Leptin and Leptin Resistance: The Fat-Cell Signal, Weight Regain and Sleep

Leptin is a hormone that fat cells release into the blood. It tells the brain how much energy the body is holding in store, so the amount in circulation rises and falls with body fat. In a study of 275 adults, serum leptin averaged 31.3 ng/ml in those who were obese and 7.5 ng/ml in those of normal weight. Most people who carry excess fat therefore have plenty of leptin; the trouble is that the brain responds to it weakly. This entry belongs to NNWA's handbook of hormones.

What does leptin do in the body?

It works as a fuel gauge for the long term. The gene behind it was cloned in 1994 from a strain of mouse that becomes profoundly obese when the gene is faulty, and the researchers proposed that its product is a signal from fat tissue that regulates the size of the body's fat depot. When fat stores are steady, leptin is steady and appetite and energy use stay roughly matched. When stores shrink, leptin drops, and the brain reads the drop as a shortage.

Leptin also follows a daily rhythm. A study that sampled blood over 24 hours found leptin highest between midnight and the early morning and lowest from noon to mid-afternoon, in lean and obese adults alike, with a swing of 75.6 per cent between peak and trough in the lean group. The hormone that pushes the other way has its own entry, ghrelin and hunger before meals.

What is leptin resistance?

It is the name for a high leptin level that fails to restrain appetite or body weight. If leptin simply switched hunger off, people with the most body fat would eat the least. The 1996 measurements showed the opposite arrangement: leptin was about four times higher in the obese group, and the authors concluded that most obese persons are insensitive to the leptin they produce themselves. The term borrows from insulin resistance, where a tissue also responds weakly to a normal or raised signal.

Two cautions belong here. Leptin resistance is a description drawn from groups of people in research; none of the sources read for this entry gives a blood value that diagnoses it in one person. And it is not a separate disease with its own treatment. It travels with excess body fat, so the practical levers are the ones that reduce fat and protect sleep.

What happens to leptin when you lose weight?

It falls, and the body responds as if it were underfed. In a controlled study, ten adults were kept in hospital at their usual weight and then at a weight 10 per cent lower. At the lower weight they burned less energy, mainly because their muscles did the same work more efficiently, and their leptin, thyroid hormones and sympathetic nervous activity were all reduced. The researchers described the weight-reduced state as a condition of relative leptin insufficiency.

The second half of that experiment is what makes it persuasive. When the same volunteers were given enough leptin by injection to bring their morning level back to where it had been before weight loss, energy expenditure, muscle efficiency and thyroid hormones returned to their earlier values. That is a research finding in ten inpatients, not a treatment anyone can ask for, but it suggests the drop in leptin is part of the cause.

Why is weight regain so common after a diet?

Because the hormonal push to eat outlasts the diet by at least a year. An Australian team put 50 overweight or obese adults without diabetes on a very-low-energy diet for 10 weeks and measured their appetite hormones before, straight after, and at 62 weeks. They lost 13.5 kg on average. Leptin, peptide YY, cholecystokinin and insulin fell, ghrelin rose, and hunger scores went up. A year later, leptin was still below its starting level and hunger was still higher.

The long-term numbers match. A meta-analysis of 29 American studies of structured programmes found that, five years on, the average person had kept off a little over 3 kg, or about 3 per cent of starting weight. People who had followed ordinary reduced-energy balanced diets had kept 2.0 kg, which was 17 per cent of what they first lost. Six of the studies reported that those who exercised more held on to more. Regain is the expected course, not a personal failing; see NNWA's article on why the scale stops moving.

How does sleep change leptin?

Short sleep lowers it, at least in the short term. In a crossover experiment in Chicago, 12 healthy young men had two days of restricted sleep and two days of extended sleep while their food and activity were held constant. After the short nights their average leptin was 18 per cent lower, hunger was 24 per cent higher, and appetite for calorie-dense, high-carbohydrate foods rose by 33 to 45 per cent. In a cohort of 1,024 adults, habitual sleep of five hours predicted leptin 15.5 per cent lower than sleep of eight hours, independent of BMI.

The evidence is not as tidy as those two papers suggest. A 2015 systematic review found only 18 randomised trials in which sleep was deliberately changed, and just four of them measured leptin or ghrelin. It concluded that sleep restriction increases food intake and also total energy expenditure, with inconsistent effects on weight. Short sleep makes eating more likely; an extra hour in bed does not guarantee fat loss.

The Indian picture: body fat, NFHS-5 and ICMR-INDIAB

No national survey measures leptin, so the Indian evidence is about the thing leptin reports on, which is body fat. NFHS-5 weighed and measured adults aged 15 to 49 across the country in 2019-21. ICMR-INDIAB examined 113,043 adults aged 20 and over in 31 states and territories between 2008 and 2020 and classified obesity by WHO Asia Pacific criteria, so the two sets of figures are not directly comparable. Both find more excess weight in towns than in villages.

Weight alone understates the matter for Indians. A consensus statement in the Journal of the Association of Physicians of India describes excess body fat, abdominal adiposity and fat stored in the liver and muscle as characteristic of Asian Indians, and estimates that its lower BMI limits would label an additional 10 to 15 per cent of the population as overweight or obese. Since leptin follows fat and not kilograms, a tape measure is the more useful home check. Try the waist to hip ratio calculator or the body fat calculator.

Which tests measure leptin, and who orders them?

The test is called serum leptin, a single blood sample, and it is seldom needed. In the studies above it was measured by immunoassay for research. Its one clear clinical use in the sources read here is in severely obese children in whom a specialist suspects congenital leptin deficiency, a rare genetic condition. The level moves with the time of day and with body fat, so a high value in an adult carrying excess fat is the expected finding, and reading it is the doctor's job.

A worked example: planning for the year after the diet

This case is illustrative and not a real person; the figures show the arithmetic and are not targets for anyone. A 41-year-old vegetarian schoolteacher in Pune weighs 80 kg, has no medical conditions and takes no medicines. She wants to reach 72 kg, a loss of 8 kg or 10 per cent of her weight, the proportion studied in the inpatient experiment above.

What does the evidence say about leptin as a treatment?

For the rare people who make no leptin, replacing it works. Three severely obese children with congenital leptin deficiency were treated with daily injections of recombinant leptin for up to four years, with sustained benefits for appetite, fat mass and blood insulin and lipids, and with normal timing of puberty. This is specialist hospital medicine for a genetic diagnosis.

For common obesity the result was modest and erratic. A randomised, placebo-controlled trial gave daily leptin injections at four dose levels to 54 lean and 73 obese adults. After 24 weeks, mean weight change in the obese groups ranged from 0.7 kg lost at the lowest dose to 7.1 kg lost at the highest, but that top group held only eight people and its standard deviation of 8.5 kg was larger than the loss. Starting leptin level did not predict who responded. Adding leptin to a body already full of it did little for most people.

Common myths and mistakes

A second mistake is to treat low leptin after a diet as damage. It is the normal response of a smaller fat store, and the reason maintenance needs its own plan. A third is to chase more leptin, when people with excess fat already have more of it than lean people. NNWA's guide to an obesity diet in India deals with the food itself.

Safety and when to see a doctor

Nothing on this page is advice about leptin injections, weight-loss medicines or doses. Recombinant leptin is a prescription treatment for rare diagnosed conditions and is not something to seek out for ordinary weight loss. ICMR-NIN's guidelines say that approaches of rapid weight loss and the use of anti-obesity drugs should be avoided; any medicine for weight, including the newer injections discussed in NNWA's piece on regain after stopping semaglutide, is a decision for a doctor who knows your history.

Learning this with NNWA

Appetite regulation is taught in NNWA's programmes, including the Weight Management course and the Diploma in Nutrition. NNWA, a private skill academy in Kolkata, runs its classes online in Hindi and in English. Graduates learn to coach healthy adults and to pass medical cases to a doctor; no NNWA course is a degree or a dietitian qualification.