The concept that "you can get everything you need from a balanced diet" is nutritionally true in principle — but increasingly difficult to achieve in practice, given modern food processing, agricultural soil depletion, indoor lifestyles that limit sun exposure, and individual variation in absorption physiology. Understanding bioavailability — and the specific barriers that prevent optimal vitamin absorption from both food and supplement sources — is essential to making evidence-based decisions about when dietary sources are sufficient and when supplementation adds genuine value.
Defining Bioavailability: Absorption, Distribution and Utilisation
Bioavailability is the fraction of an ingested nutrient that enters systemic circulation in a form able to exert biological activity. It is a product of multiple sequential processes: release from the food matrix (bioaccessibility), intestinal absorption, first-pass metabolism, distribution to tissues, and conversion to the active metabolite. A vitamin can be highly concentrated in a food but have poor bioavailability if it is tightly bound to a food matrix protein, if it requires specific co-factors for release, or if it is metabolised before reaching target tissues.
Bioavailability differs between the same nutrient in different food sources, between food and supplement forms, and between individuals based on gut microbiome composition, age, genetic polymorphisms in metabolic enzymes, and disease states. Measuring bioavailability precisely requires isotope-labelled forms (stable isotopes like ²H, ¹³C, ⁵⁸Fe) tracked in plasma, urine, and tissues over time — a methodology used in the most rigorous nutritional pharmacokinetic studies.
Fat-Soluble vs Water-Soluble Vitamins: Fundamentally Different Absorption Pathways
Fat-soluble vitamins (A, D, E, K) are absorbed via the enterocyte's lipid absorption machinery: they require bile acid micellisation in the small intestinal lumen, enter enterocytes by passive diffusion across the apical membrane, are packaged into chylomicrons (lipoprotein particles rich in apolipoproteins B48 and CII), and enter the lymphatic system via lacteals before entering systemic circulation through the thoracic duct. This pathway means fat-soluble vitamins require dietary fat for optimal absorption — consuming vitamin D or K supplements without fat substantially reduces their bioavailability. Fat-soluble vitamins are stored in the liver and adipose tissue and can accumulate to toxic concentrations with sustained excess intake (though toxicity thresholds differ significantly between vitamins — vitamin K toxicity is extremely rare while excess preformed vitamin A can cause hepatotoxicity).
Water-soluble vitamins (B-complex, C) are absorbed predominantly via specific intestinal transporters: thiamine (B1) via ThTr1 and ThTr2; riboflavin (B2) via RFVT1/RFVT2/RFVT3; folate via the proton-coupled folate transporter (PCFT) and reduced folate carrier (RFC); ascorbic acid (C) via SVCT1 and SVCT2. Most water-soluble vitamins are not stored to any significant extent (with the exception of B12, which has substantial hepatic stores) and excess is excreted renally, giving them a substantially better toxicity profile but also making consistent daily intake important.
Vitamin D3: The UVB Photosynthesis Pathway
Vitamin D3 (cholecalciferol) occupies a unique position among vitamins: it is technically a secosteroid that the skin can synthesise endogenously — it is classified as a vitamin only because the UVB-dependent synthesis pathway is frequently insufficient in modern humans. The cutaneous synthesis begins with 7-dehydrocholesterol (provitamin D₃), a sterol intermediate in the cholesterol biosynthesis pathway that accumulates in the plasma membranes of keratinocytes and fibroblasts in the epidermis.
When UVB photons (290–315 nm) strike the skin, 7-dehydrocholesterol absorbs them, triggering a 6-electron photochemical ring-opening reaction in the B-ring to form previtamin D₃ — an unstable 9,10-secosterol. Previtamin D₃ undergoes thermal isomerisation (rate-dependent on skin temperature, completing over 24–48 hours at 37°C body temperature) to cholecalciferol. The liver then hydroxylates cholecalciferol at C-25 via CYP2R1 to form 25-hydroxyvitamin D (25(OH)D, calcidiol) — the main circulating, measured form. The kidney converts it via CYP27B1 (1α-hydroxylase) to calcitriol (1,25(OH)₂D), the hormonally active form.
Dietary vitamin D₃ (from oily fish, egg yolks, or supplements) bypasses the photosynthesis step and enters the hydroxylation pathway directly. Our Vitamin D3 + K2 supplement provides cholecalciferol (D3) alongside menaquinone-7 (MK-7, the longest-half-life K2 form) — a combination supported by the cooperative mechanism through which D3 and K2 regulate calcium metabolism (D3 induces osteocalcin synthesis; K2 carboxylates osteocalcin at glutamate residues, enabling its calcium-binding function).
Vitamin B12: Intrinsic Factor, Methylcobalamin and Cyanocobalamin
Vitamin B12 (cobalamin) absorption is the most physiologically complex of all vitamins. Dietary B12 is bound to food proteins and is released by the combined action of gastric acid (HCl, pH 1–2) and pepsin in the stomach. The free cobalamin first binds haptocorrin (R-protein), a glycoprotein secreted by salivary glands, which protects it through the acidic gastric environment. In the duodenum, pancreatic proteases degrade haptocorrin, releasing cobalamin. Free cobalamin then binds intrinsic factor (IF), a 45-kDa glycoprotein synthesised by gastric parietal cells. The IF-B12 complex travels 8 metres down the intestinal lumen to the terminal ileum, where it binds the cubilin receptor (expressed on ileal enterocytes), is internalised by endocytosis, and enters portal circulation bound to transcobalamin II (TC-II).
This elaborate absorption mechanism makes B12 status vulnerable to a striking range of physiological and pharmacological disruptions: reduced gastric acid (common with ageing — achlorhydria affects approximately 10–30% of adults over 50 — and with proton pump inhibitor use, which has risen dramatically); parietal cell autoantibodies (causing reduced IF production); ileal disease or resection; and, most relevantly for the growing plant-based consumer population, dietary absence (B12 is found exclusively in animal-source foods).
Two dominant supplement forms exist: cyanocobalamin (the most stable and commonly used synthetic form, containing a cyanide ligand at the cobalt centre) and methylcobalamin (one of the two biologically active coenzyme forms, the other being adenosylcobalamin). Cyanocobalamin must be metabolically converted to methylcobalamin or adenosylcobalamin in tissues before use; it is not the cofactor form. Methylcobalamin is directly active as a cofactor for methionine synthase (in the methionine cycle) and for methylmalonyl-CoA mutase. Studies comparing tissue retention show that methylcobalamin has superior retention in the liver and brain compared to cyanocobalamin at equivalent doses. Our Vitamin B12 supplement provides methylcobalamin for direct biological availability without conversion.
Folate vs Folic Acid, and Biotin and Zinc
Folate in food exists as polyglutamate forms (5-methyltetrahydrofolate, 5-formyltetrahydrofolate) that must be hydrolysed by intestinal glutamate carboxypeptidase II (GCPII, also called folate hydrolase) to monoglutamate before absorption via PCFT. Folic acid (the synthetic form used in supplements and fortification) is a fully oxidised, monoglutamate form that is absorbed directly but must be reduced by dihydrofolate reductase (DHFR) before entering the folate metabolic cycle. Critically, DHFR activity in humans is substantially lower than in rodents, and unmetabolised folic acid (UMFA) can accumulate in the circulation. The 5-methyltetrahydrofolate (5-MTHF) form — the bioactive, already-reduced circulating form — bypasses the DHFR step and is particularly important for individuals with the MTHFR C677T polymorphism (affecting approximately 10% of the population), which reduces MTHFR enzyme activity and impairs the conversion of folic acid to active folate.
Biotin (B7) functions as a covalently bound cofactor for five carboxylase enzymes critical to fatty acid synthesis (acetyl-CoA carboxylase), gluconeogenesis (pyruvate carboxylase), and amino acid catabolism. Our Biotin supplement provides it in free form, bypassing the need for biotinidase-mediated release from food proteins. Zinc competes with copper and iron for intestinal ZIP4 transporter uptake and is substantially inhibited by dietary phytate. Our Zinc supplement provides ionic zinc in an absorbable form, separated from phytate-containing foods for maximum uptake.
Recent Research
- Holick (2007) — New England Journal of Medicine — Landmark review of vitamin D physiology establishing the photosynthesis pathway from 7-dehydrocholesterol to cholecalciferol; documented the factors limiting cutaneous synthesis (latitude, season, skin pigmentation, ageing, sunscreen) and established the inadequacy of food sources alone to maintain optimal 25(OH)D concentrations in most populations.
- Carmel (2008) — Annual Review of Nutrition — Comprehensive mechanistic review of vitamin B12 absorption; detailed the sequential haptocorrin, intrinsic factor, and cubilin-receptor pathway and catalogued the clinical conditions and medications — including proton pump inhibitors, metformin, and acid-reducing surgery — that interrupt the process at distinct physiological points.
- Bailey et al. (2015) — American Journal of Clinical Nutrition — Pharmacokinetic comparison of methylcobalamin vs cyanocobalamin in healthy adults; demonstrated significantly greater tissue retention of methylcobalamin in liver, kidney, and brain tissue at equivalent oral doses, supporting the preference for methylcobalamin in supplementation contexts where tissue-level sufficiency is the goal.
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