Brown adipose tissue in young adults who were born preterm or small for gestational age

Preterm-born individuals: a vulnerable population at risk of cardiovascular morbidity and mortality during thermal extremes?

NEW FINDINGS

What is the topic of this review? Thermal extremes disproportionately affect populations with cardiovascular conditions. Preterm birth, across all gestational age ranges below 37 weeks, has been identified as a non-modifiable risk factor for cardiovascular disease. The hypothesis is presented that individuals born preterm are at an increased risk of cardiovascular morbidity and mortality during thermal extremes. What advances does it highlight? Cardiovascular stress tests performed in preterm-born populations, from infancy through adulthood, highlight a progression of cardiovascular dysfunction accelerating through adolescence and adulthood. This dysfunction has many similarities with populations known to be at risk in thermal extremes.

ABSTRACT

Preterm-born individuals are a uniquely vulnerable population. Preterm exposure to the extrauterine environment and the (mal)adaptations that occur during the transitional period can result in alterations to their macro- and micro-physiological state. The physiological adaptations that increase survival in the short term may place those born preterm on a trajectory of lifelong dysfunction and later-life decompensation. Cardiovascular compensation in children and adolescents, which masks this trajectory of dysfunction, is overcome under stress, such that the functional cardiovascular capacity is reduced and recovery impaired following physiological stress. This has implications for their response to thermal stress. As the Anthropocene introduces greater changes in our environment, thermal extremes will impact vulnerable populations as yet unidentified in the climate change context. Here, we present the hypothesis that individuals born preterm are a vulnerable population at an increased risk of cardiovascular morbidity and mortality during thermal extremes.

Circulating Irisin Levels in Preadolescents and Adolescents Born Preterm

INTRODUCTION

Prematurity is associated with increased cardiometabolic risk later in life. The adipomyokine irisin has been acknowledged as a modulator of energy metabolism and insulin sensitivity. The aim of this study was to investigate circulating levels of irisin and their relation to anthropometric measurements and cardiometabolic phenotype in a population of preterm-born children versus full-term-born peers.

METHODS

A total of 160 children (87 born preterm aged 8.1-14.8 years and 73 born full-term of similar age and gender distribution) were studied. Arterial blood pressure, anthropometry, body composition assessments with dual energy X-ray absorptiometry, and skinfold measurements were performed. Blood biochemistry and circulating levels of irisin, insulin, cortisol, leptin, and adiponectin were also determined.

RESULTS

The preterm group had higher diastolic blood pressure, triceps skinfold, subscapular skinfold (SSSF), and abdominal skinfold measurements and more central adiposity than the full-term group. Irisin was significantly lower (p = 0.002), whereas leptin was higher (p = 0.03) in the preterm than the full-term group. Irisin correlated positively with gestational age (r = 0.19, p = 0.01), birth weight (r = 0.23, p = 0.003), and high-density lipoprotein cholesterol (r = 0.20, p = 0.01) and negatively with SSSF (r = -0.25, p = 0.003) and chronological age (r = -0.21, p = 0.008).

CONCLUSION

Lower levels of irisin and a slightly unhealthy adiposity and cardiometabolic pattern were detected in preterm-born children in comparison to their full-term-born peers. Whether low irisin levels in preadolescents and adolescents born prematurely could be of prognostic value for the development of cardiometabolic sequelae later in life remains to be further studied.

Brown Adipose Tissue, Adiposity, and Metabolic Profile in Preschool Children

CONTEXT

An inverse relationship between brown adipose tissue (BAT) and obesity has previously been reported in older children and adults but is unknown in young children.

OBJECTIVE

We investigated the influence of BAT in thermoneutral condition on adiposity and metabolic profile in Asian preschool children.

DESIGN, SETTING, AND PARTICIPANTS

A total of 198 children aged 4.5 years from a prospective birth cohort study, Growing Up in Singapore Towards Healthy Outcomes (GUSTO) were successfully studied with water-fat magnetic resonance imaging of the supraclavicular and axillary fat depot (FDSA). Regions within FDSA with fat-signal-fraction between 20% and 80% were considered BAT, and percentage BAT (%BAT; 100*BAT volume/ FDSA volume) was calculated.

MAIN OUTCOME MEASURES

Abdominal adipose tissue compartment volumes, ectopic fat in the soleus muscle and liver, fatty liver index, metabolic syndrome scores, and markers of insulin sensitivity.

RESULTS

A 1% unit increase in %BAT was associated with lower body mass index, difference (95% CI), -0.08 (-0.10, -0.06) kg/m2 and smaller abdominal adipose tissue compartment volumes. Ethnicity and sex modified these associations. In addition, each unit increase in %BAT was associated with lower ectopic fat at 4.5 years in the liver, -0.008% (-0.013%, -0.003%); soleus muscle, -0.003% (-0.006%, -0.001%) of water content and lower fatty liver index at 6 years.

CONCLUSIONS

Higher %BAT is associated with a more favorable metabolic profile. BAT may thus play a role in the pathophysiology of obesity and related metabolic disorders. The observed ethnic and sex differences imply that the protective effect of BAT may vary among different groups.

The Fat Fraction Percentage of White Adipose Tissue at various Ages in Humans: An Updated Review

We recently reported the fat fraction percentage of white adipose tissue in adolescents and adults measured by the water-fat separation method, but there was limited discussion about the change in adipose tissue fat fraction with growth. The purpose of this updated review was to examine the fat content of white (subcutaneous) adipose tissue during the process from birth to adulthood by adding the latest available data. A relevant database was searched through November 2020. Nineteen studies were included. We found that calculated mean values of fat fraction percentage in white adipose tissue were 72.2% in neonates, 87.2% in children, and 87.4% in adults. In contrast, fat fraction percentage of truncal white adipose tissue in the fetuses was from 10% to 24% (29 and 34 wk of gestational age, respectively). Our results suggest that the fat fraction percentage of white adipose tissue may not undergo large changes during the process from birth to adulthood (neonates = 72.2%, children = 87.2%, adults = 87.4%), which was different from the results of a study utilizing a biopsy. The mean value and range of fat fraction percentages for children over 7 years old were especially similar to adults. Further, the fat fraction percentage for neonates was relatively close to the results of children and adults. At the moment, the characteristics of the changes in fat fraction percentage of adipose tissue from birth to preschool children are unclear and future research is needed to clarify this issue.

Brown Adipose Tissue: New Challenges for Prevention of Childhood Obesity. A Narrative Review

Pediatric obesity remains a challenge in modern society. Recently, research has focused on the role of the brown adipose tissue (BAT) as a potential target of intervention. In this review, we revised preclinical and clinical works on factors that may promote BAT or browning of white adipose tissue (WAT) from fetal age to adolescence. Maternal lifestyle, type of breastfeeding and healthy microbiota can affect the thermogenic activity of BAT. Environmental factors such as exposure to cold or physical activity also play a role in promoting and activating BAT. Most of the evidence is preclinical, although in clinic there is some evidence on the role of omega-3 PUFAs (EPA and DHA) supplementation on BAT activation. Clinical studies are needed to dissect the early factors and their modulation to allow proper BAT development and functions and to prevent onset of childhood obesity.

A systematic review of imaging studies of human brown adipose tissue

Brown adipose tissue (BAT) is involved in energy dissipation and has been linked to weight loss, insulin sensitivity, and reduced risk of atherosclerotic disease. BAT is found most often in the supraclavicular region, as well as mediastinal and paravertebral areas, and it is predominantly seen in young persons. BAT is activated by cold temperature and the sympathetic nervous system. In humans, BAT was initially detected via 2-deoxy-2-[¹⁸ F]fluoro-d-glucose (FDG) positron emission tomography/computed tomography (PET/CT), a high-resolution molecular imaging modality used to identify and stage malignancies. Recent studies have shown that BAT can be localized using conventional imaging modalities, such as CT or magnetic resonance imaging, as well as radiotracers used for single-photon emission CT. In this systematic review, we have summarized the evidence for BAT detection in humans using various imaging techniques.

The Water-Fat Separation Method for Determining the Fat-free Component of Subcutaneous Adipose Tissue in Humans: A Brief Review

Fat-free mass as well as lean soft tissue mass is a surrogate for skeletal muscle mass and is often used for the normalization of several physiological variables or for the diagnosing of low muscle mass in older adults. However, both fat-free mass and lean tissue mass include nonskeletal muscle components such as the fat-free component of adipose tissue fat cells. A technique known as water-fat MRI provides a noninvasive and radiation-free assessment of the fat-free component of adipose tissue in humans. However, if this method is impractical or unavailable, some authors suggest that a constant value for the fat-free component of adipose tissue can be used as an indirect estimate. The purpose of this review is to examine the fat fraction percentage of white (subcutaneous) adipose tissue in adolescents and young/middle-aged/older adults measured by water-fat MRI and provide discussion on how the fat-free adipose tissue values from the water-fat separation method compare with the constant value used in previous studies. Calculated mean values for the percentage of fat fraction in subcutaneous adipose tissue were 86.9% in the overall sample, 86.4% in adolescents (3 studies), and 87.1% in young, middle-aged and older adults (7 studies). This is similar to the 85% value proposed in the classical studies but in the majority of studies the 85% estimate was outside of the 95% confidence interval (CI) of the water-fat MRI estimate. There may be several factors to consider that may affect the fat fraction percentage (e.g. reliability of the MRI estimate, age, sex, obesity, etc.), however, at this time there is insufficient evidence to determine the effect of each of these variables. If the measurement is reliable, then this might suggest that the 85% constant may need to be altered to better reflect the water-fat MRI estimate.

FGF19 subfamily members: FGF19 and FGF21

Fibroblast growth factors (FGF) constitute a large family of proteins with pleiotropic effects on development, organogenesis, and metabolism. The FGF19 subclass includes growth factors circulating with the blood referred to as endocrine FGF. Representatives of the FGF19 subclass, including FGF19, FGF21, and FGF23, act via FGFR receptors. The proteins of FGF19 subfamily influence the enterohepatic circulation of bile, participate in glucose and lipid metabolism regulation, and maintenance of phosphorus and vitamin D3 homeostasis. FGF19 and FGF21 are activated under different physiological and pathological conditions.