call for further studies exploring hypertrophy-targeted strategies in the management of CT.

Challenges of considering both extremities of the weight status spectrum to better understand obesity: insights from the NUTRILEAN project in constitutionally thin individuals

BACKGROUND/OBJECTIVES

While the physiology of obesity has been so extensively investigated to date, only an extremely small number of studies (less than 50) have focused on the other extremity of the weight spectrum: constitutional thinness. Yet, this important state of underweight in the absence of any eating disorders provides a mirror model of obesity that might be particularly insightful in understanding obesity. Nevertheless, important methodological and recruitment-related issues appear when it comes to this complex constitutionally thin phenotype, as experienced by our research group with the realization of the ongoing NUTRILEAN clinical trial. To face this challenge, the present paper aims at identifying, analyzing, and discussing the quality of such recruitment processes in publications about constitutional thinness.

METHODS

In this order, a group of experts collectively created a new grading system to assess the level of rigour and quality achieved by each study based on different criteria.

RESULTS

The main results were that (i) metabolic-related biasing criteria were poorly observed despite being crucial, (ii) recruitment processes were not detailed enough and with sufficient explicitness, and (iii) recruiting among already identified patients would be associated with both higher sample sizes and better scores of quality.

CONCLUSIONS

The present work encourages investigators to adopt a high level of rigour despite the complexity and duration of recruitment processes for this specific population, and readers to pay close attention to the quality of recruitment when interpreting the data. To better understand obesity and its physiological adaptations, it seems essential not only to compare it to normal-weight conditions, but also to the other extremity of the weight status spectrum represented by constitutional thinness.

Proteomics reveals unique plasma signatures in constitutional thinness

PURPOSE

Studying the plasma proteome of control versus constitutionally thin (CT) individuals, exposed to overfeeding, may give insights into weight-gain management, providing relevant information to the clinical entity of weight-gain resistant CT, and discovering new markers for the condition.

EXPERIMENTAL DESIGN

Untargeted protein relative quantification of 63 CT and normal-weight individuals was obtained in blood plasma at baseline, during and after an overfeeding challenge using mass spectrometry-based proteomics.

RESULTS

The plasma proteome of CT subjects presented limited specificity with respect to controls at baseline. Yet, CT showed lower levels of inflammatory C-reactive protein and larger levels of protective insulin-like growth factor-binding protein 2. Differences were more marked during and after overfeeding. CT plasma proteome showed larger magnitude and significance in response, suggesting enhanced "resilience" and more rapid adaptation to changes. Four proteins behaved similarly between CT and controls, while five were regulated in opposite fashion. Ten proteins were differential during overfeeding in CT only (including increased fatty acid-binding protein and glyceraldehyde-3-phosphate dehydrogenase, and decreased apolipoprotein C-II and transferrin receptor protein 1).

CONCLUSIONS AND CLINICAL RELEVANCE

This first proteomic profiling of a CT cohort reveals different plasma proteomes between CT subjects and controls in a longitudinal clinical trial. Our molecular observations further support that the resistance to weight gain in CT subjects appears predominantly biological.

CLINICALTRIALS

gov Identifier: NCT02004821.

New Insights on Bone Tissue and Structural Muscle-Bone Unit in Constitutional Thinness

While few studies pointed out low bone mineral densities in constitutionally thin women, little is known about potential explanations. The objective was to further explore bone architecture in both women and men with constitutional thinness to investigate their mechanical muscle-bone coupling (or uncoupling). Thirty constitutionally thin people and 31 normal weight controls participated in the study. Body composition, hip structural analysis, and trabecular bone score were assessed by dual-energy X-ray absorptiometry, bone architecture using high-resolution peripheral quantitative computed tomography, and muscle explorations through histological staining on muscle biopsies. Thirty-two out of the 48 indexes relative to density, geometry, texture, and architecture of bones were found significantly lower (p < 0.05) in constitutionally thin individuals compared with controls. This observation was particularly pronounced in constitutionally thin men. Bone microarchitecture was more altered in weight-supporting bone (tibia) than in non-weight-supporting (radius) bone, which might refer to a normal physiological adaptation (Frost’s mechanostat theory). Yet, the heat-maps of correlations analyses showed many alterations of body weight or muscle associations with bone parameters in constitutionally thin individuals contrary to controls. Present results might support the idea of intrinsic disturbances of bone cells independently to the small muscle structure, particularly in men.

Is constitutional thinness really different from anorexia nervosa? A systematic review and meta-analysis

A growing interest in constitutional thinness has been observed in the last decades, but the publications however cover various fields of study and report equivocal results. The present work systematically reviewed any clinical trials enrolling participants with constitutional thinness and bibliographic researches were performed between December 2018 and June 2020. From a total of 1 212 records initially identified, 402 records were removed as duplicates, 381 articles were excluded based on titles or abstracts and 390 references were excluded against eligibility criteria. Thirty-nine articles were finally included in the systematic review. The results showed that constitutionally thin people seem to be underweight but not underfat and present a fat-free mass as blunted as anorexic patients, despite being a little less underweight. The meta-analysis confirmed that constitutionally thin people present normal energy intake and revealed a trend toward a higher resting metabolic rate to fat-free mass ratio which suggests a highly metabolic fat-free mass. Contrary to patients with anorexia nervosa, constitutionally thin people present normal levels of insulin-like growth factor 1, estradiol, growth hormone, follicle-stimulating hormone, and luteinizing hormone. An intermediate level of leptin between anorexic and control participants was however observed in constitutional thinness. While all the studies reported normal free triiodothyronine and cortisol levels in constitutionally thin individuals, a higher fasting free triiodothyronine level (p = 0.033) and a lower 24 h mean cortisol level (p = 0.005) were observed for the first time. Present results give robust evidence that constitutionally thin people present an atypical phenotype highly different from anorexia nervosa.

Underweight but not underfat: is fat-free mass a key factor in constitutionally thin women?

Constitutional thinness is defined as a state of severe underweight with a body mass index similar to anorectic patients (BMI < 17.5 kg/m²), in the absence of any eating disorders or other obvious disruptive factors impacting energy balance. The analysis of body composition is essential as a first approach to characterize constitutional thinness and might help identify new discriminating differences between constitutional thinness and anorexia nervosa. A meta-analytical approach was performed to compare body composition of constitutionally thin, anorectic, and normal-weight subjects from all available studies found in the literature. The statistical analysis was carried out on large sample sizes: n = 205 females with constitutional thinness, n = 228 normal-weight control females, and n = 258 females with anorexia nervosa. Despite being as underweight as anorectic patients, constitutionally thin participants paradoxically presented higher percentages of fat mass than anorectic patients (18.9% vs. 11.4%, respectively; SMD [95% CI]: 1.62 [1.16; 2.08]), even found in the normal healthy ranges. Constitutionally thin people, however, display as low fat-free mass as anorectic patients. These observations question the use of high-fat diets in this population and bring new insights for nutrition and/or training strategies directed toward muscle mass gain. The present results give new elements to further distinguish constitutional thinness from anorexia nervosa and reinforce the need to better investigate the atypical phenotype of constitutional thinness.

Increased 1,25(OH)2-Vitamin D Concentrations after Energy Restriction Are Associated with Changes in Skeletal Muscle Phenotype

The influence of energy restriction (ER) on muscle is controversial, and the mechanisms are not well understood. To study the effect of ER on skeletal muscle phenotype and the influence of vitamin D, rats (n = 34) were fed a control diet or an ER diet. Muscle mass, muscle somatic index (MSI), fiber-type composition, fiber size, and metabolic activity were studied in tibialis cranialis (TC) and soleus (SOL) muscles. Plasma vitamin D metabolites and renal expression of enzymes involved in vitamin D metabolism were measured. In the ER group, muscle weight was unchanged in TC and decreased by 12% in SOL, but MSI increased in both muscles (p < 0.0001) by 55% and 36%, respectively. Histomorphometric studies showed 14% increase in the percentage of type IIA fibers and 13% reduction in type IIX fibers in TC of ER rats. Decreased size of type I fibers and reduced oxidative activity was identified in SOL of ER rats. An increase in plasma 1,25(OH)₂-vitamin D (169.7 ± 6.8 vs. 85.4 ± 11.5 pg/mL, p < 0.0001) with kidney up-regulation of CYP27b1 and down-regulation of CYP24a1 was observed in ER rats. Plasma vitamin D correlated with MSI in both muscles (p < 0.001), with the percentages of type IIA and type IIX fibers in TC and with the oxidative profile in SOL. In conclusion, ER preserves skeletal muscle mass, improves contractile phenotype in phasic muscles (TC), and reduces energy expenditure in antigravity muscles (SOL). These beneficial effects are closely related to the increases in vitamin D secondary to ER.

Resistance to lean mass gain in constitutional thinness in free-living conditions is not overpassed by overfeeding

BACKGROUND

Constitutional thinness (CT), a non-malnourished underweight state with no eating disorders, is characterized by weight gain resistance to high fat diet. Data issued from muscle biopsies suggested blunted anabolic mechanisms in free-living state. Weight and metabolic responses to protein caloric supplementation has not been yet explored in CT.

METHODS

A 2 week overfeeding (additional 600 kcal, 30 g protein, 72 g carbohydrate, and 21 g fat) was performed to compare two groups of CTs (12 women and 11 men) to normal-weight controls (12 women and 10 men). Bodyweight, food intake, energy expenditure, body composition, nitrogen balance, appetite hormones profiles, and urine metabolome were monitored before and after overfeeding.

RESULTS

Before overfeeding, positive energy gap was found in both CT genders (309 ± 370 kcal in CT-F and 332 ± 709 kcal in CT-M) associated with higher relative protein intake per kilo (1.74 ± 0.32 g/kg/day in CT-F vs. 1.16 ± 0.23 in C-F, P < 0.0001; 1.56 ± 0.36 in CT-M vs. 1.22 ± 0.32 in C-M, P = 0.03), lower nitrogen (7.26 ± 2.36 g/day in CT-F vs. 11.41 ± 3.64 in C-F, P = 0.003; 9.70 ± 3.85 in CT-M vs. 14.14 ± 4.19 in C-M, P = 0.02), but higher essential amino acids urinary excretion (CT/C fold change of 1.13 for leucine and 1.14 for arginine) in free-living conditions. After overfeeding, CTs presented an accentuated positive energy gap, still higher than in controls (675 ± 540 in CTs vs. 379 ± 427 in C, P = 0.04). Increase in lean mass was induced in both controls genders but not in CTs (a trend was noticed in CT women), despite a similar nitrogen balance after overfeeding (5.06 ± 4.33 g/day in CTs vs. 4.28 ± 3.15 in controls, P = 0.49). Higher anorectic gut hormones' tone, glucagon-like peptide 1 and peptide tyrosine tyrosine, during test meal and higher snacking frequency were noticed before and after overfeeding in CTs.

CONCLUSIONS

The blunted muscle energy mechanism, previously described in CTs in free-living state, is associated with basal saturated protein turn over suggested by the concordance of positive nitrogen balance and an increased urine excretion of several essential amino acids. This saturation cannot be overpassed by increasing this spontaneous high-protein intake suggesting a resistance to lean mass gain in CT phenotype.

Skeletal muscle of females and males with constitutional thinness: a low intramuscular lipid content and oxidative profile

Constitutional thinness (CT) is a nonpathological state of underweight. The current study aimed to explore skeletal muscle energy storage in individuals with CT and to further characterize muscle phenotype at baseline and in response to overfeeding. Thirty subjects with CT (15 females, 15 males) and 31 normal-weight control subjects (16 females, 15 males) participated in the study. Histological and enzymological analyses were performed on muscle biopsy specimens before and after overfeeding. In the skeletal muscle of CT participants compared with controls, we observed a lower content of intramuscular triglycerides for type I (-17%, p < 0.01) and type IIA (-14%, p < 0.05) muscle fibers, a lower glycogen content for type I (-6%, p < 0.01) and type IIA (-5%, p < 0.05) muscle fibers, a specific fiber-type distribution, a marked muscle hypotrophy (-20%, p < 0.001), a low capillary-to-fiber ratio (-19%, p < 0.001), and low citrate synthase activity (-18%, p < 0.05). In response to overfeeding, CT participants increased their intramuscular triglycerides content in type I (+10%, p < 0.01) and type IIA (+9%, p < 0.01) muscle fibers. CT individuals seem to present an unusual muscle phenotype and different adaptations to overfeeding compared with normal-weight individuals, suggesting a specific energy metabolism and muscle adaptations. ClinicalTrials.gov registration no. NCT02004821. Novelty Low intramuscular triglycerides and glycogen content in skeletal muscle of constitutionally thin individuals. Low oxidative capacity, low capillary supply, and fiber hypotrophy in skeletal muscle of constitutionally thin individuals. Increase in intramuscular triglycerides in constitutional thinness in response to overfeeding.

Definition and diagnosis of constitutional thinness: a systematic review

The existing literature about the definition and diagnostic criteria of constitutional thinness (CT) appears equivocal. The present work systematically reviewed the criteria used in the diagnosis of adult individuals with CT (PROSPERO registration number: CRD42019138236). Five electronic bibliographic databases were searched between December 2018 and November 2019: MEDLINE, Embase, CENTRAL (Cochrane Library), Google Scholar and Clinical Trials. Search terms were combined with Medical Subject Headings terms. The search strategy included any clinical trials that enrolled adults with CT. Studies were systematically excluded if the state of thinness was not due to a well-identified constitutional origin. From the 689 references after duplicate removal, 199 studies were excluded based on title and 164 based on abstract. According to the inclusion and exclusion criteria, 291 other studies were removed. Finally, thirty-five studies remained at the end of the process. The analysis of these studies showed high heterogeneity in the diagnostic criteria of CT. A real need emerged to adopt a common terminology and to systematically exclude potential non-constitutional origins of thinness such as eating disorders, associated pathology or over-exercising, with validated tools. Weight history, physiological menses and weight gain resistance are also important criteria to consider. The present systematic review revealed that our medical and scientific approaches of CT need to be harmonised in terms of terminology and diagnostic criteria. Although further studies are needed, we finally proposed recommendations and a decision tree to help in the recognition and diagnosis of CT.

Persistent low body weight in humans is associated with higher mitochondrial activity in white adipose tissue

BACKGROUND

Constitutional thinness (CT) is a state of low but stable body weight (BMI ≤18 kg/m2). CT subjects have normal-range hormonal profiles and food intake but exhibit resistance to weight gain despite living in the modern world's obesogenic environment.

OBJECTIVE

The goal of this study is to identify molecular mechanisms underlying this protective phenotype against weight gain.

METHODS

We conducted a clinical overfeeding study on 30 CT subjects and 30 controls (BMI 20-25 kg/m2) matched for age and sex. We performed clinical and integrative molecular and transcriptomic analyses on white adipose and muscle tissues.

RESULTS

Our results demonstrate that adipocytes were markedly smaller in CT individuals (mean ± SEM: 2174 ± 142 μm 2) compared with controls (3586 ± 216 μm2) (P < 0.01). The mitochondrial respiratory capacity was higher in CT adipose tissue, particularly at the level of complex II of the electron transport chain (2.2-fold increase; P < 0.01). This higher activity was paralleled by an increase in mitochondrial number (CT compared with control: 784 ± 27 compared with 675 ± 30 mitochondrial DNA molecules per cell; P < 0.05). No evidence for uncoupled respiration or "browning" of the white adipose tissue was found. In accordance with the mitochondrial differences, CT subjects had a distinct adipose transcriptomic profile [62 differentially expressed genes (false discovery rate of 0.1 and log fold change >0.75)], with many differentially expressed genes associating with positive metabolic outcomes. Pathway analyses revealed an increase in fatty acid oxidation ( P = 3 × 10-04) but also triglyceride biosynthesis (P = 3.6 × 10-04). No differential response to the overfeeding was observed in the 2 groups.

CONCLUSIONS

The distinct molecular signature of the adipose tissue in CT individuals suggests the presence of augm ented futile lipid cycling, rather than mitochondrial uncoupling, as a way to increase energy expenditure in CT individuals. We propose that increased mitochondrial function in adipose tissue is an important mediator in sustaining the low body weight in CT individuals. This knowledge could ultimately allow more targeted approaches for weight management treatment strategies. This trial was registered at clinicaltrials.gov as NCT02004821.