By Fady Hannah-Shmouni, MD, FRCPC, as told to Stephanie Booth
Familial hyperchylomicronemia syndrome (FCS) is a rare genetic condition that doesn't let your body clear extra fat from your bloodstream.
Usually, after you eat, your body changes any extra calories into a type of fat called triglycerides. They're stored in your fat cells so they can later be used as fuel. If you have FCS, triglycerides stay in your blood, where they can cause serious issues over time.
Fady Hannah-Shmouni, MD, an endocrinologist and geneticist in Vancouver, British Columbia, explains more about FCS.
What's an easy way to describe familial hyperchylomicronemia syndrome?
Imagine your blood is supposed to be a clear, flowing liquid. But after you eat, it turns milky, almost like a strawberry smoothie. That's essentially what happens in FCS. Your body can't break down a type of fat called triglycerides the way it should, so they build up to dangerous levels in your bloodstream.
It's a rare genetic condition, meaning you're born with it, and it affects only about one or two people in every million. Most people go years without knowing they have it, which is part of what makes it so tricky.
What causes FCS?
FCS is caused by mutations in genes responsible for producing an enzyme called lipoprotein lipase, or LPL, which is what your body normally uses to clear fat from the blood after a meal. When that enzyme is missing or doesn't work properly, fat just builds up.
Do we know yet what triggers these mutations?
The mutations themselves aren't "triggered" by anything you did. They're inherited, meaning you received a faulty copy of the gene from both parents. It's not caused by diet, lifestyle, or environment. You were simply born with the blueprint that way.
What can trigger symptoms to flare, though, are things like eating a high-fat diet, pregnancy, alcohol, or certain medications – those don't cause the disease, but they can make it much worse.
What are common symptoms of FCS?
The symptoms can be all over the place, which is one reason it's so hard to diagnose. The most serious is sudden, severe belly pain – the kind that can land you in the emergency room. People also experience nausea, vomiting, fatigue, and a kind of "brain fog" that makes it hard to think clearly. Some develop small yellowish bumps on their skin called xanthomas, or notice their eyes look a little cloudy.
How long should someone wait before getting them checked out by a doctor?
Here's my advice: Don't wait. If you're having recurring abdominal pain, especially after meals, or if a blood test has ever shown very high triglycerides, get it checked out right away. These aren't symptoms to watch and see.
How is FCS diagnosed?
Diagnosis usually starts with a blood test showing severely elevated triglycerides. We're talking levels above 1,000 mg/dL, sometimes even above 10,000, when normal is under 150. From there, genetic testing can confirm which specific gene mutation is involved.
What are some challenges people face when getting a diagnosis?
The challenge is that most doctors simply don't think of FCS because it's so rare. People often spend years bouncing between specialists, being told they have pancreatitis or that they just need to "eat better." There's a real emotional toll to that.
It's not uncommon to hear patients say they were dismissed or misdiagnosed for a decade before finally getting an answer.
What's the treatment for FCS?
For a long time, the main treatment was an extremely low-fat diet – and I mean extreme, like keeping fat under 20 grams per day, which is genuinely hard to live with. That's still a cornerstone of management.
Standard cholesterol-lowering drugs like statins or fibrates don't really work for FCS the way they might for other conditions. Two newer medications — olezarsen (Tryngolza) and plozasiran (Redemplo) — target the genetic pathway involved and can significantly reduce triglyceride levels. Both are FDA-approved for adults with FCS. As of June 2026, Tryngolza is also approved for use with diet in adults with severe hypertriglyceridemia to reduce triglycerides and lower the risk of acute pancreatitis.
Gene therapy is another area of active research that holds real promise for the future.
Beyond standard care, how can people with FCS best take care of their health?
Diet is everything. Keeping fat intake very low is the single most powerful thing someone with FCS can do day to day. That means reading labels obsessively, meal prepping, and working closely with a registered dietitian who understands the condition.
Alcohol should be avoided entirely. It raises triglycerides significantly. Same goes for certain medications like estrogens or beta-blockers if at all possible, but always talk to your doctor before changing anything.
Pregnancy requires extra monitoring because triglycerides naturally rise and can reach dangerous levels.
I also always encourage patients to carry a medical alert card explaining their condition, because if they end up in an ER, the doctors there may never have seen FCS before.
What are the risks of not treating FCS?
The biggest risk is acute pancreatitis, a sudden, potentially life-threatening inflammation of the pancreas. It's excruciatingly painful and can require hospitalization. Repeated bouts of pancreatitis can permanently damage the pancreas, leading to chronic pain, digestive problems, and even diabetes. There's also a risk of organ damage over time and a significant impact on quality of life – the fatigue, the cognitive issues, the constant dietary restrictions.
Untreated or poorly managed FCS is not something that just quietly sits in the background. It has real consequences.
What advice do you have for someone recently diagnosed with FCS?
I always tell patients: a diagnosis, even a scary one, is actually the beginning of getting better because now we know what we're dealing with.
FCS is manageable. Yes, it requires real changes, and yes, it's a lifelong condition. But with the right diet, the right medical team, and the newer treatments available, people with FCS can live full, meaningful lives.
You're also not alone. There are patient communities and advocacy organizations specifically for FCS where people share experiences and support each other. Finding those connections can make an enormous difference.
This is not a sentence. It's a starting point.