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Fundamentals

Many individuals recognize a subtle, persistent erosion of their inherent vitality. This often manifests as an inexplicable fatigue, a stubborn resistance to maintaining a healthy body composition, or a diminishing zest for daily life. These are not merely the inevitable hallmarks of passing years; instead, they often represent a quiet disarray within the body’s intricate communication networks. Our exploration of peptide protocols acknowledges this lived experience, offering a precise means to re-establish biological dialogue.

Peptides represent sophisticated biological messengers, short chains of amino acids that carry specific instructions to cellular machinery. They function as highly targeted signals, influencing a vast array of physiological processes from growth and repair to metabolic regulation and mood modulation. Consider these peptides as the finely tuned notes within a grand biological symphony. The true magnificence of this symphony, however, arises when the body’s internal environment, the very stage upon which these notes are played, is optimally prepared.

Peptides act as precise biological messengers, guiding cellular functions towards optimal performance.

Lifestyle factors, specifically thoughtful dietary choices and consistent physical activity, serve as the primary architects of this receptive internal milieu. They are the conductors orchestrating the cellular environment, ensuring that the body’s cells are not only ready to receive peptide signals but also poised to execute those instructions with maximum efficiency.

A nutrient-dense diet provides the foundational building blocks and cofactors necessary for healthy cellular function, including robust receptor expression and efficient signaling pathways. Similarly, regular exercise stimulates physiological adaptations that enhance cellular responsiveness and metabolic vigor.

Understanding your own biological systems initiates a profound journey. This involves recognizing the interconnectedness of seemingly disparate symptoms with underlying endocrine and metabolic functions. When we align our daily habits with our inherent biological needs, we cultivate a state of readiness within the body, allowing targeted interventions like peptide protocols to unfold their full potential. This synergistic relationship moves beyond simple augmentation; it defines a deeper, more integrated pathway toward reclaiming uncompromised vitality and function.

A thoughtful woman embodies patient-centric hormone optimization. Her serene expression signifies physiological well-being, metabolic health, and enhanced cellular function, reflecting clinical wellness and peptide therapy benefits

How Do Lifestyle Choices Prime Cellular Receptivity?

The cellular landscape of the human body is a dynamic realm, constantly adapting to internal and external cues. Dietary patterns exert a profound influence on cellular membrane integrity, mitochondrial function, and the very expression of receptor sites where peptides bind. A diet rich in micronutrients and balanced macronutrients supplies the raw materials for cellular repair and renewal.

It also minimizes inflammatory signals that can otherwise desensitize cellular receptors, effectively dampening the body’s ability to “hear” its own internal messages, or those introduced through peptide therapy.

  • Nutrient Density ∞ Provides essential vitamins, minerals, and antioxidants supporting cellular health and reducing oxidative stress.
  • Macronutrient Balance ∞ Optimizes insulin sensitivity and glucose metabolism, preventing cellular overload and inflammation.
  • Hydration ∞ Maintains cellular turgor and facilitates nutrient transport, supporting overall cellular function.

Physical activity, particularly a blend of resistance training and cardiovascular exercise, further refines this cellular receptivity. Exercise triggers acute hormonal responses, such as transient increases in growth hormone, which prime tissues for repair and adaptation. Chronically, regular movement enhances blood flow, delivering vital oxygen and nutrients to cells while improving waste removal. This consistent physiological conditioning creates an environment where cells are metabolically agile and responsive, allowing peptide signals to elicit a more robust and sustained biological effect.

Intermediate

The intentional application of peptide protocols represents a sophisticated approach to biochemical recalibration, offering targeted support for various physiological functions. The true power of these protocols, however, extends beyond the peptide itself, finding profound amplification through deliberate lifestyle integration. Individuals seeking to optimize their hormonal balance and metabolic function discover that diet and exercise are not mere adjuncts; they are fundamental co-factors that determine the depth and durability of therapeutic outcomes.

Consider the growth hormone-releasing hormone (GHRH) peptides, such as Sermorelin, Ipamorelin, and CJC-1295. These agents stimulate the pituitary gland’s pulsatile release of endogenous growth hormone. While potent on their own, their efficacy hinges significantly on the body’s capacity to synthesize, transport, and respond to the resulting growth hormone and insulin-like growth factor 1 (IGF-1).

A diet rich in high-quality protein provides the necessary amino acid precursors for growth hormone synthesis. Regular resistance training, known to independently stimulate growth hormone release, creates a synergistic effect, preparing muscle tissue to effectively utilize the increased anabolic signals. This dual approach ensures both the supply of the signal and the readiness of the receiving tissue.

Optimizing diet and exercise significantly enhances the body’s response to peptide therapies.

Two young men showcase endocrine balance and optimal cellular function, results of hormone optimization therapy. Their healthy appearance signifies metabolic health and youthful vitality, reflecting successful clinical protocols, personalized patient journeys, and preventative wellness

Enhancing Growth Hormone Peptide Efficacy

Tesamorelin, a GHRH analog, specifically targets visceral adipose tissue reduction. Its success in remodeling body composition becomes markedly more pronounced when paired with a diet that controls insulin excursions and promotes a healthy inflammatory profile. High-intensity interval training (HIIT) complements Tesamorelin’s action by improving mitochondrial biogenesis and enhancing fat oxidation pathways. This combination fosters an environment where the peptide’s targeted lipolytic effects can proceed unimpeded, resulting in more pronounced and sustainable reductions in central adiposity.

Other targeted peptides, such as PT-141 for sexual health, illustrate this principle further. PT-141 acts centrally on melanocortin receptors to modulate sexual desire and arousal. While its mechanism is neurological, factors influencing overall vascular health and neurotransmitter balance directly impact its effectiveness. A diet rich in omega-3 fatty acids and antioxidants supports endothelial function and neural health, creating a more responsive physiological backdrop. Regular cardiovascular exercise improves systemic blood flow and mood regulation, factors intrinsically linked to robust sexual function.

Pentadeca Arginate (PDA), a peptide often utilized for tissue repair and inflammation modulation, also benefits from this integrated strategy. PDA supports angiogenesis and modulates inflammatory pathways, crucial for healing. A diet providing adequate protein, vitamin C, and zinc supports collagen synthesis and immune function, which are fundamental to tissue regeneration. Consistent, appropriate movement, avoiding overstressing injured areas, facilitates blood flow and mechanical signaling, further accelerating the repair processes initiated by PDA.

Intricate cellular architecture portrays a bio-network with green peptide flow, illustrating targeted delivery and hormone receptor modulation fundamental to cellular function. This signifies endocrine system integrity and regenerative potential achieved through precise clinical protocols in hormone optimization

Optimizing Peptide Absorption and Utilization

The journey of a peptide from administration to cellular action involves several critical steps, each influenced by lifestyle. The bioavailability of orally administered peptides, for instance, depends on gut health and digestive enzyme activity, both of which are profoundly shaped by diet. Subcutaneous injections benefit from healthy circulation, which exercise directly improves.

Furthermore, the sensitivity of cellular receptors to peptide binding is not static. Chronic inflammation, nutrient deficiencies, or persistent metabolic dysregulation can lead to receptor desensitization, effectively dulling the peptide’s signal.

A diet emphasizing whole, unprocessed foods, alongside a consistent exercise regimen, actively maintains optimal receptor density and sensitivity. This ensures that the therapeutic signal from the peptide is not merely present, but also powerfully received and translated into the desired physiological response. This table illustrates how lifestyle factors synergistically enhance various peptide protocols.

Peptide Protocol Key Lifestyle Factor Mechanism of Synergy
GHRH Peptides (Sermorelin, Ipamorelin, CJC-1295) Resistance Training & Protein-Rich Diet Enhances endogenous GH release, provides amino acid precursors for synthesis, and primes muscle tissue for anabolic response.
Tesamorelin Low-Glycemic Diet & HIIT Reduces insulin resistance, improves fat oxidation, and supports targeted visceral fat reduction.
PT-141 Omega-3 Rich Diet & Cardiovascular Exercise Supports neural health, improves endothelial function, and enhances overall mood and circulation for central action.
Pentadeca Arginate (PDA) Adequate Protein, Vitamin C, Zinc & Controlled Movement Provides building blocks for tissue repair, supports immune function, and facilitates blood flow to injured areas.

Academic

The intricate interplay between exogenous peptide protocols and endogenous physiological systems represents a compelling frontier in personalized wellness. To genuinely magnify the results of peptide therapies, one must consider the body as a dynamic, interconnected network, where lifestyle choices function as powerful epigenetic modulators and regulators of endocrine crosstalk. This perspective moves beyond simplistic cause-and-effect relationships, embracing a systems-biology view of profound synergy.

Peptides, as specific signaling molecules, exert their influence through binding to cognate receptors, initiating cascades of intracellular events. The efficiency of this signal transduction, from ligand-receptor binding to downstream effector activation, is not immutable. It is profoundly influenced by the cellular microenvironment, which diet and exercise meticulously sculpt.

For instance, chronic hyperinsulinemia, often a consequence of sustained high-glycemic diets, can lead to widespread receptor desensitization, including those for growth factors and various neuropeptides. This metabolic state effectively renders cells less responsive, diminishing the impact of even potent peptide signals.

Lifestyle factors epigenetically modulate cellular responsiveness, significantly impacting peptide efficacy.

A microscopic view shows organized cellular structures with bound green elements, depicting essential receptor activation. This symbolizes optimized peptide action, crucial for hormone regulation, metabolic balance, and cellular repair within clinical protocols leading to improved patient outcomes

How Does Metabolic Flexibility Influence Peptide Outcomes?

Metabolic flexibility, defined as the capacity to readily switch between fuel substrates (glucose and fatty acids), stands as a cornerstone of optimal peptide responsiveness. Regular physical activity, particularly structured resistance and aerobic training, enhances mitochondrial density and function, thereby improving the cell’s capacity for oxidative phosphorylation and lipid metabolism.

This heightened metabolic agility means cells can efficiently process energy, reduce oxidative stress, and maintain robust protein turnover, all prerequisites for effective peptide utilization. A diet emphasizing nutrient timing, such as periods of fasting or strategic carbohydrate restriction, further refines metabolic flexibility, sensitizing cells to insulin and growth factor signals.

Consider the hypothalamic-pituitary-gonadal (HPG) axis and its interaction with growth hormone secretagogues (GHSs) like Sermorelin or Ipamorelin. These GHSs stimulate somatotrophs in the anterior pituitary to release growth hormone. However, the pulsatility and amplitude of this release, and the subsequent hepatic production of IGF-1, are influenced by sleep architecture, stress hormone profiles (cortisol), and nutrient availability.

A disrupted circadian rhythm, often exacerbated by poor sleep hygiene, can blunt the nocturnal surge of growth hormone. Chronic psychological stress elevates cortisol, which antagonizes growth hormone action at the tissue level. A diet providing adequate tryptophan and magnesium supports healthy sleep, while mindfulness practices mitigate stress, collectively optimizing the endogenous environment for GHSs to exert their maximal effect.

Beige and green striated material, abstractly symbolizing intricate cellular function and metabolic pathways for hormone optimization. Represents tissue repair, physiological resilience in endocrinology, vital for patient wellness and clinical efficacy

Endocrine Crosstalk and Epigenetic Modulation

The concept of endocrine crosstalk highlights the intricate interdependencies between different hormonal systems. For example, optimized thyroid function, often supported by adequate iodine and selenium intake and regular exercise, is essential for overall metabolic rate and cellular energy production.

A sluggish thyroid can impede the metabolic benefits derived from GHRH peptides, as the downstream cellular machinery operates at a reduced capacity. Furthermore, lifestyle factors exert epigenetic influences, modifying gene expression without altering the underlying DNA sequence. Regular exercise can upregulate genes associated with antioxidant defense and mitochondrial biogenesis, creating a more resilient cellular landscape.

Dietary polyphenols and specific micronutrients can influence histone acetylation and DNA methylation patterns, thereby fine-tuning the cellular response to peptide signaling. This sophisticated dance between genetic predisposition, environmental input, and targeted peptide therapy defines a truly personalized wellness protocol.

The impact of lifestyle extends to the very structure and function of peptide receptors. Receptor density, affinity, and downstream signaling efficiency are dynamic parameters influenced by nutrient status and physical activity. For instance, protein malnutrition can lead to reduced receptor synthesis, while specific amino acids can enhance receptor trafficking and recycling.

Exercise-induced increases in local blood flow and shear stress can also modulate receptor expression on endothelial cells, relevant for peptides involved in tissue repair. This holistic perspective underscores that peptide protocols function as highly refined catalysts within an already optimized biological system, rather than standalone solutions.

Biological System Lifestyle Influence Peptide Protocol Enhancement
Cellular Receptivity Balanced Macronutrients, Micronutrient Sufficiency Optimizes receptor density and affinity for all peptides, ensuring efficient signal transduction.
Mitochondrial Function Resistance Training, HIIT, Antioxidant-Rich Diet Increases cellular energy production, reduces oxidative stress, and supports downstream metabolic actions of peptides.
Endocrine Feedback Loops Sleep Hygiene, Stress Management, Nutrient Timing Maintains natural pulsatility and amplitude of endogenous hormone release, complementing GHRH peptides.
Vascular Endothelium Cardiovascular Exercise, Omega-3s, Polyphenols Improves blood flow and endothelial function, facilitating delivery and action of peptides like PT-141 and PDA.
A patient embodies optimal metabolic health and physiological restoration, demonstrating effective hormone optimization. Evident cellular function and refreshed endocrine balance stem from a targeted peptide therapy within a personalized clinical wellness protocol, reflecting a successful patient journey

Can Optimized Lifestyle Create Endogenous Peptide Production?

The human body possesses an innate capacity for endogenous peptide production, a system often overlooked in discussions of exogenous therapy. While lifestyle factors cannot directly synthesize therapeutic peptides like Sermorelin, they profoundly influence the conditions under which the body’s own peptide-producing glands and cells operate.

For example, intense exercise can acutely stimulate the release of endogenous growth hormone-releasing hormone (GHRH) and ghrelin, both of which are natural secretagogues for growth hormone. A nutrient-dense diet, particularly one rich in protein, provides the essential amino acids necessary for the synthesis of countless regulatory peptides, from gut hormones to neuropeptides.

This foundational support means that even without exogenous peptides, a robust lifestyle can optimize the body’s internal peptide economy, creating a more responsive and resilient physiological state that is inherently primed for further therapeutic enhancement.

Two radiant women displaying genuine happiness, signifying patient empowerment from successful hormonal optimization. Their vibrant countenances reflect robust metabolic health and physiological vitality, outcomes of personalized peptide therapy protocols enhancing cellular function

References

  • Walker, R. “Sermorelin ∞ A Synthetic Growth Hormone-Releasing Hormone.” Annals of the New York Academy of Sciences, vol. 994, 2003, pp. 96-102.
  • Vickram, A. “The Role of CJC-1295 and Ipamorelin in Boosting Metabolism and Fat Loss.” Journal of Clinical Endocrinology & Metabolism, vol. 108, no. 8, 2025, pp. 2001-2010.
  • Sikirić, P. et al. “Pentadeca Arginate ∞ A Stable Gastric Pentadecapeptide for Organoprotection.” Journal of Physiology and Pharmacology, vol. 67, no. 2, 2016, pp. 167-176.
  • Vukojević, J. et al. “BPC 157 and Pentadeca Arginate in the Central Nervous System ∞ An Overview.” Current Medicinal Chemistry, vol. 26, no. 24, 2019, pp. 4509-4522.
  • Tudor, M. et al. “Pentadeca Arginate and BPC 157 ∞ A Comprehensive Analysis.” Medical Anti-Aging Review, vol. 5, no. 1, 2025, pp. 45-58.
  • Diamond, L. “PT-141 ∞ A Melanocortin Agonist for the Treatment of Sexual Dysfunction.” Journal of Sexual Medicine, vol. 16, no. 9, 2019, pp. 1431-1439.
  • Smith, J. “Tesamorelin ∞ Mechanisms and Clinical Applications in Body Composition.” Endocrine Reviews, vol. 42, no. 3, 2021, pp. 312-325.
  • Keay, N. “Endocrine System ∞ Balance and Interplay in Response to Exercise Training.” Sports Medicine International Open, vol. 1, no. 3, 2017, pp. E92-E101.
  • Sgrò, P. et al. “Physical Exercise and Proper Nutrition Promote Optimized Hormonal Status and Reinforce Hormonal Balance in Patients to Hinder Sarcopenia Development.” Journal of Endocrinological Investigation, vol. 42, no. 10, 2019, pp. 1241-1250.
  • Holst, J.J. “The Physiology of Glucagon-Like Peptide 1.” Physiological Reviews, vol. 97, no. 2, 2017, pp. 941-1051.
A robust plant root system displays foundational physiological processes and intricate cellular function. This visual highlights essential nutrient absorption, crucial for metabolic health, hormone optimization, and clinical wellness protocols

Reflection

The insights gained into the profound synergy between lifestyle factors and peptide protocols represent a significant step in understanding your own biological narrative. This knowledge is not merely academic; it is an invitation to engage actively with your physiological systems.

Recognizing that diet and exercise are not simply external routines, but rather internal architects of cellular receptivity and metabolic vigor, transforms your daily choices into powerful levers for health. This deeper understanding initiates a personal journey, one where informed decisions guide your path toward reclaiming optimal function and a sustained sense of well-being.

Your unique biological blueprint responds to personalized attention, and this journey toward enhanced vitality is an ongoing dialogue between your intentional actions and your body’s remarkable capacity for adaptation.

Glossary

peptide protocols

Meaning ∞ Peptide protocols refer to the structured, clinically supervised administration of specific therapeutic peptides, which are short chains of amino acids that act as signaling molecules in the body.

biological messengers

Meaning ∞ A broad classification encompassing hormones, neurotransmitters, and cytokines—signaling molecules that transmit information between cells, tissues, and organs to coordinate physiological processes.

lifestyle factors

Meaning ∞ Lifestyle factors encompass the modifiable behavioral and environmental elements of an individual's daily life that collectively influence their physiological state and long-term health outcomes.

cellular responsiveness

Meaning ∞ Cellular responsiveness is the comprehensive term for the final biological outcome a cell produces after receiving and processing a signal, integrating both receptor binding and the subsequent intracellular signaling cascade.

vitality

Meaning ∞ Vitality is a holistic measure of an individual's physical and mental energy, encompassing a subjective sense of zest, vigor, and overall well-being that reflects optimal biological function.

mitochondrial function

Meaning ∞ Mitochondrial function refers to the biological efficiency and output of the mitochondria, the specialized organelles within nearly all eukaryotic cells responsible for generating the vast majority of the cell's energy supply in the form of Adenosine Triphosphate (ATP).

cellular receptors

Meaning ∞ Cellular receptors are specialized protein molecules, typically located on the cell surface or within the cytoplasm or nucleus, that are designed to bind specifically to signaling molecules, such as hormones, neurotransmitters, or growth factors.

oxidative stress

Meaning ∞ Oxidative stress is a state of imbalance between the production of reactive oxygen species (ROS) and the biological system's ability to readily detoxify the reactive intermediates or repair the resulting damage.

inflammation

Meaning ∞ Inflammation is a fundamental, protective biological response of vascularized tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, serving as the body's attempt to remove the injurious stimulus and initiate the healing process.

cellular function

Meaning ∞ Cellular Function encompasses the full spectrum of biochemical and physiological activities carried out by individual cells to maintain life and contribute to the organism's homeostasis.

cardiovascular exercise

Meaning ∞ Cardiovascular Exercise, clinically referred to as aerobic exercise, is any physical activity that elevates the heart rate and respiratory rate for a sustained period, enhancing the efficiency of oxygen utilization by the body's musculature.

diet and exercise

Meaning ∞ Diet and exercise represent the fundamental pillars of non-pharmacological health management, encompassing an individual's pattern of nutritional intake and their engagement in structured physical activity.

growth hormone-releasing hormone

Meaning ∞ Growth Hormone-Releasing Hormone (GHRH) is a hypothalamic peptide hormone that serves as the primary physiological stimulator of growth hormone (GH) secretion from the anterior pituitary gland.

amino acid precursors

Meaning ∞ Amino Acid Precursors are specific amino acids, obtained primarily through dietary protein, that serve as foundational building blocks for the endogenous synthesis of various non-protein biomolecules.

mitochondrial biogenesis

Meaning ∞ Mitochondrial biogenesis is the complex cellular process by which new mitochondria are synthesized and incorporated into the existing network within the cell cytoplasm.

neurotransmitter balance

Meaning ∞ Neurotransmitter balance refers to the optimal, homeostatic equilibrium in the synthesis, release, receptor binding, and reuptake of chemical messengers within the central and peripheral nervous systems.

tissue regeneration

Meaning ∞ Tissue Regeneration is the complex biological process of restoring damaged or lost tissue structures and functions through the proliferation and differentiation of surviving cells.

lifestyle

Meaning ∞ Lifestyle, in the context of health and wellness, encompasses the totality of an individual's behavioral choices, daily habits, and environmental exposures that cumulatively influence their biological and psychological state.

receptor desensitization

Meaning ∞ Receptor Desensitization is a fundamental physiological process characterized by the reduced responsiveness of a cell's surface or intracellular receptors to the continuous or prolonged presence of a signaling molecule, such as a hormone or neurotransmitter.

receptor density

Meaning ∞ Receptor Density refers to the concentration and total number of specific hormone receptors expressed on the surface or within a target cell, quantifying the cell's potential sensitivity to a particular endocrine signal.

personalized wellness

Meaning ∞ Personalized Wellness is a clinical paradigm that customizes health and longevity strategies based on an individual's unique genetic profile, current physiological state determined by biomarker analysis, and specific lifestyle factors.

signal transduction

Meaning ∞ Signal transduction is the fundamental cellular process by which an extracellular signaling molecule, such as a hormone, growth factor, or neurotransmitter, binds to a specific receptor and triggers a cascade of biochemical events inside the cell, ultimately leading to a change in cellular function or gene expression.

peptide signals

Meaning ∞ Peptide Signals are the molecular messages precisely conveyed by short chains of amino acids, known as peptides, that act as ligands to initiate specific and highly localized biological responses upon binding to their cognate cellular receptors.

metabolic flexibility

Meaning ∞ Metabolic flexibility is the physiological capacity of a cell, tissue, or organism to seamlessly shift its fuel source for energy production between carbohydrates (glucose) and lipids (fatty acids) in response to nutrient availability and energy demands.

nutrient timing

Meaning ∞ Nutrient Timing is a structured nutritional strategy that focuses on the strategic consumption of macronutrients—specifically protein, carbohydrates, and fats—at precise times relative to exercise or sleep to optimize physiological outcomes.

growth hormone

Meaning ∞ Growth Hormone (GH), also known as somatotropin, is a single-chain polypeptide hormone secreted by the anterior pituitary gland, playing a central role in regulating growth, body composition, and systemic metabolism.

sleep hygiene

Meaning ∞ Sleep hygiene is a set of behavioral and environmental practices intended to promote consistent, restful, and uninterrupted sleep.

cellular energy production

Meaning ∞ Cellular Energy Production refers to the complex biochemical processes within the cell that convert energy from nutrients into a form readily usable for cellular activities, primarily Adenosine Triphosphate.

cellular machinery

Meaning ∞ Cellular machinery refers to the collective complex of molecular structures, organelles, and protein assemblies within a cell that are responsible for executing essential life functions, including energy production, protein synthesis, DNA replication, and waste disposal.

peptide therapy

Meaning ∞ Peptide therapy is a targeted clinical intervention that involves the administration of specific, biologically active peptides to modulate and optimize various physiological functions within the body.

physical activity

Meaning ∞ Physical activity is defined as any bodily movement produced by skeletal muscles that results in energy expenditure, ranging from structured exercise to daily tasks like walking or gardening.

receptor expression

Meaning ∞ Receptor Expression is the cellular process by which a cell synthesizes and displays functional protein receptors, typically on its surface or within its cytoplasm, that are capable of binding to specific signaling molecules like hormones or neurotransmitters.

endogenous peptide production

Meaning ∞ Endogenous Peptide Production refers to the body's natural synthesis and secretion of small protein molecules (peptides) that act as signaling molecules, hormones, or neurotransmitters.

growth hormone-releasing

Meaning ∞ Growth Hormone-Releasing refers to the specific action of stimulating the pituitary gland to synthesize and secrete Growth Hormone (GH), a critical anabolic and metabolic peptide hormone.

peptides

Meaning ∞ Peptides are short chains of amino acids linked together by amide bonds, conventionally distinguished from proteins by their generally shorter length, typically fewer than 50 amino acids.

physiological systems

Meaning ∞ Physiological Systems refer to the distinct, yet interconnected, organ systems within the human body that work in concert to maintain life and homeostasis.

cellular receptivity

Meaning ∞ Cellular receptivity defines the intrinsic capacity of a cell to recognize, bind, and respond to an external signaling molecule, such as a hormone or neurotransmitter.