Toward reconciling the roles of FGF21 in protein appetite, sweet preference, and energy expenditure

Dietary protein restriction diminishes sucrose reward and reduces sucrose-evoked mesolimbic dopamine signaling in mice

A growing literature suggests manipulating dietary protein status decreases sweet consumption in rodents and in humans. Underlying neurocircuit mechanisms have not yet been determined, but previous work points towards hedonic rather than homeostatic pathways. Here we hypothesized that a history of protein restriction reduces sucrose seeking by altering mesolimbic dopamine signaling in mice. We tested this hypothesis using established behavioral tests of palatability and conditioned reward, including the palatability contrast and conditioned place preference (CPP) tests. We used modern optical sensors for measuring real-time nucleus accumbens (NAc) dopamine dynamics during voluntary sucrose consumption, via fiber photometry, in male C57/Bl6J mice maintained on low-protein high-carbohydrate (LPHC) or control (CON) diet for ∼5 weeks. Our results showed that a history of protein restriction decreased the consumption of a sucrose 'dessert' in sated mice by ∼50% compared to controls [T-test, p < 0.05]. The dopamine release in NAc during sucrose consumption was reduced, also by ∼50%, in LPHC-fed mice compared to CON [T-test, p < 0.01]. Furthermore, LPHC-feeding blocked the sucrose-conditioned place preference we observed in CON-fed mice [paired T-test, p < 0.05], indicating reduced sucrose reward. This was accompanied by a 33% decrease in neuronal activation of the NAc core, as measured by c-Fos immunolabeling from brains collected directly after the CPP test [T-test, p < 0.05]. Together, these findings advance our mechanistic understanding of how dietary protein restriction decreases the consumption of sweets-by inhibiting the incentive salience of a sucrose reward, together with reduced sucrose-evoked dopamine release in NAc.

Macronutrient interactions and models of obesity: Insights from nutritional geometry

The global obesity epidemic results from a complex interplay of genetic and environmental factors, with diet being a prominent modifiable element driving weight gain and adiposity. Although excess intake of energetic macronutrients is implicated in causing obesity, ongoing debate centers on whether sugar or fat or both are driving the rising obesity rates. This has led to competing models of obesity such as the "Carbohydrate Insulin Model", the "Energy Balance Model", and the "Fructose Survival Hypothesis". Conflicting evidence from studies designed to focus on individual energetic macronutrients or energy rather than macronutrient mixtures underlies this disagreement. Recent research in humans and animals employing the nutritional geometry framework (NGF) emphasizes the importance of considering interactions among dietary components. Protein interacts with carbohydrates, fats, and dietary energy density to influence both calorie intake ("protein leverage") and, directly and indirectly, metabolic physiology and adiposity. Consideration of these interactions can help to reconcile different models of obesity, and potentially cast new light on obesity interventions.

Distinct genetic signals at the FGF21 locus complicate studies of FGF21's role in diet regulation using human cohort data

OBJECTIVES

Experimental and genetic studies suggest that fibroblast growth factor 21 (FGF21) modulates macronutrient and alcohol preferences, but evidence of such regulation in humans remains scarce. To address this gap in translation, we aimed to map the relationships between plasma FGF21 levels, FGF21 genetic variation and habitual macronutrient intake in a large human population.

METHODS

We fine-mapped and performed colocalization of the FGF21 genetic region in GWAS summary statistics of plasma FGF21 levels and macronutrient intake. UK Biobank data were used to investigate the associations between FGF21 genetic variants, plasma FGF21 protein levels, and macronutrient intake (including alcohol) assessed with repeated 24-hour recalls. One- and two-sample mendelian randomization were performed to estimate the effects of plasma FGF21 on macronutrient intake.

RESULTS

We show that the main macronutrient-associated variant rs838133 and the FGF21 cis-pQTL rs838131, both in the FGF21 gene, are distinct genetic signals. Effect directions also suggest that the influence of FGF21 variation on macronutrient intake appear more complex than by direct mediation through plasma FGF21. Only when considering this complexity at FGF21, is plasma FGF21 estimated to reduce alcohol and increase protein and fat intake using mendelian randomization. Importantly, plasma FGF21 levels also appear markedly elevated by primarily high alcohol and low protein intake.

CONCLUSIONS

These findings support the feedback diet-regulatory mechanism of FGF21 in humans, but highlights the need for mechanistic characterization of the complex FGF21 genetic region.

FGF21 as a mediator of adaptive changes in food intake and macronutrient preference in response to protein restriction

Free-feeding animals navigate complex nutritional landscapes in which food availability, cost, and nutritional value can vary markedly. Animals have thus developed neural mechanisms that enable the detection of nutrient restriction, and these mechanisms engage adaptive physiological and behavioral responses that limit or reverse this nutrient restriction. This review focuses specifically on dietary protein as an essential and independently defended nutrient. Adequate protein intake is required for life, and ample evidence exists to support an active defense of protein that involves behavioral changes in food intake, food preference, and food motivation, likely mediated by neural changes that increase the reward value of protein foods. Available evidence also suggests that the circulating hormone fibroblast growth factor 21 (FGF21) acts in the brain to coordinate these adaptive changes in food intake, making it a unique endocrine signal that drives changes in macronutrient preference in the context of protein restriction. This article is part of the Special Issue on "Food intake and feeding states".

FGF21 acts in the brain to drive macronutrient-specific changes in behavioral motivation and brain reward signaling

Dietary protein restriction induces adaptive changes in food preference, increasing protein consumption over carbohydrates or fat. We investigated whether motivation and reward signaling underpin these preferences. In an operant task, protein-restricted male mice responded more for liquid protein rewards, but not carbohydrate, fat, or sweet rewards compared to non-restricted mice. The protein restriction-induced increase in operant responding for protein was absent in Fgf21-KO mice and mice with neuron-specific deletion of the FGF21 co-receptor beta-Klotho (Klb Cam2ka ) mice. Fiber photometry recording of VTA dopamine neurons revealed that oral delivery of maltodextrin triggered a larger activation as compared to casein in control-fed mice, whereas casein triggered a larger activation in protein-restricted mice. This restriction-induced shift in nutrient-specific VTA dopamine signaling was lost in Fgf21-KO mice. These data strongly suggest that the increased FGF21 during protein restriction acts in the brain to induce a protein-specific appetite by specifically enhancing the reward value of protein-containing foods and the motivation to consume them.

FGF21 mediating the Sex-dependent Response to Dietary Macronutrients

Sex is key variable influencing body composition and substrate utilization. At rest, females maintain greater adiposity than males and resist the mobilization of fat. Males maintain greater lean muscle mass and mobilize fat readily. Determining the mechanisms that direct these sex-dependent effects is important for both reproductive and metabolic health. Here, we highlight the fundamental importance of sex in shaping metabolic physiology and assess growing evidence that the hepatokine fibroblast growth factor-21 (FGF21) plays a mechanistic role to facilitate sex-dependent responses to a changing nutritional environment. First, we examine the importance of sex in modulating body composition and substrate utilization. We summarize new data that point toward sex-biased effects of pharmacologic FGF21 administration on these endpoints. When energy is not limited, metabolic responses to FGF21 mirror broader sex differences; FGF21-treated males conserve lean mass at the expense of increased lipid catabolism, whereas FGF21-treated females conserve fat mass at the expense of reduced lean mass. Next, we examine the importance of sex in modulating the endogenous secretion of FGF21 in response to changing macronutrient and energy availability. During the resting state when energy is not limited, macronutrient imbalance increases the secretion of FGF21 more so in males than females. When energy is limited, the effect of sex on both the secretion of FGF21 and its metabolic actions may be reversed. Altogether, we argue that a growing literature supports FGF21 as a plausible mechanism contributing to the sex-dependent mobilization vs preservation of lipid storage and highlight the need for further research.

Rapid benefits in older age from transition to whole food diet regardless of protein source or fat to carbohydrate ratio: Arandomised control trial

Plant-based diets reduces the risk of chronic conditions. The interaction between protein source and other macronutrients-fat (F) and carbohydrate (C)-has yet to be investigated. The aim was to assess the main and interactive effects of protein-source (plant vs. animal) and F:C (high or low) and the transition from an Australian diet to a whole food diet on various health markers in older individuals. This single-blinded, parallel, randomised experimental trial used a 2 × 2 factorial design to compare pro-vegetarian (70:30 plant to animal) versus omnivorous (50:50 plant to animal) diets at 14% protein and varying fat-to-carbohydrate ratios (high fat ~40% vs. low fat ~30%) over 4 weeks. Study foods were provided, alcohol consumption was discouraged, and dietary intake was determined through food records. Analysis included both RCT and observational data. Changes in appetite, palatability of diets, and dietary intake were assessed. Body composition, muscle strength, function, gut microbiome, and cardiometabolic health parameters were measured. Data from 113 (of the 128 randomised) individuals aged 65-75 years were analysed. Pro-vegetarian diets reduced diastolic blood pressure, total cholesterol and glucose levels. Moreover, the overall sample exhibited increased short-chain fatty acids and FGF21 levels, as well as improvements in body composition, function, and cardio-metabolic parameters irrespective of dietary treatment. Transitioning to a diet rich in fruit, vegetables, fibre, and moderate protein was associated with improved health markers in older age, with added benefits from pro-vegetarian diets. Further research on long-term effects is needed.

Dietary protein restriction diminishes sucrose reward and reduces sucrose-evoked mesolimbic dopamine signaling

Objective

A growing literature suggests manipulating dietary protein status decreases sweet consumption in rodents and in humans. Underlying neurocircuit mechanisms have not yet been determined, but previous work points towards hedonic rather than homeostatic pathways. Here we hypothesized that a history of protein restriction reduces sucrose seeking by altering mesolimbic dopamine signaling.

Methods

We tested this hypothesis using established behavioral tests of palatability and motivation, including the 'palatability contrast' and conditioned place preference (CPP) tests. We used modern optical sensors for measuring real-time nucleus accumbens (NAc) dopamine dynamics during sucrose consumption, via fiber photometry, in male C57/Bl6J mice maintained on low-protein high-carbohydrate (LPHC) or control (CON) diet for ∼5 weeks.

Results

A history of protein restriction decreased the consumption of a sucrose 'dessert' in sated mice by ∼50% compared to controls [T-test, p< 0.05]. The dopamine release in NAc during sucrose consumption was reduced, also by ∼50%, in LPHC-fed mice compared to CON [T-test, p< 0.01]. Furthermore, LPHC-feeding blocked the sucrose-conditioned place preference we observed in CON-fed mice [paired T-test, p< 0.05], indicating reduced motivation. This was accompanied by a 33% decrease in neuronal activation of the NAc core, as measured by c-Fos immunolabeling from brains collected directly after the CPP test.

Conclusions

Despite ongoing efforts to promote healthier dietary habits, adherence to recommendations aimed at reducing the intake of added sugars and processed sweets remains challenging. This study highlights chronic dietary protein restriction as a nutritional intervention that suppresses the motivation for sucrose intake, via blunted sucrose-evoke dopamine release in NAc.

Mice Regulate Dietary Amino Acid Balance and Energy Intake by Selecting between Complementary Protein Sources

BACKGROUND

A balanced intake of protein and constituent amino acids (AAs) requires adjustments to total food intake (protein leverage [PL]) and food selection to balance deficits and excesses (complementary feeding). We provided mice with choices of casein and whey, 2 protein sources that are complementary in AA balance, across a range of protein concentrations (P%) of digestible energy (DE).

OBJECTIVES

We aimed to determine if: 1) PL operates similarly for casein and whey; 2) one protein source is preferred at control P%; 3) the preference changes as P% falls; and 4) AA intakes under control and low P% levels identify AAs that drive changes in protein selection.

METHODS

Food intake and plasma fibroblast growth factor-21 (FGF21) concentrations were measured in mice at various P% (P7.5%-P33%). For direct comparisons, defined diets were used in which the protein source was either casein or whey. In food choice studies, mice had access to foods in which both casein and whey were provided at the same P% level at the same time.

RESULTS

PL operated at different P% thresholds in casein (13%)- and whey (10%)-based diets, and the magnitude of PL was greater for casein. Although mice preferred casein under control conditions (P23%), a pronounced preference shift to whey occurred as P% fell to P13% and P10%. At low P%, increases in food intake were accompanied by increases in plasma FGF21, a protein hunger signal. Among AAs deficient in casein and enriched in whey, the intake of Cys was the most invariant as P% changed between P23% and P10%, appearing to drive the switch in protein preference.

CONCLUSIONS

Mice selected between complementary protein sources, casein and whey, achieving stable total energy intake and regulated intake of AAs as P% varied. Supplementation of low P% casein diets with one whey-enriched AA, Cys, suppressed plasma FGF21 and total food intake.

Hepatic interoception in health and disease

The liver is a large organ with crucial functions in metabolism and immune defense, as well as blood homeostasis and detoxification, and it is clearly in bidirectional communication with the brain and rest of the body via both neural and humoral pathways. A host of neural sensory mechanisms have been proposed, but in contrast to the gut-brain axis, details for both the exact site and molecular signaling steps of their peripheral transduction mechanisms are generally lacking. Similarly, knowledge about function-specific sensory and motor components of both vagal and spinal access pathways to the hepatic parenchyma is missing. Lack of progress largely owes to controversies regarding selectivity of vagal access pathways and extent of hepatocyte innervation. In contrast, there is considerable evidence for glucose sensors in the wall of the hepatic portal vein and their importance for glucose handling by the liver and the brain and the systemic response to hypoglycemia. As liver diseases are on the rise globally, and there are intriguing associations between liver diseases and mental illnesses, it will be important to further dissect and identify both neural and humoral pathways that mediate hepatocyte-specific signals to relevant brain areas. The question of whether and how sensations from the liver contribute to interoceptive self-awareness has not yet been explored.

Context matters for addressing controversies in FGF21 biology

Recent discoveries by Solon-Biet and colleagues highlight the importance of nutritional context for addressing current controversies in Fibroblast Growth Factor 21 (FGF21) biology. Through a series of complex studies, the authors explored the physiological and pharmacological effects of FGF21 on feeding behavior and energy balance under differing nutritional and metabolic statuses.