Metabolic Responses to 24-Hour Fasting and Mild Cold Exposure in Overweight Individuals Are Correlated and Accompanied by Changes in FGF21 Concentration

Impaired metabolic flexibility to fasting is associated with increased ad libitum energy intake in healthy adults

OBJECTIVE

We investigated how changes in 24-h respiratory exchange ratio (RER) and substrate oxidation during fasting versus an energy balance condition influence subsequent ad libitum food intake.

METHODS

Forty-four healthy, weight-stable volunteers (30 male and 14 female; mean [SD], age 39.3 [11.0] years; BMI 31.7 [8.3] kg/m2) underwent 24-h energy expenditure measurements in a respiratory chamber during energy balance (50% carbohydrate, 30% fat, and 20% protein) and 24-h fasting. Immediately after each chamber stay, participants were allowed 24-h ad libitum food intake from computerized vending machines.

RESULTS

Twenty-four-hour RER decreased by 9.4% (95% CI: -10.4% to -8.5%; p < 0.0001) during fasting compared to energy balance, reflecting a decrease in carbohydrate oxidation (mean [SD], -2.6 [0.8] MJ/day; p < 0.0001) and an increase in lipid oxidation (2.3 [0.9] MJ/day; p < 0.0001). Changes in 24-h RER and carbohydrate oxidation in response to fasting were correlated with the subsequent energy intake such that smaller decreases in fasting 24-h RER and carbohydrate oxidation, but not lipid oxidation, were associated with greater energy intake after fasting (r = 0.31, p = 0.04; r = 0.40, p = 0.007; and r = -0.27, p = 0.07, respectively).

CONCLUSIONS

Impaired metabolic flexibility to fasting, reflected by an inability to transition away from carbohydrate oxidation, is linked with increased energy intake.

Critical role of FGF21 in diabetic kidney disease: from energy metabolism to innate immunity

Diabetic kidney disease (DKD) stands as the predominant cause of chronic kidney disease (CKD) on a global scale, with its incidence witnessing a consistent annual rise, thereby imposing a substantial burden on public health. The pathogenesis of DKD is primarily rooted in metabolic disorders and inflammation. Recent years have seen a surge in studies highlighting the regulatory impact of energy metabolism on innate immunity, forging a significant area of research interest. Within this context, fibroblast growth factor 21 (FGF21), recognized as an energy metabolism regulator, assumes a pivotal role. Beyond its role in maintaining glucose and lipid metabolism homeostasis, FGF21 exerts regulatory influence on innate immunity, concurrently inhibiting inflammation and fibrosis. Serving as a nexus between energy metabolism and innate immunity, FGF21 has evolved into a therapeutic target for diabetes, nonalcoholic steatohepatitis, and cardiovascular diseases. While the relationship between FGF21 and DKD has garnered increased attention in recent studies, a comprehensive exploration of this association has yet to be systematically addressed. This paper seeks to fill this gap by summarizing the mechanisms through which FGF21 operates in DKD, encompassing facets of energy metabolism and innate immunity. Additionally, we aim to assess the diagnostic and prognostic value of FGF21 in DKD and explore its potential role as a treatment modality for the condition.

How can we assess "thrifty" and "spendthrift" phenotypes?

PURPOSE OF REVIEW

There is a large inter-individual variability in the magnitude of body weight change that cannot be fully explained by differences in daily energy intake and physical activity levels and that can be attributed to differences in energy metabolism. Measuring the short-term metabolic response to acute changes in energy intake can better uncover this inter-individual variability and quantify the degree of metabolic thriftiness that characterizes an individual's susceptibility to weight gain and resistance to weight loss. This review summarizes the methods used to identify the individual-specific metabolic phenotype (thrifty vs. spendthrift) in research and clinical settings.

RECENT FINDINGS

The metabolic responses to short-term fasting, protein-imbalanced overfeeding, and mild cold exposure constitute quantitative factors that characterize metabolic thriftiness.

SUMMARY

The energy expenditure response to prolonged fasting is considered the most accurate and reproducible measure of metabolic thriftiness, likely because the largest energy deficit best captures interindividual differences in the extent of metabolic slowing. However, all the other dietary/environmental challenges can be used to quantify the degree of thriftiness using whole-room indirect calorimetry. Efforts are underway to identify alternative methods to assess metabolic phenotypes in clinical and outpatient settings such as the hormonal response to low-protein meals.

Hepatic FGF21: Its Emerging Role in Inter-Organ Crosstalk and Cancers

Fibroblast growth factor (FGF) 21 is one of the FGF members with special endocrine properties. In the last twenty years, it has attracted intense research and development for its physiological functions that respond to dietary manipulation, pharmacological benefits of improving the macronutrient metabolism, and clinical values as a biomarker of various human diseases. Generally, FGF21 can be produced by major metabolic organs, but only the subgroup from the liver shows canonical endocrine properties, which emphasizes the special value of delineating the unique secretory and functional characteristics of hepatic FGF21. There has been a growth in literature to address the extra-hepatic activities of FGF21, and many striking findings have therefore been published. Yet, they are fragmented and scattered, and controversies are raised from divergent findings. For this reason, there is a need for a systematic and critical evaluation of current research in this aspect. In this review, we focus on the current knowledge about the molecular biology of endocrine FGF21, especially present details on the regulation of circulating levels of FGF21. We also emphasize its emerging roles in inter-organ crosstalk and cancer development.

Ketogenic Diet and Weight Loss: Is There an Effect on Energy Expenditure?

A dysregulation between energy intake (EI) and energy expenditure (EE), the two components of the energy balance equation, is one of the mechanisms responsible for the development of obesity. Conservation of energy equilibrium is deemed a dynamic process and alterations of one component (energy intake or energy expenditure) lead to biological and/or behavioral compensatory changes in the counterpart. The interplay between energy demand and caloric intake appears designed to guarantee an adequate fuel supply in variable life contexts. In the past decades, researchers focused their attention on finding efficient strategies to fight the obesity pandemic. The ketogenic or “keto” diet (KD) gained substantial consideration as a potential weight-loss strategy, whereby the concentration of blood ketones (acetoacetate, 3-β-hydroxybutyrate, and acetone) increases as a result of increased fatty acid breakdown and the activity of ketogenic enzymes. It has been hypothesized that during the first phase of KDs when glucose utilization is still prevalent, an increase in EE may occur, due to increased hepatic oxygen consumption for gluconeogenesis and for triglyceride-fatty acid recycling. Later, a decrease in 24-h EE may ensue due to the slowing of gluconeogenesis and increase in fatty acid oxidation, with a reduction of the respiratory quotient and possibly the direct action of additional hormonal signals.

Protein-carbohydrate interaction effects on energy balance, FGF21, IGF-1, and hypothalamic gene expression in rats

Amino acids are involved in energy homeostasis, just as are carbohydrates and lipids. Therefore, mechanisms controlling protein intake should operate independently and in combination with systems controlling overall energy intake to coordinate appropriate metabolic and behavioral responses. The objective of this study was to quantify the respective roles of dietary protein and carbohydrate levels on energy balance, plasma fibroblast growth factor 21 (FGF21) and insulin growth factor 1 (IGF-1) concentrations, and hypothalamic neurotransmitters (POMC, NPY, AgRP, and CART). In a simplified geometric framework, 7-wk-old male Wistar rats were fed 12 diets containing 3%-30% protein for 3 wk, in which carbohydrates accounted for 30%-75% of the carbohydrate and fat part of the diet. As a result of this study, most of the studied parameters (body composition, energy expenditure, plasma FGF21 and IGF-1 concentrations, and Pomc/Agrp ratio) responded mainly to the protein content and to a lesser extent to the carbohydrate content in the diet.NEW & NOTEWORTHY As mechanisms controlling protein intake can operate independently and in combination with those controlling energy intakes, we investigated the metabolic and behavioral effects of the protein-carbohydrate interaction. With a simplified geometric framework, we showed that body composition, energy balance, plasma FGF21 and IGF-1 concentrations, and hypothalamic Pomc/Agrp ratio were primarily responsive to protein content and, to a lesser extent, to carbohydrate content of the diet.

Energy Balance and Control of Body Weight: Possible Effects of Meal Timing and Circadian Rhythm Dysregulation

Conservation of the energy equilibrium can be considered a dynamic process and variations of one component (energy intake or energy expenditure) cause biological and/or behavioral compensatory changes in the other part of the system. The interplay between energy demand and caloric intake appears designed to guarantee an adequate food supply in variable life contexts. The circadian rhythm plays a major role in systemic homeostasis by acting as “timekeeper” of the human body, under the control of central and peripheral clocks that regulate many physiological functions such as sleep, hunger and body temperature. Clock-associated biological processes anticipate the daily demands imposed by the environment, being synchronized under ideal physiologic conditions. Factors that interfere with the expected demand, including daily distribution of macronutrients, physical activity and light exposure, may disrupt the physiologic harmony between predicted and actual behavior. Such a desynchronization may favor the development of a wide range of disease-related processes, including obesity and its comorbidities. Evidence has been provided that the main components of 24-h EE may be affected by disruption of the circadian rhythm. The sleep pattern, meal timing and meal composition could mediate these effects. An increased understanding of the crosstalk between disruption of the circadian rhythm and energy balance may shed light on the pathophysiologic mechanisms underlying weight gain, which may eventually lead to design effective strategies to fight the obesity pandemic.

Reduced adaptive thermogenesis during acute protein-imbalanced overfeeding is a metabolic hallmark of the human thrifty phenotype

BACKGROUND

The human thrifty phenotype is characterized by a greater decrease in 24-h energy expenditure (24EE) during fasting due to relatively higher eucaloric 24EE in sedentary conditions, both of which are indicative of greater propensity to weight gain. Thriftiness is also associated with a smaller increase in 24EE (i.e., reduced adaptive thermogenesis) during overfeeding.

OBJECTIVES

We investigated whether short-term measures of adaptive thermogenesis during overfeeding with low/normal/high protein content characterize thriftiness.

METHODS

In this secondary cross-sectional analysis of a single-arm crossover study, 24EE was measured using whole-room indirect calorimetry during energy balance, fasting, and different overfeeding conditions (low/3% protein, high/30% protein, and 3 normal/20% protein diets) with 200% of eucaloric requirements in 77 healthy individuals [63 men; BMI (in kg/m2): 26.4 ± 4.3; body fat by DXA: 27.7% ± 9.4%, mean ± SD] with normal glucose regulation. Relations between the 24EE during energy balance (adjusted for body composition) and 24EE during each overfeeding diet were analyzed using separate linear regression models. Participants were arbitrarily categorized as thrifty/spendthrift based on the median value (-177 kcal/d) of the difference in 24EE between fasting and energy balance conditions.

RESULTS

Differences in 24EE during low/high-protein overfeeding diets (regression line slope = 0.76 and 0.68, respectively, both P < 0.05 compared with slope = 1) but not during the normal-protein overfeeding diets (all P > 0.05 compared with slope = 1) were dependent on baseline 24EE during energy balance. Specifically, individuals with higher eucaloric 24EE (thriftier phenotype) showed smaller increases in 24EE during protein-imbalanced overfeeding. Analyzed by group, thrifty individuals had smaller increases in 24EE by 42 and 237 kcal/d during low- and high-protein overfeeding, respectively, compared with spendthrift individuals who showed greater increases in 24EE by 100 and 302 kcal/d (P ≤ 0.03 compared with thrifty group).

CONCLUSIONS

During acute overfeeding conditions with low/high-protein content, thrifty participants have limited capacity to increase 24EE, indicating that impaired adaptive thermogenesis during protein-imbalanced diets further characterizes the thrifty phenotype and its susceptibility to weight gain. This trial was registered at clinicalTrials.gov as NCT00523627.

Hepatic AKT orchestrates adipose tissue thermogenesis via FGF21-dependent and -independent mechanisms

Organismal stressors such as cold exposure require a systemic response to maintain body temperature. Brown adipose tissue (BAT) is a key thermogenic tissue in mammals that protects against hypothermia in response to cold exposure. Defining the complex interplay of multiple organ systems in this response is fundamental to our understanding of adipose tissue thermogenesis. In this study, we identify a role for hepatic insulin signaling via AKT in the adaptive response to cold stress and show that liver AKT is an essential cell-nonautonomous regulator of adipocyte lipolysis and BAT function. Mechanistically, inhibition of forkhead box O1 (FOXO1) by AKT controls BAT thermogenesis by enhancing catecholamine-induced lipolysis in the white adipose tissue (WAT) and increasing circulating fibroblast growth factor 21 (FGF21). Our data identify a role for hepatic insulin signaling via the AKT-FOXO1 axis in regulating WAT lipolysis, promoting BAT thermogenic capacity, and ensuring a proper thermogenic response to acute cold exposure.

Reduced brown adipose tissue activity during cold exposure is a metabolic feature of the human thrifty phenotype

BACKGROUND

We recently demonstrated that thrifty subjects, characterized by a greater decrease in 24 h energy expenditure (24hEE) during short-term fasting, have less capacity for cold-induced thermogenesis (CIT) during 24 h of mild cold exposure.

OBJECTIVE

As cold-induced brown adipose tissue activation (CIBA) is a determinant of CIT, we sought to investigate whether thrifty individuals also have reduced CIBA.

METHODS

Twenty-four healthy subjects (age: 29.8 ± 9.5y, body fat: 27.3 ± 12.4%, 63% male) were admitted to our clinical research unit and underwent two 24hEE assessments in a whole-room indirect calorimeter during energy balance and fasting conditions at thermoneutrality to quantify their degree of thriftiness. Positron emission tomography/computed tomography scans were performed after exposure to 16 °C for 2 h to quantify peak CIBA.

RESULTS

A greater decrease in 24hEE during fasting was associated with lower peak CIBA (r = 0.50, p = 0.01), such that a 100 kcal/day greater reduction in 24hEE related to an average 3.2 g/mL lower peak CIBA.

CONCLUSION

Our results indicate that reduced CIBA is a metabolic trait of the thrifty phenotype which might explain reduced CIT capacity and greater predisposition towards weight gain in individuals with a thrifty metabolism.

Beneficial effects of whole-body cryotherapy on glucose homeostasis and amino acid profile are associated with a reduced myostatin serum concentration

The study investigated the effect of single and chronic (10 sessions) whole-body cryotherapy (WBC; 3-min, − 110 °C) on amino acid (AA) profile, myostatin, fibroblast growth factor 21 (FGF21), and concentrations of brain-derived neurotrophic factor (BDNF), irisin and adiponectin in relation to glucose homeostasis. Thirty-five, healthy men were randomly split into experimental (young: 28 ± 7 years and middle-aged: 51 ± 3 years) and control groups. Blood samples were taken before and 1 h after the first and last (10th) WBC session. Baseline myostatin correlated significantly with visceral fat area, glucose, insulin, HOMA-IR and irisin (all p < 0.05). The single session of WBC induced temporary changes in AA profile, whereas chronic exposure lowered valine and asparagine concentrations (p < 0.01 and p = 0.01, respectively) compared to the baseline. The chronic WBC reduced fasting glucose (p = 0.04), FGF21 (− 35.8%, p = 0.06) and myostatin (-18.2%, p = 0.06). Still, the effects were age-dependent. The decrease of myostatin was more pronounced in middle-aged participants (p < 0.01). Concentrations of irisin and adiponectin increased in response to chronic WBC, while BDNF level remained unchanged. By improving the adipo-myokine profile, chronic WBC may reduce effectively the risk of the metabolic syndrome associated with hyperinsulinemia, increased levels of valine and asparagine, and muscle atrophy.

Resting Energy Expenditure: From Cellular to Whole-Body Level, a Mechanistic Historical Perspective

The basis of heat generated by the human body has been a source of speculation and research for more than 2,000 years. Basal heat production, now usually referred to as resting energy expenditure (REE), is currently recognized as deriving from biochemical reactions at subcellular and cellular levels that are expressed in the energy expended by the body's 78 organs and tissues. These organs and tissues, and the 11 systems to which they belong, influence body size and shape. Connecting these subcellular-/cellular-level reactions to organs and tissues, and then on to body size and shape, provides a comprehensive understanding of individual differences in REE, a contemporary topic of interest in obesity research and clinical practice. This review critically examines these linkages, their association with widely used statistical and physiological REE prediction formulas, and often-unappreciated aspects of measuring basal heat production in humans.

Free Thyroxine Levels are Associated with Cold Induced Thermogenesis in Healthy Euthyroid Individuals

Thyroid hormone (TH) is an important regulator of mammalian metabolism and facilitates cold induced thermogenesis (CIT) in brown adipose tissue (BAT). Profound hypothyroidism or hyperthyroidism lead to alterations in BAT function and CIT. In euthyroid humans the inter-individual variation of thyroid hormones is relatively large. Therefore, we investigated whether levels of free thyroxine (T4) or free triiodothyronine (T3) are positively associated with CIT in euthyroid individuals. We performed an observational study in 79 healthy, euthyroid volunteers (mean age 25.6 years, mean BMI 23.0 kg · m^-2). Resting energy expenditure (REE) was measured by indirect calorimetry during warm conditions (EE_warm) and after a mild cold stimulus of two hours (EE_cold). CIT was calculated as the difference between EE_cold and EE_warm. BAT activity was assessed by ¹⁸F-FDG-PET after a mild cold stimulus in a subset of 26 participants. EE_cold and CIT were significantly related to levels of free T4 (R² = 0.11, p=0.0025 and R² = 0.13, p=0.0011, respectively) but not to free T3 and TSH. Cold induced BAT activity was also associated with levels of free T4 (R² = 0.21, p=0.018). CIT was approximately fourfold higher in participants in the highest tertile of free T4 as compared to the lowest tertile. Additionally, free T4 was weakly, albeit significantly associated with outdoor temperature seven days prior to the respective study visit (R² = 0.06, p=0.037). These finding suggests that variations in thyroid hormone levels within the euthyroid range are related to the capability to adapt to cool temperatures and affect energy balance.