Nutrition & Supplements

Bioavailability

The proportion of an ingested substance (nutrient, supplement, or drug) that reaches systemic circulation in an active form. High bioavailability is essential for oral supplements to have their claimed biological effects.

Bioavailability describes the fraction of an administered substance that reaches the systemic circulation in an active form and is therefore available to exert its biological effect. For oral supplements, bioavailability is primarily determined by gastrointestinal absorption, first-pass hepatic metabolism, and stability in the gut environment.

Why Bioavailability Matters

A supplement may contain an active ingredient at the labeled dose, but if that ingredient is poorly absorbed or rapidly degraded before reaching the target tissue, the labeled dose is functionally meaningless. Bioavailability is why:

  • Some nutrients taken orally are equivalent to IV delivery (well-absorbed)
  • Others require special formulation or higher doses to achieve tissue concentrations
  • Some ingredients work well in lab tests but fail in vivo

Factors Affecting Oral Bioavailability in Dogs

Molecular size: large molecules (proteins, polysaccharides) are generally poorly absorbed intact. Glucosamine (small molecule) absorbs reasonably well; chondroitin (large polysaccharide) has variable and debated absorption.

Lipid solubility: fat-soluble compounds (vitamins A, D, E, K; curcumin; CBD) require co-ingestion with dietary fat for absorption. Water-soluble compounds are generally better absorbed but not always.

First-pass metabolism: absorbed compounds pass through the liver before entering systemic circulation. Extensive first-pass metabolism can reduce effective systemic availability despite good gut absorption.

Gut pH and enzyme environment: dog gastric pH is more acidic than human (~1.5–2.5) and small intestine enzyme profiles differ. Human bioavailability data does not always translate to dogs.

Formulation: enteric coating protects acid-sensitive compounds; liposomal formulations enhance lipid-soluble compound absorption; nanoparticle formulations increase surface area for some compounds.

Common Supplement Bioavailability Concerns

  • Curcumin (turmeric): notoriously poor bioavailability in standard form; phospholipid complexes (Meriva) or piperine combinations significantly improve absorption
  • Chondroitin: large molecular size raises absorption questions; studies in dogs show it does reach joint tissue, but at what effective dose is debated
  • Oral hyaluronic acid: large molecule; limited evidence for joint-space bioavailability
  • Fish oil (omega-3): triglyceride form from whole fish is better absorbed than ethyl ester form found in some concentrates; consider form when comparing products