HPC › Bio › Glossary
Leptin is an adipocyte-derived peptide hormone that signals energy reserves to the hypothalamus, suppressing appetite and increasing energy expenditure when fat stores are sufficient. Secreted in direct proportion to body fat mass, it inhibits orexigenic neuropeptide Y pathways and activates anorexigenic pro-opiomelanocortin neurons. In obesity, chronically elevated leptin produces receptor desensitisation, rendering the signal functionally silent.
Beyond appetite regulation, leptin also modulates immune function, reproduction, and thyroid and growth hormone secretion through receptors expressed on multiple non-hypothalamic tissues.
How it works
Leptin was identified in 1994 through positional cloning of the mouse ob gene, which revealed a 167-amino-acid peptide secreted exclusively by adipocytes 1. Mice homozygous for ob mutations develop extreme obesity and metabolic dysfunction, fully reversed by leptin replacement, establishing the hormone's causal role. In humans, adipocytes synthesise leptin in direct proportion to fat mass: circulating concentrations rise as adipose tissue accumulates and fall during caloric restriction or weight loss 2.
At the hypothalamic level, leptin crosses the blood-brain barrier via a saturable transport system and binds LepR receptors in the arcuate nucleus 2. Activation of LepR inhibits orexigenic NPY/AgRP neurons, which drive hunger, and simultaneously activates anorexigenic POMC/CART neurons, suppressing appetite and raising energy expenditure. The net result is a coherent signal from the adipose reservoir to the brain's control system: when fat stores are sufficient, food-seeking is reduced and metabolic rate rises.
Leptin's actions extend beyond appetite. The hormone activates the hypothalamic-pituitary-gonadal axis, modulates thyroid and growth hormone secretion, and exerts pro-inflammatory and immune-regulatory effects through receptors on T lymphocytes and macrophages 2. This pleiotropic profile explains why congenital leptin deficiency causes not only severe early-onset obesity but also failure of pubertal development; recombinant leptin replacement normalises body weight and initiates pubertal progression, demonstrating the hormone's indispensable roles beyond energy balance.
Example
A professional who regularly sleeps five hours a night to accommodate long working days notices persistent hunger despite eating normal portions. Leptin is chronically suppressed by the shortened sleep, while ghrelin rises in tandem. The combination drives preference for energy-dense foods and increases total caloric intake, compounding over months into measurable weight gain without any change in exercise or deliberate dietary behaviour.
Reduced sleep systematically suppresses leptin, elevates ghrelin, and shifts the hunger set-point upward, making caloric restraint progressively harder to sustain.
Why it matters
For most people who struggle with their weight, the problem is not insufficient leptin but leptin resistance: chronically elevated circulating leptin has desensitised hypothalamic receptors through impaired JAK2-STAT3 signalling, reduced blood-brain barrier transport, and endoplasmic reticulum stress 4. The appetite-suppression signal is broadcast continuously but cannot be received. This paradox explains why obesity tends to perpetuate itself; the very hormone that should limit fat accumulation becomes less effective precisely when fat stores are highest.
Sleep deprivation compounds this vulnerability. Restricting sleep to four hours for two nights reduces circulating leptin by 18% and raises ghrelin by 28%, elevating hunger ratings by 24% even without additional physical activity 3. For practitioners optimising body composition or managing energy intake, both leptin resistance and sleep-driven leptin suppression represent addressable levers. Correcting chronic sleep restriction is among the most evidence-supported, non-pharmacological means of restoring leptin signalling to baseline.
What does leptin do in the body?+
Leptin is a hormone secreted by fat cells that signals to the hypothalamus when energy stores are adequate. It inhibits appetite-driving neurons and activates neurons that suppress hunger and raise metabolic rate, producing a coordinated satiety signal. Circulating leptin concentrations rise with fat mass and fall with caloric restriction or weight loss.
What is leptin resistance and why does it occur?+
Leptin resistance describes the state in which chronically elevated leptin fails to suppress appetite because hypothalamic receptors have become desensitised. It arises through several mechanisms: impaired JAK2-STAT3 signalling, reduced transport of leptin across the blood-brain barrier, and endoplasmic reticulum stress. The result is a broken feedback loop in which high fat stores cannot communicate effectively with the brain.
How does sleep deprivation affect leptin levels?+
Restricting sleep to four hours for two nights suppresses circulating leptin by approximately 18% and raises ghrelin by 28%, increasing hunger ratings by 24% with no change in physical activity. Chronically short sleep therefore creates a hormonal environment that systematically increases appetite and preference for energy-dense foods, independent of caloric need.
Can leptin supplements help with weight loss?+
Over-the-counter leptin products have no peer-reviewed efficacy evidence and are not bioidentical to the human hormone. The approved recombinant form, metreleptin, is effective only for rare conditions such as congenital leptin deficiency and generalised lipodystrophy. For common obesity, endogenous leptin is already elevated; the therapeutic priority is restoring receptor sensitivity, not supplementing the hormone.
