Adipsin (Complement Factor D)
Adipsin, also called complement factor D, is a protein made largely by fat cells that drives part of the immune complement system and generates signals promoting fat storage; its levels change in obesity and diabetes.
Plain English
Patient-Friendly Explanation
Adipsin, also known as complement factor D, is a protein produced in large amounts by fat cells and released into the bloodstream. Its day job belongs to the immune system: factor D is the rate-limiting enzyme of the alternative complement pathway, an early warning system against infection. Where weight is concerned, it matters because the fragment it generates acts on fat cells to encourage them to take up glucose and build triglycerides - so this immune protein is also part of the machinery of fat storage. Levels change with body weight and metabolism: adipsin is raised in many people with obesity and with type 2 diabetes, tracks with insulin resistance, triglycerides and inflammation, and yet has been linked in some long-running studies with a lower chance of developing diabetes later, so the picture is not simply more is worse. Adipsin is a research and mechanistic marker, not a routine blood test; it is useful for understanding how fat tissue talks to the immune and metabolic systems, not for diagnosing or monitoring individual patients.
Medical
Clinical Definition
Adipsin (complement factor D) is an adipocyte-secreted serine protease that cleaves factor B within the C3bBb complex, making it the rate-limiting enzyme of the alternative complement pathway; its expression is regulated by PPAR-gamma and by nutritional state, being reduced by fasting and restored by refeeding in rodent models, and it was among the first adipocyte secretory products to be identified. Adipose-derived adipsin generates C3a, which is cleaved by carboxypeptidase to acylation-stimulating protein (C3adesArg), a stimulant of adipocyte glucose uptake and triglyceride synthesis; animals deficient in this axis are lean and show delayed triglyceride clearance, giving adipsin a direct mechanistic link to lipid storage. Human studies generally report higher circulating adipsin in obesity, with positive associations with insulin resistance, fasting glucose, HbA1c, triglycerides, inflammatory markers, visceral adiposity and hepatic steatosis, and elevated levels in type 2 diabetes in several cohorts; levels fall with weight loss. Counter-intuitively, higher baseline adipsin has been associated with a lower incidence of future diabetes in some prospective cohorts, and adipsin has appeared favourable in other metabolic contexts, so the net relationship is cohort- and context-dependent and should not be presented as a simple marker of harm. Adipsin is measured by immunoassay and in multiplex proteomic research panels, not in routine clinical practice. Its clinical value is conceptual: adipsin, adiponectin, leptin, chemerin, retinol-binding protein 4 and related adipokines demonstrate that adipose tissue is an endocrine organ that modulates complement activation, inflammation and insulin sensitivity, and that weight loss shifts the adipokine profile toward a more favourable state.
Key Insight
Why It Matters
Adipsin shows that fat tissue is not a passive store of energy - it makes proteins that influence immunity, inflammation and how the body handles sugar and fat. You will not have it measured in a clinic, but it explains why losing fat changes far more than the number on the scales.
Sources
Reviewed against standard medical references.
Medically reviewed by
Family Physician & Registered Acupuncturist, Accord Medical Clinic. MBBS (NUS), Graduate Diploma in Family Medicine (NUS), Graduate Diploma in Acupuncture (Singapore College of TCM).
Last reviewed: September 2026
This page is general health education, not medical advice. Whether any treatment is appropriate for you is a decision made in consultation after assessment.
