SAR1B Genotype

If you or a family member has had unexplained chronic diarrhea, fat in the stool, very low cholesterol on routine labs, or trouble growing or absorbing fat-soluble vitamins, this test looks at one of the rare but treatable genetic causes. SAR1B (Secretion-Associated Ras-Related GTPase 1B) sits at the center of how the small intestine packages dietary fat for delivery to the rest of the body. When both copies of this gene carry damaging variants, fat gets stuck inside intestinal cells instead of moving into the bloodstream.

This is a one-time test of your inherited DNA. The result does not change over your lifetime, but it can reshape decisions about diet, vitamin supplementation, and family planning. For most people with normal cholesterol and no symptoms, this gene will not be a factor. For the small group with severely low cholesterol or unexplained fat malabsorption, identifying a SAR1B variant can finally name what is happening.

What This Gene Actually Does

SAR1B carries the recipe for a small transport protein (a 198-amino-acid molecule called a GTPase, an enzyme that uses cellular energy to do work) that acts like a shuttle helper inside cells. Its main job is to bundle large fat-carrying particles (chylomicrons) into vesicles that move them from one part of the cell to the next, eventually releasing them into the bloodstream. The gene sits on chromosome 5 and is expressed across many tissues, with expression actually higher in skeletal muscle, liver, heart, kidneys, and placenta than in the small intestine, even though the intestine is where the clinical effects of a damaged gene show up most clearly.

When SAR1B works, fat you eat gets absorbed, packaged, and delivered as energy and as raw material for cell membranes and hormones. When both copies of the gene are damaged, the packaging step fails. Fat accumulates inside intestinal cells, blood cholesterol drops to very low levels, and fat-soluble vitamins (especially vitamin E) become hard to absorb.

Chylomicron Retention Disease

The clearest reason to test SAR1B is to confirm or rule out chylomicron retention disease (also called Anderson's disease), an inherited fat malabsorption syndrome. Children with this condition typically present with chronic diarrhea, failure to thrive, severely low cholesterol, and vitamin E deficiency. Endoscopy shows intestinal cells stuffed with fat droplets that cannot be released.

In one 15-year clinical laboratory study of 44 patients with primary low cholesterol, 23 received a confirmed genetic diagnosis. Of those, 3 patients carried two damaging copies of SAR1B. The remaining cases were explained by other genes (APOB in 17 patients, ANGPTL3 in 2, MTTP in 1), which is why SAR1B is usually tested after APOB comes back negative. Across the published literature, more than 20 distinct SAR1B mutations have been described to date, in a patient population that has grown beyond 50 cases as more reports have accumulated.

How the Damage Happens at the Gene Level

Most known disease-causing SAR1B variants fall into a few categories. Some are frameshift or nonsense mutations (such as c.142delG or c.49C>T) that cut the protein short, producing a stub that is only about a third of normal length and cannot function. Others are missense changes (such as p.Asp137His and p.Asp137Asn) that swap a single building block at a critical site where the protein binds its energy source. A few are large deletions that remove an entire section of the gene including the start signal.

In intestinal biopsies from affected patients, SAR1B messenger RNA was significantly lower than normal. The closely related SAR1A gene was increased, and laboratory studies show that SAR1A and SAR1B have overlapping functions, but this backup is not enough to fully rescue fat transport in the human disease. The overall amount of Sar1 protein in intestinal cells was reduced, which fits the clinical picture of failed fat transport.

What Routine Lipid Tests Miss

A standard lipid panel can flag the downstream consequences of a damaged SAR1B (very low total cholesterol, low LDL, low apolipoprotein B, normal or low triglycerides), but it cannot tell you why your numbers look that way. Familial hypobetalipoproteinemia from APOB variants, abetalipoproteinemia from MTTP variants, and chylomicron retention disease from SAR1B variants all produce overlapping lipid patterns. Only sequencing the gene itself separates them.

Identifying a SAR1B mutation also opens the door for testing biological relatives, who may carry one copy without symptoms or have mild signs that have been missed. It also informs the targeted treatment plan (a low long-chain fat diet, aggressive vitamin E supplementation) that differs from the management of other causes of low cholesterol.

Limits of Genetic Testing for This Condition

SAR1B sequencing is not perfect. A Japanese patient with the full clinical picture of chylomicron retention disease was found to have a completely normal SAR1B protein-coding sequence, showing that other genes or regulatory regions can produce the same phenotype. The relationship between specific SAR1B mutations and disease severity is also imperfect. Patients with the same variant can have noticeably different symptoms and lab findings, which means a positive result confirms the condition but does not predict exactly how severe it will be.

One-Time Test, Lifetime Decisions

Because SAR1B is part of your inherited DNA, this is a one-time test. The result does not change with diet, age, exercise, or supplements. What changes over time is what you do with the result. If you carry two damaging copies, your downstream lipid panel, vitamin E level, liver tests, and creatine kinase (a muscle enzyme often elevated in this condition) should be monitored regularly. If you carry one copy without symptoms, biological children inherit a risk that depends on your partner's genotype.

For most people the test is informative once and then becomes a piece of background knowledge that shapes choices for life: which symptoms to take seriously, which supplements to prioritize, which family members to encourage to test, and which specialists to consult if symptoms develop.

What an Abnormal Result Should Prompt

If two damaging SAR1B variants are found, the next steps are not more genetic testing but a targeted clinical workup. That typically includes a full lipid panel, vitamin E and other fat-soluble vitamins (A, D, K), liver enzymes and imaging to assess for fatty liver, creatine kinase to check for muscle involvement, and consultation with a lipidologist or metabolic specialist. A registered dietitian familiar with fat malabsorption can build a low long-chain fat eating plan that minimizes symptoms while keeping nutrition adequate.

If only one damaging variant is found, you are likely a carrier without disease. Carrier status still has implications for biological relatives, especially if your partner has unexplained low cholesterol. A genetic counselor can help map out what testing makes sense for parents, siblings, and children.

What This Test Does Not Tell You

This test looks specifically at SAR1B. It does not tell you about your overall cardiovascular risk, your standard cholesterol numbers, or your risk of common conditions like heart attack or stroke. It does not replace a lipid panel, ApoB, or Lp(a). If you have routine cholesterol concerns rather than a suspicion of inherited fat malabsorption, this is not the right starting point.