Can suboptimal fatty-acid, vitamin D, or vitamin B12 biomarkers persist despite supplementation?

Persistently suboptimal omega-3, vitamin D, or vitamin B12 biomarkers despite reported supplementation are well explained by an intake-to-biologic-response gap. Supplement use alone does not establish adequate absorbed, transported, or functionally available exposure.

Clinical and practical evidence

• Omega-3: EPA+DHA dose explained ~68% of erythrocyte omega-3 index response (~70% after body-weight adjustment). Doses ≤600 mg/day often did not reliably produce an omega-3 index ≥8%; body size, dietary exposure, and adherence further modify response. Plasma measures recent intake, whereas erythrocyte EPA+DHA reflects longer-term incorporation.
• Vitamin D: A low 25(OH)D during treatment can result from inadequate consumed dose, missed doses, low-dose combination products, malabsorption, or enhanced catabolism. Obesity/body fat can reduce bioavailability through adipose sequestration, and malabsorptive syndromes may require substantially higher dosing.
• Vitamin B12: In older adults, 500–1,000 µg/day oral cyanocobalamin produced the greatest MMA reductions; estimated near-maximal biochemical response required ~647–1,032 µg/day. Benefit was not sustained after stopping 1,000 µg/day, supporting the importance of continued exposure.

Mechanistic considerations

• EPA/DHA absorption depends on lipase activity, bile-salt micelles, enterocyte uptake, and chylomicron export; meal fat and formulation can affect uptake. Tissue partitioning, oxidation, and genetic variation can modify incorporation.
• Vitamin D requires fat- and bile-dependent intestinal absorption, protein-bound transport, hepatic 25-hydroxylation, adipose storage, and regulated catabolism.
• B12 requires intrinsic-factor-mediated ileal uptake and cellular transport. Metformin may impair calcium-dependent intrinsic-factor–B12 uptake; high-dose oral B12 can still work through passive absorption.

Bottom line

• Persistent abnormalities should prompt verification of regimen, dose, continuity, administration, and biomarker choice, followed by assessment of absorption risks, body composition, medications, and metabolism—not an assumption that supplementation has failed.