In this pilot clinical trial, the investigators will test whether a carefully supervised ketogenic diet can safely help people with Marfan syndrome who already have a chronic or residual aortic dissection. Fifteen participants will work with a study coordinator and dietitian to learn how to follow the diet and will have regular guidance and check-ins. The participants will measure their blood ketone levels at home with a simple finger-stick device and relay their readings to the research team. Every three months, blood samples will also be collected to confirm the body's level of ketosis.

Participants will continue their usual heart and blood-pressure medicines and regular visits with their doctors. Standard of care computed tomography (CT) scans of the aorta will be performed at the start of the study and then again at 6 and 12 months to see if the aorta changes in size.

The main goal is to see how many patients can avoid needing aortic surgery within one year, compared with what is normally expected from past studies. The investigators will also track how well patients follow the diet, how their aorta changes, and whether inflammation in their blood decreases.

Inclusion Criteria:

1. Adults between 18 and 50 years old.
2. Ability to understand and willingness to sign an Institutional Review Board (IRB) approved written informed consent document.
3. Genetically or clinically confirmed Marfan syndrome
4. Chronic descending thoracic aortic dissection present for at least three months
5. Maximal descending thoracic aortic diameter must measure between 4.0 and 5.0 cm at baseline on contrast-enhanced computed tomography angiography (CTA)
6. Have been clinically stable and consistent antihypertensive regimen for at least four weeks

Exclusion Criteria:

1. Acute or rapidly enlarging dissection (>0.5 cm increase over the preceding three months)
2. Prior descending thoracic aortic repair or endograft placement
3. Advanced renal or hepatic dysfunction
4. Poorly controlled diabetes requiring insulin
5. Active malignancy, pregnancy, or other metabolic or nutritional disorders that would contraindicate ketogenic therapy
6. Body mass index below 18 kg/m²
7. Inability to comply with dietary restrictions or follow-up imaging
8. Current enrollment in another interventional study, which in the opinion of the principal investigator, may confound study results.
9. Patients who, in the opinion of the principal investigator, are not suitable candidates for study participation.