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The neural circuitry of the hypothalamus does not operate in isolation. To regulate energy balance, the brain must be continuously informed about the body's nutritional state — how recently food has been eaten, how full the stomach is, and how large the body's energy reserves are. This information is conveyed largely by hormones: chemical messengers released into the bloodstream by the gut, the pancreas and the body's fat stores, which travel to the hypothalamus and modulate hunger and satiety. This lesson examines the two hormones the specification names: ghrelin, the "hunger hormone," secreted principally by the stomach and acting to stimulate appetite; and leptin, the "satiety hormone," secreted by adipose (fat) tissue and acting over the longer term to suppress appetite. It sets out where each hormone is produced, how it acts on the arcuate nucleus of the hypothalamus, the research of Cummings and colleagues on ghrelin's time-course, the important phenomenon of leptin resistance, and a structured evaluation. The hormonal account completes the picture begun in the neural mechanisms lesson: the arcuate nucleus is precisely the point at which these hormonal signals act on the neural circuitry. Throughout, the control of eating is treated as a physiological signalling system to be analysed scientifically.
Key Definition: A hormone is a chemical messenger secreted into the bloodstream by an endocrine gland or tissue, acting on distant target cells. In the control of eating, hormones such as ghrelin and leptin carry information about the body's nutritional state to the hypothalamus, where they adjust hunger and satiety.
This lesson addresses the following point from the AQA A-Level Psychology (7182) specification, Paper 3 — Eating Behaviour:
It develops the named content — the source, action and time-course of ghrelin and leptin, their action on the hypothalamus, and leptin resistance — and prepares you to describe (AO1) and evaluate (AO3) the hormonal account. It pairs directly with the neural mechanisms lesson (the arcuate nucleus is the shared interface) and the two are frequently examined together, and connect forward to the biological explanations of obesity. Because these questions rarely include a scenario stem, the assessment objectives are typically split AO1/AO3 only, with no AO2 application required unless a stem is provided.
Both ghrelin and leptin exert their effects principally on the arcuate nucleus of the hypothalamus, so it is worth establishing this common target first. The arcuate nucleus sits at the base of the hypothalamus, in a position with relatively permeable access to the bloodstream, allowing it to "sample" circulating hormones. It contains two opposing populations of neurons that together set the level of appetite:
Hunger and satiety reflect the balance between these two populations. Ghrelin and leptin act in opposite directions on this balance: ghrelin activates the appetite-stimulating (NPY/AgRP) neurons (and inhibits the POMC neurons), tipping the balance towards hunger; leptin activates the appetite-suppressing (POMC) neurons (and inhibits the NPY/AgRP neurons), tipping the balance towards satiety. This shared mechanism is why the hormonal and neural lessons describe a single integrated system rather than two separate ones.
It is helpful to situate ghrelin and leptin within the broader endocrine regulation of eating, even though they are the two the specification names. The body uses a family of signals operating over different timescales: ghrelin initiates meals; insulin (from the pancreas) both enables cellular glucose uptake and acts at the arcuate nucleus as an adiposity signal much like leptin; and a set of gut satiety hormones released during and after a meal contribute to terminating it. The general design principle is a division of labour between orexigenic signals that promote eating and anorexigenic signals that stop it, with the arcuate nucleus integrating them into a single output. Understanding this design clarifies why ghrelin and leptin are the headline hormones: ghrelin is the clearest meal-initiation signal and leptin the clearest long-term energy-reserve signal, so between them they bracket the two ends of the timescale over which appetite is regulated. Keeping this framing in mind prevents the common error of treating the two hormones as a closed, two-part system rather than as the most important members of a larger integrated network.
Exam Tip: A high-band answer names the arcuate nucleus as the site of action and explains that ghrelin and leptin act on opposing neuron populations there. This converts a vague "the hormone makes you hungry/full" into a precise mechanistic account that examiners reward.
Ghrelin is a peptide hormone secreted principally by cells in the lining of the stomach (with smaller contributions from elsewhere in the gut and pancreas). It is the body's main appetite-stimulating hormone and is often called the "hunger hormone." Its defining behavioural feature is its time-course in relation to meals: ghrelin levels rise before a meal, when the stomach is empty, and fall after eating, once food has been consumed. This pattern makes ghrelin an ideal meal-initiation signal — a chemical announcement that the stomach is empty and that it is time to eat — and it accounts for the experience of hunger building in the period before an anticipated meal.
Mechanistically, ghrelin secreted by the empty stomach enters the bloodstream, crosses to the arcuate nucleus, and activates the NPY/AgRP (appetite-stimulating) neurons, increasing the drive to eat and to seek food. Because ghrelin is produced by the stomach itself, it provides a direct gut-to-brain signal of nutritional status, exemplifying the gut–brain axis. Beyond meal initiation, ghrelin also appears to influence longer-term energy balance and to interact with reward circuitry, increasing the incentive value of food — one reason it is implicated in the difficulty of maintaining weight loss, since reduced body mass can be accompanied by elevated ghrelin and heightened hunger.
The classic demonstration of ghrelin's meal-related dynamics comes from Cummings et al. (2004). Aim: to investigate how circulating ghrelin levels change in relation to eating and to the experience of hunger across the day. Method: they took repeated blood samples from participants over an extended period, measuring ghrelin concentrations and tracking their relationship to meals and to self-reported hunger. Findings: ghrelin levels rose sharply shortly before each meal and fell rapidly after eating, and these fluctuations tracked participants' subjective ratings of hunger — hunger peaked as ghrelin peaked, before a meal, and subsided as ghrelin fell afterwards. Conclusion: ghrelin functions as a physiological hunger/meal-initiation signal, with its pre-meal surge and post-meal fall closely corresponding to the felt experience of hunger and satiety. This study provided strong, direct human evidence for ghrelin's proposed role and is the key citation for the hormone.
| Feature | Ghrelin | Leptin |
|---|---|---|
| Main source | Stomach (gut) | Adipose (fat) tissue |
| Effect on appetite | Stimulates (orexigenic) | Suppresses (anorexigenic) |
| Timescale | Short-term: meal initiation | Long-term: fat-store / energy reserves |
| Action at arcuate nucleus | Activates NPY/AgRP neurons | Activates POMC neurons |
| Level when stomach empty / fasting | Rises (pre-meal surge) | Falls with falling fat stores |
| Level after eating / with high fat stores | Falls | Rises |
| Key research / phenomenon | Cummings et al. (2004) — tracks hunger | Leptin resistance in obesity |
Leptin is a peptide hormone secreted by adipose (fat) tissue, and its level in the blood is broadly proportional to the amount of body fat. This makes leptin a long-term signal of the body's energy reserves: when fat stores are large, leptin is high; when fat stores fall (for example during prolonged energy deficit), leptin falls. Where ghrelin signals the short-term, meal-to-meal state of the stomach, leptin signals the longer-term, accumulated state of the body's energy bank. Its principal action is to suppress appetite and so to defend against excessive fat gain.
Mechanistically, leptin released from fat tissue travels to the arcuate nucleus and activates the POMC (appetite-suppressing) neurons while inhibiting the NPY/AgRP (appetite-stimulating) neurons, reducing hunger and increasing energy expenditure. In the homeostatic logic of the previous lesson, leptin is a lipostatic signal: it informs the brain about the size of the fat reserve so that the system can defend a set point. The behavioural importance of leptin is most starkly revealed by its absence: rare cases of congenital leptin deficiency, in which a person cannot produce functional leptin, are characterised by severe early-onset over-eating and obesity, because the brain receives no "satiety" signal from the fat stores and behaves as though the body is perpetually starved. Such cases respond to leptin administration, providing compelling evidence that leptin is genuinely a satiety signal.
The discovery of leptin illustrates how the hormone's role was established. Long before leptin was identified, a strain of severely obese mice (the ob/ob mouse) was known to over-eat compulsively. Classic physiological experiments showed that when the circulation of an ob/ob mouse was surgically joined to that of a normal mouse, the obese mouse reduced its eating — implying that the normal mouse's blood carried a satiety factor that the obese mouse lacked. The eventual identification of that missing factor as leptin, the product of the ob gene, confirmed the long-suspected existence of a blood-borne signal from fat tissue to the brain that suppresses appetite. This history matters for evaluation because it shows leptin's satiety role was established through converging lines of evidence — genetic, physiological and, later, the human congenital-deficiency cases — rather than resting on a single study. It also explains the initial therapeutic optimism: a hormone whose absence caused over-eating seemed an obvious candidate for treating obesity, which is precisely why the discovery of leptin resistance in common obesity was so significant and, at first, so unexpected.
A central and initially paradoxical phenomenon is leptin resistance. When leptin was discovered, it was hoped that it might be a treatment for common obesity — if leptin suppresses appetite, perhaps giving more of it would reduce over-eating. In fact, most people with obesity have high, not low, leptin levels (because they have large fat stores), yet they do not experience the expected appetite suppression. The explanation is that the brain has become resistant to leptin's signal: despite high circulating levels, leptin is less able to act effectively on the arcuate nucleus (through reduced transport across to the brain and/or impaired downstream signalling), so the "satiety" message is not received. The consequence is a vicious cycle: the brain effectively perceives a state of energy deficit despite ample reserves, maintaining hunger and frustrating the system's ability to defend against further fat gain. Leptin resistance is therefore a key concept for understanding why simply having high leptin does not prevent obesity, and why leptin supplementation is not an effective treatment for the common form of the condition — a theme developed in the biological explanations of obesity.
Leptin resistance also illuminates a deeper and asymmetric feature of the body's energy regulation. The system appears to be far more vigorous in defending against weight loss than against weight gain: when fat stores fall, leptin drops, hunger rises and metabolic rate slows, mounting a powerful corrective response; but when fat stores rise, the expected leptin "brake" is blunted by resistance, so the system fails to defend the upper boundary with equal force. From an evolutionary standpoint (recalling the food-preference lesson) this asymmetry makes sense: in the EEA, the recurrent threat was starvation, not surplus, so selection prioritised mechanisms that protect against energy deficit over those that limit energy storage. The practical implication is sobering: the same hormonal system that so effectively resists weight loss offers comparatively weak resistance to weight gain in a modern environment of abundance, which helps explain at the level of mechanism why obesity is common and why deliberate, sustained weight loss is so often opposed by the body. This is a clear instance of an evolved physiological system operating in an environment it was not designed for, and it connects the hormonal account directly to the dieting and obesity lessons that follow.
Key Definition: Leptin resistance is a state in which, despite high circulating leptin levels, the brain responds inadequately to leptin's appetite-suppressing signal, so that satiety is not effectively triggered. It is common in obesity and helps explain why high leptin does not prevent over-eating.
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