Why do low-intake pigs have higher insulin levels?

Elevated insulin despite low feed intake reflects metabolic dysregulation rather than nutrient surplus. By the end of the nursery these pigs show hyperinsulinaemia together with elevated leptin, triglycerides and TNF-alpha, a profile consistent with reduced insulin sensitivity and cytokine-driven leptin resistance. Energy is diverted toward fat deposition and immune activity instead of lean growth.

Which tissues best explain post-weaning feed intake problems?

The ileum and the hypothalamus, studied together. The ileum is where most nutrient absorption has already occurred and where much of the gut-associated lymphoid tissue sits, which makes it the right place to read barrier integrity and local immune status. The hypothalamus is the central appetite regulator. Cross-tissue integration shows the two are tightly coupled: ileal barrier and metabolism genes correlate strongly with hypothalamic immune and appetite genes.

How does the gut-brain axis suppress appetite after weaning?

It works in three steps. First, nutrient deprivation strips the enterocyte of substrate, mitochondrial ATP production falls, and the intestinal barrier loosens because it is too energetically expensive to maintain. Second, more microbially derived signals cross the compromised barrier and reach the hypothalamus, which responds with innate immune activation including TLR, IL-17 and IL-1 signalling, a well-documented driver of appetite suppression. Third, mTORC1 reports nutrient sufficiency while GLP-1 and PYY add satiety signals, so the pig does not eat even though orexigenic AgRP and NPY neurons are activated.

What is leptin resistance in nursery pigs and why does it matter?

Leptin is secreted by adipose tissue in proportion to fat stores and normally tells the brain to stop eating. Chronically elevated leptin alongside continued appetite dysregulation indicates the brain has stopped responding to it. By day 39, low-growth pigs show leptin concentrations roughly 20-fold higher than fast-growing pigs, together with elevated TNF-alpha. Because leptin receptors share structural features with IL-6 receptors, inflammatory signalling and leptin signalling become entangled, which impairs insulin sensitivity and diverts energy toward fat and immune activation rather than lean growth.

Low Feed Intake Syndrome in Weanling Pigs: Gut-Brain Link

Q: What is low feed intake syndrome in weanling pigs?

Low feed intake syndrome is a self-reinforcing physiological state that begins when a piglet fails to eat in the first days after weaning. Nutrient deprivation suppresses mitochondrial function and antioxidant capacity in the gut, the intestinal barrier loosens, and microbial and immune signals reach the brain. The hypothalamus responds by switching on immune pathways and appetite-suppressing signalling, especially mTORC1. Hormonal changes, including elevated insulin and leptin and altered PYY, then stop normal appetite from returning even after feed becomes freely available.

Q: Does low feed intake cause gut inflammation in weaned pigs?

In the early phase the evidence points the other way. Pigs with low early growth show lower circulating TNF-alpha and reduced expression of pro-inflammatory pathways, including TLR-2, TLR-4 and IL-1beta, in the ileum. The gut immune system appears to enter an energy-conserving state because it lacks the substrate to mount a response. Inflammatory signalling is more prominent in the hypothalamus at day 7, and systemic TNF-alpha only rises later, by day 39.

Q: Do low-intake pigs catch up in growth?

Only partially. The growth gap narrows from roughly 60% at day 7 to about 12% by day 39 through compensatory growth. But transcriptomic and endocrine markers do not normalise: gut barrier-related genes stay misexpressed, hypothalamic mTOR and insulin receptor signalling stay activated, and mitochondrial function remains suppressed. Growth recovery outpaces physiological recovery.

Q: When is the intervention window for low feed intake in weanling pigs?

The first 72 hours after weaning. Physiological imprinting is largely established by day 7, and markers of impairment remain detectable at day 39 despite five weeks of ad libitum access to a complete diet. Interventions applied in week two or three are trying to reverse a cascade that hormonal feedback has already locked in.

Low feed intake syndrome in weanling pigs is the cascade that starts when a piglet fails to eat in the first days after weaning. Nutrient deprivation suppresses mitochondrial function and antioxidant capacity in the gut. The intestinal barrier loosens. Microbial and immune signals reach the brain, and the hypothalamus switches on immune and appetite-suppressing pathways. Hormones like PYY, insulin and leptin fall out of step. The pig stays small, stays off feed, and even when growth partially catches up weeks later, the physiology never fully resets.

Somewhere between a quarter and half of all pigs entering the nursery go through some version of this.

That is the short version. The longer version matters more, because the details change what you should actually do about it.

The first 72 hours decide the next five weeks

Weaning is not one event. It is a pile-up. The diet changes from liquid milk to dry pelleted feed, the litter gets broken up, unfamiliar pigs are mixed, the environment changes, and the pig meets a wall of new antigens and microbes. For roughly one pig in ten, that pile-up produces a true fasting period that can stretch to 30 hours.

Thirty hours without eating is a long time for a 6 kg animal with almost no body reserve. But the fast itself is not the real problem. The real problem is what happens afterwards, when some pigs resume eating promptly and others never really switch on.

Here is the part that should bother every nursery manager. Weaning weight has always been the standard explanation for this variability: heavier pigs eat more, grow faster, end heavier. It is a clean story, and it is incomplete. When researchers pulled electronic feeding station data across large populations, heavier weaning weight only translated into higher feed intake and better growth from about the second week post-weaning onward. Not from day one.

So something else decides whether a piglet starts eating in the first 72 hours, and it has very little to do with how much that piglet weighed when it left the farrowing crate. That something else is the gut-brain axis.

Why pigs that look identical end up completely different

Take two piglets with the same weaning weight, the same genetics, the same litter and the same pre-weaning management. One is eating 200 g/day by day three. The other is eating 80 g/day. Fast forward a week and the second pig is gaining 60% slower. Fast forward to the end of the nursery and it is still 12% behind.

Weigh them at weaning and you cannot tell them apart. That is the frustrating part, and it is also the clue. The divergence is not written into the pig’s size. It is written into how its gut and its brain respond to the stress of weaning.

To read that response you have to look at two tissues at once. The ileum, where most nutrient absorption has already happened and where a large share of the gut’s immune tissue sits. And the hypothalamus, the brain’s appetite control centre. Work combining transcriptomics on both tissues with blood biomarkers, sampled at day 7 and again at day 39 post-weaning, has produced a remarkably consistent picture.

Day 7: two tissues, two different problems

The ileum is running on empty

At one week post-weaning, pigs in the low-growth group show around 40 differentially expressed genes in the ileum. The striking thing is the direction: more genes are switched off than switched on. This is not an inflamed, activated gut. It is a gut that has gone quiet.

The pathways that go quiet are the ones you would expect from a tissue starved of substrate.

Oxidative balance collapses. Glutathione metabolism drops sharply, along with antioxidant enzyme systems and HMOX1-mediated cytoprotection. Glutathione is the gut’s main redox buffer, and when it falls, everything downstream gets harder.

Mitochondrial function stalls. Mitochondrial biogenesis is suppressed, and with it the energy supply that every other enterocyte function depends on.

Nutrient metabolism shuts down broadly. Cholesterol biosynthesis and cysteine synthesis are among the hardest hit. Genes like HMGCS1, IDI1 and HSD17B7 all drop, and all three correlate positively with early growth.

Barrier integrity falters. Tight junction genes including claudins CLDN10 and CLDN11 are reduced, and apoptosis and chemical-stress pathways are strongly inhibited.

That last point deserves emphasis, because it reframes the whole problem. The intestinal barrier is the most energetically expensive structure in the gut to maintain. Enterocytes turn over every three to five days, and that renewal runs on ATP. Cut the energy supply and the barrier is the first thing to degrade. Not because of inflammation. Because of starvation.

The hypothalamus is doing the opposite

While the ileum goes quiet, the hypothalamus lights up. Around 46 genes are differentially expressed, and the pattern is close to a mirror image.

Immune pathways come on strong: Toll-like receptor signalling (TLR2/10, TLR4, TLR7/8), IL-17 and IL-1 signalling, leukocyte migration, and the complement cascade. At the same time, adaptive immune functions such as MHC class II antigen presentation and T-cell signalling are suppressed. It is an innate, alarmed, non-specific immune posture.

Then there is the appetite machinery. mTORC1 and mTOR signalling show the strongest activation scores of any pathway measured, alongside GLP-1 signalling. mTORC1 is the cell’s nutrient-sufficiency sensor, and in the arcuate nucleus it also acts as a satiety brake. When it is switched on, the pig should feel full. Energy-preservation pathways go the same way: galanin signalling, adipocytokine and LKB1-AMPK signalling, thermogenesis.

So the picture at day 7 is a starved, leaking gut sending signals to a brain that is behaving as if the animal were infected and already full.

The blood tells the same story, with one twist

Blood biomarkers at day 7 line up with the tissue data.

Marker	Direction in low-growth pigs	What it points to
IGF-I	Down (significant)	Anabolic signalling is blunted; the growth axis is off
Insulin	Up (significant)	Paradoxical, given how little the pig is eating
TNF-α	Down (significant)	No systemic inflammatory response
PYY	Trending up	A satiety signal the pig cannot afford
Serotonin	Numerically up	Altered gut-derived signalling

The TNF-α result is the twist, and it matters. The conventional assumption has been that low feed intake after weaning triggers gut inflammation, and that the inflammation then suppresses appetite. This data says the opposite. Low-growth pigs had lower circulating TNF-α than fast-growing pigs, and in the ileum only the complement cascade and B-cell signalling were increased. The classic pro-inflammatory TLR and IL-1β pathways were not.

That is not a one-off. Other groups have reported pigs with barrier dysfunction and no overt intestinal inflammation, and reduced expression of immune-activation pathways in low-intake pigs during the first days after weaning.

The mechanism makes sense once you think about energy economics. Mounting an immune response is expensive. A pig already short of substrate cannot afford it, so the gut immune system drops into an energy-conserving posture. That leaves the barrier weaker and less defended, which means more microbial products cross into circulation. Those products do not cause local inflammation. They travel.

Day 39: growth recovers. The pig does not.

By the end of the nursery the growth gap has narrowed from 60% to 12%. On paper that looks like a success story, the classic compensatory growth response where refeeding partially offsets an early deficit.

The transcriptome says otherwise. At day 39 there are more differentially expressed genes than at day 7: 119 in the ileum and 80 in the hypothalamus. Physiological recovery is clearly lagging behind growth recovery.

The ileum pushes back, but the foundation stays cracked

The day-39 ileum is working harder. Digestion and absorption pathways are up, including water-soluble vitamin absorption, micro-mineral absorption and protein digestion enzymes. Nutrient metabolism is up too: beta-oxidation of long-chain fatty acids, phospholipid metabolism, glycogen breakdown. Genes like SLC52A3, which mediates riboflavin uptake, and ASB15, a regulator of growth-related processes, are more highly expressed.

That is a genuine compensatory response. Underneath it, gut integrity is still impaired. Cell replication is suppressed across mitotic control, mitotic cell cycle and DNA synthesis. Cellular stress responses are blunted: DNA repair, chemical stress, hypoxia response. Cellular senescence and apoptosis are reduced.

Read that last bit carefully. Apoptosis going down sounds like good news until you remember that controlled cell death is how the intestinal lining renews itself. A gut that cannot complete the normal cycle of proliferation, differentiation and shedding is a gut stuck in a low-functioning steady state. Genes involved in epithelial barrier organisation and mucosal protection, SIPA1, ST3GAL1 and ARSG, are still misexpressed 39 days after weaning.

There is a longer-term parallel worth keeping in mind. Pigs exposed to early-life adversity at weaning have been shown to still carry a weakened gastrointestinal barrier at 20 weeks of age. The nursery is not where this story ends.

The hypothalamus never fully switches back

Hypothalamic changes persist as well. mTOR signalling and insulin receptor signalling stay activated. T-cell signalling is now up, along with BCL10 and IL17RD, which suggests a shift from innate immune sensing toward adaptive immune-related signalling. Respiratory electron transport is strongly suppressed and deubiquitination is inhibited.

Meanwhile the orexigenic neurons are screaming. AgRP and NPY mRNA are elevated, the classic fasting signature, the brain shouting “eat”. And the appetite-suppressing machinery stays on anyway. The pig is hungry and full at the same time.

Leptin, insulin, and the closed loop

The day-39 blood profile is where the long-term cost becomes explicit. Insulin is elevated, which is hyperinsulinaemia in an animal that is not overeating. Leptin is elevated dramatically, roughly 20-fold higher than in the high-growth group. Triglycerides are elevated. TNF-α is elevated. PYY is lower.

Leptin is secreted by adipose tissue in proportion to fat stores, and it normally tells the brain to stop eating. Chronically elevated leptin alongside continued appetite dysregulation is the signature of leptin resistance: the brain has stopped listening. Leptin receptors belong to the class I cytokine receptor family and share structural features with IL-6 receptors, which is one reason inflammatory signalling and leptin signalling get tangled together. Elevated TNF-α, which correlates with leptin and IL-6 in these pigs, fits the picture of cytokine-driven leptin resistance.

That closes a loop. Leptin resistance impairs insulin sensitivity, which pushes insulin higher, which diverts energy toward fat deposition and immune activation rather than lean growth. The pig gains weight again, but the composition of that gain has shifted, and the regulatory system that should be steering appetite is now mis-calibrated.

The three-step cascade

Put it together and low feed intake syndrome looks less like a single failure and more like a self-reinforcing sequence.

Step 1. Energy crisis in the gut. Early nutrient deprivation strips the enterocyte of substrate. Mitochondrial ATP production falls, oxidative stress rises because glutathione and HMOX1 cytoprotection are down, and cellular renewal slows. The barrier loosens because the gut cannot afford to maintain it. Local immunity adopts an energy-conserving posture, so innate defences weaken further.

Step 2. The brain gets the wrong message. A compromised barrier with impaired immunity means more microbially derived signals reach circulation. The hypothalamus responds with innate immune activation: TLR signalling, IL-17, IL-1, complement. That neuroimmune activation is a well-documented driver of appetite suppression. At the same time mTORC1 reports “sufficient”, and GLP-1 and PYY add satiety signals. The pig does not eat.

Step 3. Hormonal dysregulation locks it in. Insulin climbs paradoxically, PYY signals false satiety early on, and by the end of the nursery leptin resistance has set in alongside elevated TNF-α. The feedback loops that should restore normal appetite are themselves broken.

Breaking any one of those three links changes the trajectory. Waiting until day 15 means trying to break all three at once.

What this means in the barn

Stop using weaning weight as your only sorting tool

Weaning weight tells you about the sow and the lactation. It does not tell you whether a piglet will start eating in the first three days. If your pen allocation, diet complexity and intervention thresholds are driven entirely by weight, you are sorting on the wrong variable for the problem that actually costs you money in week one.

The first week is the whole ballgame

Every finding in this body of work points the same direction. The physiological imprinting happens early, and it does not fully reverse. The pigs that diverged at day 7 were still divergent at day 39, after five weeks of ad libitum access to a properly formulated diet. Compensatory growth narrowed the weight gap. It did not close the physiological one.

Practically, that means three things.

Measure early feed intake, not just weekly weights. Pen-level intake from day 0 to day 3 is the single most informative number you can collect in a nursery, and it is the one most operations do not have.

Treat the first 72 hours as a distinct management phase. Whatever gets a pig eating quickly, be it feed form, flavour, water availability, thermal comfort, social stability or supplemental gruel, is worth more per unit of effort than anything you do in week three.

Watch the low-intake pens specifically. The pigs that fast for 24 to 30 hours are the ones that end up with impaired barrier function, and they are the ones most likely to succumb to dysbiosis once the barrier is compromised and local immunity is suppressed.

Rethink the anti-inflammatory instinct

If the early problem is an energy-conserving, hypofunctional gut rather than an inflamed one, then blanket anti-inflammatory strategies aimed at the gut may be aiming at the wrong target, in the wrong tissue, at the wrong time. In this syndrome the inflammatory signature at day 7 shows up in the hypothalamus, not the ileum. That is a different intervention logic entirely.

None of this means inflammation is irrelevant. By day 39, TNF-α is elevated and clearly implicated in leptin resistance. It means the timing and the tissue matter.

Be careful what you call recovery

A low-intake pig that regains a normal growth curve by day 39 has not recovered. Its ileum is still showing suppressed cell replication and impaired DNA repair. Its hypothalamus is still running appetite-suppressing pathways alongside fasting-induced orexigenic signalling. Its leptin and insulin are dysregulated.

Those pigs carry hidden costs into the finishing phase, in feed efficiency, in body composition, and in resilience to the next challenge. Partial catch-up growth is not the same as a clean bill of health, and managing as if it were is how a week-one problem quietly becomes a close-out problem.

Key takeaways

Low feed intake syndrome affects an estimated 25% to 50% of weaned pigs. It originates in the first days after weaning, not in weaning weight.
At day 7 the ileum of low-growth pigs is functionally suppressed, with oxidative balance, mitochondrial function, nutrient metabolism and barrier integrity all down, while the hypothalamus shows innate immune activation and strong mTOR-mediated appetite suppression.
Early low feed intake is not accompanied by gut inflammation. TNF-α is lower and local immune pathways are down. The immune activation appears centrally, in the brain.
Compensatory growth narrows the weight gap to 12% by day 39, but transcriptomic, endocrine and metabolic disruption persists: barrier impairment, mTOR activation, hyperinsulinaemia, and leptin resistance.
The cascade is self-reinforcing. Starvation damages the barrier, barrier failure activates hypothalamic immune signalling, and hormonal dysregulation stops normal appetite from returning.
The highest-leverage intervention window is the first 72 hours. The highest-leverage metric is early feed intake.

Frequently asked questions

What is low feed intake syndrome in weanling pigs?

It is a self-reinforcing physiological state that begins when a piglet fails to eat in the first days after weaning. Nutrient deprivation impairs mitochondrial function and gut barrier integrity, microbial and immune signals activate the hypothalamus, and appetite-suppressing pathways stay switched on. Hormonal changes, including elevated insulin and leptin and altered PYY, then prevent normal appetite from returning even after feed becomes freely available.

How many pigs are affected?

Current estimates place it between 25% and 50% of pigs entering the nursery. Around 10% of piglets experience a true fasting period lasting up to 30 hours immediately after weaning, and a substantial share of the rest settle into sustained low intake that fails to meet nutrient requirements for growth.

Is weaning weight the main cause?

No. Weaning weight correlates with later performance, but analyses of electronic feeding station data show it only influences feed intake and growth from about the second week post-weaning onward. It does not explain why pigs of identical weaning weight diverge so sharply in the first 72 hours. The gut-brain response to weaning stress explains far more.

Does low feed intake cause gut inflammation in weaned pigs?

The evidence points the other way in the early phase. Pigs with low early growth show lower circulating TNF-α and reduced expression of pro-inflammatory pathways, including TLR-2, TLR-4 and IL-1β, in the ileum. The gut immune system appears to enter an energy-conserving state because it lacks the substrate to mount a response. Inflammatory signalling is more prominent in the hypothalamus at day 7, and systemic TNF-α only rises later, by day 39.

Why do low-intake pigs have higher insulin?

Elevated insulin despite low feed intake reflects metabolic dysregulation rather than nutrient surplus. By the end of the nursery, low-growth pigs show hyperinsulinaemia together with elevated leptin, triglycerides and TNF-α, a profile consistent with reduced insulin sensitivity and cytokine-driven leptin resistance. Energy gets diverted toward fat deposition and immune activity rather than lean growth.

Do low-intake pigs catch up?

Partially. The growth gap narrows from roughly 60% at day 7 to about 12% by day 39 through compensatory growth. But transcriptomic and endocrine markers do not normalise. Gut barrier-related genes stay misexpressed, hypothalamic mTOR and insulin receptor signalling stay activated, and mitochondrial function remains suppressed. Growth recovery outpaces physiological recovery.

Which tissues are most useful for understanding the problem?

The ileum and the hypothalamus, studied together. The ileum is where most absorption has occurred and where much of the gut-associated lymphoid tissue sits, which makes it the right place to read barrier and local immune status. The hypothalamus is the central appetite regulator. Cross-tissue integration shows the two are tightly coupled: ileal barrier and metabolism genes correlate strongly with hypothalamic immune and appetite genes.

What is the single most useful thing a producer can measure?

Pen-level average daily feed intake from day 0 to day 3 post-weaning. It identifies the at-risk population earlier and more accurately than weaning weight, and it is the earliest point at which intervention still changes the trajectory.

When is the intervention window?

The first 72 hours. Physiological imprinting is largely established by day 7, and markers of impairment are still detectable at day 39 despite five weeks of ad libitum access to a complete diet. Interventions applied in week two or three are trying to reverse a cascade already locked in by hormonal feedback.

Why is PYY described as a “false satiety” signal?

PYY is a satiety hormone released by the distal gut in response to nutrients. In low-intake pigs it trends higher early on despite the pigs eating less, probably because impaired absorption in the small intestine lets undigested nutrients reach the hindgut, where fermentation triggers L-cell release. The brain receives a “full” signal from an animal that is actually underfed.