GI Hormone Disorders

BCH 130 — Advanced Human Biochemistry · Dr. Radi

build Jul 18 · 08:35 · CC BY-NC-SA 4.0 · owned figures (RDKit / matplotlib)
Dr. Radi

By the end of this unit, you can…

  • Diagram the gut-brain appetite axis
  • Explain leptin resistance and the defended-set-point model of why obesity resists dietary weight loss.
  • Justify obesity pharmacotherapy and bariatric surgery mechanistically (GLP-1 / GIP agonists, incretin remodeling).
  • Match functional GI/pancreatic neuroendocrine tumors to their hormone and syndrome
Dr. Radi

Today's route 🗺️

  1. Obesity & the Gut-Brain Appetite Axis
  2. Neuroendocrine Tumors
Dr. Radi

1 · Obesity & the Gut-Brain Appetite Axis

"Appetite isn't willpower — it's biochemistry! A whole cast of gut and fat hormones reports to one hub in your hypothalamus, and obesity is what happens when that hub stops listening. Let's meet the signals, see why diets rebound, and turn the whole axis into a treatment."

Dr. Radi

One hub hears every hormone

Hunger runs on hormones — a conversation between gut, fat, and brain. Leptin (from fat) and the post-meal crew — PYY, GLP-1, CCK — push hypothalamic POMC toward satiety. Ghrelin, from an empty stomach, is the lone hunger voice, firing up AgRP. Both report to one integrator: MC4R.

Dr. Radi

Two clocks, one axis

Watch the timing — it tells you each hormone's job. Ghrelin climbs before every meal, the biochemical growl that says "feed me," then crashes once you eat. PYY and GLP-1 do the opposite, rising after a meal to call it quits. And leptin? Nearly flat — a slow, tonic readout of how much fat you're carrying.

Dr. Radi

Why the diet always wins

Here's the cruel twist of obesity. More fat means more leptin — yet the brain stops hearing it. SOCS3, PTP1B, and poor transport across the blood-brain barrier jam the LepR signal, so the hypothalamus reads starvation despite plenty. Diet down and leptin drops further, and biology fights back — more hunger, slower metabolism — defending the set-point.

Dr. Radi

Prescribing the axis

Now the payoff: the best obesity therapies don't fight the axis, they work through it. GLP-1 agonists (semaglutide) and dual GIP/GLP-1 drugs (tirzepatide) amplify satiety and slow the stomach. Bariatric surgery rewires the gut so it pours out more GLP-1 and PYY and less ghrelin — an incretin remodel that resets appetite for good.

Dr. Radi

2 · Neuroendocrine Tumors

"A neuroendocrine tumor is a hormone gland gone rogue — it picks one signal and screams it. The beautiful part for us: each hormone writes its own unmistakable syndrome. Learn the pairings, meet carcinoid's tryptophan heist, spot the inherited clusters, and you can diagnose from the labs alone."

Dr. Radi

One hormone, one syndrome

Each functional NET over-secretes a single hormone, and that hormone is the diagnosis. Gastrinoma floods gastrin → Zollinger-Ellison ulcers. Insulinoma → fasting hypoglycemia with high C-peptide. VIPoma → the watery-diarrhea WDHA syndrome. Glucagonoma → a migratory rash and hyperglycemia. Somatostatinoma → diabetes, gallstones, steatorrhea.

Dr. Radi

Carcinoid steals your tryptophan

Carcinoid tumors pump out serotonin, giving the classic triad — flushing, diarrhea, wheezing — plus right-sided valve fibrosis (symptoms need to bypass the liver first). We catch it by urinary 5-HIAA. And the sneaky part: the tumor diverts so much tryptophan into serotonin that little is left for niacin — so patients can slide into pellagra.

Dr. Radi

When it runs in the family

Sometimes these tumors travel in inherited packs. MEN1 (mutated menin) is the "3 P's" — parathyroid, pituitary, and pancreatic islet tumors. MEN2 (an activated RET oncogene) always brings medullary thyroid carcinoma, with pheochromocytoma in about half. Spot one germline case and you screen the whole family.

Dr. Radi

The universal handle

However exotic the hormone, every NET shares two features you can grab. They spill chromogranin A into the blood (a pan-NET marker) and stud their surface with somatostatin receptors. That second one is a gift: Ga-68 DOTATATE PET lights them up for imaging, and octreotide binds the same receptor to switch the secretion — and the syndrome — right off.

Dr. Radi

Can you…?

  • ☐ diagram the gut-brain appetite axis?
  • ☐ explain leptin resistance and the defended-set-point model of why obesity resists dietary weight loss.?
  • ☐ justify obesity pharmacotherapy and bariatric surgery mechanistically (GLP-1 / GIP agonists, incretin remodeling).?
  • ☐ match functional GI/pancreatic neuroendocrine tumors to their hormone and syndrome?

If any box stays empty, the practice site has a drill for it. 🧪

Dr. Radi