What proportion of total body protein should be albumin?

Proportion of Albumin in Total Body Protein

Albumin constitutes approximately 50% of the total plasma protein mass, but only about 10-15% of total body protein. 1

Albumin's Role in the Body

Albumin is the most abundant protein in plasma and plays several critical physiological roles:

1. Primary functions:

   • Generation of colloid-oncotic pressure
   • Binding and transport of various substances (hormones, drugs, etc.)
   • Neutralization of free radicals
   • Anticoagulant properties 2

2. Production and turnover:

   • Synthesized exclusively by the liver
   • Normal turnover time is approximately 25 days
   • Synthesis rate is about 10.5 g/day in healthy adults 3
   • Clearance occurs through renal (≈6%), gastrointestinal (≈10%), and catabolic (≈84%) pathways 3

Albumin in Plasma vs. Total Body Protein

While albumin is the predominant plasma protein, it represents a smaller fraction of total body protein:

• Plasma composition: Albumin makes up approximately 50% of total plasma proteins 4, 5
• Total body protein: Albumin constitutes only about 10-15% of total body protein mass

Clinical Significance of Albumin Levels

Serum albumin concentration is a strong prognostic indicator of morbidity and mortality. Hypoalbuminemia can result from various mechanisms:

1. Acute hypoalbuminemia:

   • Often caused by increased capillary permeability during sepsis or trauma
   • Results in redistribution of albumin from vascular to interstitial space 3

2. Chronic hypoalbuminemia:

   • Decreased synthesis (liver disease)
   • Increased catabolism
   • Enhanced losses (nephrotic syndrome, protein-losing enteropathy) 3

Albumin in Special Populations

Patients with Renal Disease

• Serum albumin is an important predictor of outcomes in peritoneal dialysis patients
• Low serum albumin strongly predicts mortality and morbidity 1

Patients with Liver Disease

• In cirrhotic patients with ascites, albumin administration helps restore plasma oncotic pressure
• Albumin is effective in preventing circulatory dysfunction after large-volume paracentesis 5
• Current guidelines recommend albumin infusion (8 g albumin/L of ascites removed) after paracentesis of >5L 1

Pitfalls in Interpreting Albumin Levels

1. Not a reliable marker of malnutrition:

   • Contrary to common belief, serum albumin is not a useful marker of malnutrition
   • A calorie- and protein-deficient diet does not necessarily reduce albumin synthesis 3

2. Multiple factors affect levels:

   • Inflammation
   • Fluid status
   • Liver function
   • Renal function
   • Gastrointestinal losses

3. Cause vs. effect:

   • Reduced albumin concentration appears to be a result rather than a cause of ill-health
   • Therapy designed solely to increase albumin concentration has limited benefit 3

In conclusion, while albumin is the predominant plasma protein (50% of plasma proteins), it represents only about 10-15% of total body protein. Its concentration in serum serves as an important prognostic indicator in various disease states, but understanding the mechanisms behind hypoalbuminemia is crucial for appropriate clinical management.