Most vitamins are absorbed in the small intestine, but different vitamins have preferred zones along the roughly 20-foot stretch of gut between your stomach and colon. Fat-soluble vitamins like A, D, E, and K enter the bloodstream primarily in the upper small intestine, packaged alongside dietary fat. Water-soluble vitamins such as C, folate, and most B vitamins rely on dedicated carrier proteins concentrated in the jejunum. Vitamin B12, famously, can only be absorbed in the very last section of the small intestine. Even the colon plays a minor role, picking up vitamins synthesized by resident bacteria. The story is more segmented than people realize, and understanding which vitamin gets absorbed where helps explain why certain diseases, surgeries, and medications cause specific deficiencies rather than blanket malnutrition.
The Stomach Prepares Vitamins but Does Not Absorb Them
Your stomach is not an absorption site for vitamins, but it does critical prep work. Gastric acid and pepsin break food apart, freeing vitamins from the proteins they are bound to. This release step is especially important for vitamin B12. In food, B12 is tightly bound to animal proteins, and stomach acid is needed to liberate it. Once free, B12 immediately binds to a protein called haptocorrin (also known as R protein), which is secreted by salivary glands and the stomach lining. This binding protects B12 from the harsh acidic environment as it moves downstream.
When stomach acid is reduced, the liberation of B12 from food proteins suffers. Research on people taking omeprazole, a common acid-suppressing drug, found that absorption of protein-bound B12 dropped to roughly 0.5% compared to about 1.2% in people with normal acid levels.1Taylor & Francis Online. Effect of hypochlorhydria due to omeprazole treatment or atrophic gastritis on protein-bound vitamin B12 absorption That may sound like a tiny absolute number either way, but over months of acid suppression the cumulative shortfall can become clinically meaningful. Adding an acidic drink like cranberry juice partially restored absorption in that same study, and dilute hydrochloric acid improved it further, confirming that the problem is really about pH.
The stomach also begins preparing fat-soluble vitamins for later absorption by churning dietary fat into smaller droplets, a process called emulsification. Gastric lipase starts breaking down some triglycerides, and the resulting mixture moves into the duodenum as a semi-liquid called chyme. Without this mechanical and chemical processing, the downstream absorption of vitamins A, D, E, and K would be far less efficient.
The Duodenum and Upper Small Intestine
The duodenum is the first and shortest segment of the small intestine, only about 10 inches long, but it is one of the busiest absorption zones for micronutrients. Iron, calcium, and several vitamins are taken up here, largely because the duodenal lining has a high density of specialized transport proteins and the slightly acidic conditions coming from the stomach favor the uptake of certain minerals.
For vitamin B12, the duodenum is where a crucial handoff occurs. The haptocorrin that protected B12 in the stomach gets broken down by pancreatic enzymes, freeing B12 to bind to a different escort called intrinsic factor (IF), which was secreted by stomach cells earlier.2Vitamins and Hormones. Vitamin B12 absorption and malabsorption This transfer from haptocorrin to intrinsic factor is essential. Without it, B12 never reaches the receptors that can actually pull it across the intestinal wall. Research has shown that in the acid conditions of the stomach, B12 binds almost exclusively to haptocorrin rather than intrinsic factor, and only after pancreatic proteases degrade haptocorrin in the alkaline duodenum does B12 shift over to intrinsic factor.3PubMed Central. Effect of proteolytic enzymes on the binding of cobalamin to R protein and intrinsic factor People with pancreatic insufficiency can develop B12 deficiency precisely because this handoff fails.
Fat-soluble vitamins also begin their absorption journey here. Bile salts from the gallbladder mix with partially digested fat in the duodenum to form tiny structures called micelles, which ferry vitamins A, D, E, and K across the watery environment of the intestinal lumen to the absorptive cells lining the gut wall. Without adequate bile, fat-soluble vitamin absorption drops sharply.
How Fat-Soluble Vitamins Enter the Body
Vitamins A, D, E, and K follow a fundamentally different absorption pathway from water-soluble vitamins. Because they dissolve in fat rather than water, they hitch a ride with dietary lipids and share much of the same transport machinery that handles cholesterol and fatty acids. After micelles deliver these vitamins to the surface of intestinal cells (enterocytes), a set of broad-specificity membrane proteins pulls them inside. These include the scavenger receptor SR-BI, the protein CD36, and a cholesterol transporter called NPC1L1, among others.4PubMed. Proteins involved in fat-soluble vitamin and carotenoid transport across the intestinal cells: New insights from the past decade
Once inside the enterocyte, fat-soluble vitamins are packaged into large lipoprotein particles called chylomicrons, along with triglycerides and cholesterol.5Life Metabolism. Transporters in vitamin uptake and cellular metabolism: impacts on health and disease – Section: Absorption and transport of vitamin A These chylomicrons are too large to enter the blood capillaries directly, so they are exported into the lymphatic system first, eventually draining into the bloodstream via a duct near the left collarbone.6PubMed Central. Intestinal lipid absorption This lymphatic detour is unique to fat-soluble vitamins and dietary fat; water-soluble vitamins go straight into the portal blood heading for the liver.
The practical consequence of this fat-dependent route is simple: if you take a fat-soluble vitamin supplement on an empty stomach or with a fat-free meal, substantially less of it may be absorbed. Conversely, eating even a modest amount of fat alongside these vitamins improves uptake. Some research has also found that vitamin E can be secreted from enterocytes in a second type of lipoprotein particle that does not contain the usual protein scaffold of chylomicrons, suggesting the body has backup routes for getting at least some fat-soluble vitamins into circulation.6PubMed Central. Intestinal lipid absorption
The Jejunum Is the Main Hub for Water-Soluble Vitamins
The jejunum, the middle segment of the small intestine, is where the bulk of water-soluble vitamin absorption takes place. Vitamins C, B1 (thiamin), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin), and folate are all taken up here through dedicated carrier proteins embedded in the intestinal lining.7PubMed Central. Intestinal absorption of water-soluble vitamins in health and disease Each vitamin has its own transport system, and these carriers are not interchangeable: the transporter that moves folate will not accept vitamin C, and vice versa.
Folate absorption illustrates the specificity well. A protein called the proton-coupled folate transporter sits on the surface of jejunal cells and actively pulls folate from the gut lumen. Mutations in the gene for this transporter cause hereditary folate malabsorption, a rare but serious condition that leads to severe folate deficiency starting in infancy.8PubMed Central. Membrane transporters and folate homeostasis: intestinal absorption and transport into systemic compartments and tissues Vitamin C, meanwhile, relies on two sodium-dependent transporters called SVCT-1 and SVCT-2. In the intestine, SVCT-1 sits on the side of the cell facing the gut lumen and does the heavy lifting of pulling vitamin C inward, while SVCT-2 operates on the opposite side, helping move the vitamin into the bloodstream.9PubMed Central. Molecular mechanism(s) involved in differential expression of vitamin C transporters along the intestinal tract
Because these carriers can become saturated, there is a ceiling to how much of a water-soluble vitamin you can absorb from a single dose. For vitamin C, absorption efficiency drops from about 70-90% at moderate dietary intakes to well under 50% at very high single doses. This is why splitting supplement doses through the day tends to yield better blood levels than taking one large pill. Interestingly, high-dose vitamin C may transiently increase the permeability of the spaces between intestinal cells, which could in theory allow some additional nutrients to slip through a secondary route.10World Journal of Gastroenterology. Higher doses of ascorbic acid may have the potential to promote nutrient delivery via intestinal paracellular absorption Whether this is practically significant for nutrient absorption in real-life diets remains unclear.
Vitamin B12 and the Terminal Ileum
B12 stands apart from every other vitamin because of its extraordinarily specific absorption site. After traveling through the stomach and duodenum bound to intrinsic factor, the B12-IF complex reaches the terminal ileum, the very last stretch of the small intestine, where a receptor called cubam is waiting. Cubam is actually two proteins working together: cubilin, a large protein that recognizes and grabs the B12-intrinsic factor complex, and amnionless, a smaller transmembrane protein that anchors cubilin in the cell membrane and drives the entire package into the cell through a process called endocytosis.11PubMed. Structural basis for receptor recognition of vitamin-B(12)-intrinsic factor complexes12PubMed Central. AMN directs endocytosis of the intrinsic factor-vitamin B(12) receptor cubam by engaging ARH or Dab2
This setup means that B12 absorption depends on a long chain of events working in sequence: stomach acid to free B12 from food, haptocorrin to protect it, pancreatic enzymes to release it from haptocorrin, intrinsic factor to bind it, and finally the cubam receptor in the ileum to pull it in. A failure at any link in that chain leads to deficiency. People who lack intrinsic factor (as in pernicious anemia), who have had their terminal ileum surgically removed, or who have ileal inflammation from Crohn’s disease all face high B12 deficiency risk. A study of inflammatory bowel disease patients found that Crohn’s disease with ileal involvement carried roughly a threefold increase in the odds of B12 deficiency, and ileocecal resection was associated with a similar increase.13International Journal of Translational Medicine. Disease Localization and Bowel Resections as Predictors of Vitamin B12 and Vitamin D Status in Patients with Inflammatory Bowel Disease
No other vitamin has such a narrow absorption window. If the jejunum is damaged but the ileum is intact, B12 absorption may remain fine while folate and vitamin C absorption suffers. The reverse is also true: ileal disease can wipe out B12 absorption while leaving other vitamin uptake relatively unscathed.
What the Colon Contributes
The colon has traditionally been considered irrelevant to vitamin absorption, since the small intestine handles the vast majority of dietary nutrient uptake. But bacteria in the large intestine produce several B vitamins and a substantial amount of vitamin K2 (menaquinone). The question has always been whether the colon can actually absorb these microbially produced vitamins in meaningful quantities.
Emerging evidence suggests the answer is at least a qualified yes. Researchers now recognize that the colon has specific transport mechanisms that can take up microbiota-derived vitamins, and that this microbial contribution to the host vitamin pool is increasingly viewed as functionally relevant to overall vitamin balance.14PubMed Central. Approaches to dissect the vitamin biosynthetic network of the gut microbiota For vitamin K in particular, the interplay between dietary intake and gut bacterial production appears to matter for overall status, especially as people age. The diet-microbiome connection is bidirectional: the composition of your diet shapes which bacteria thrive in the colon, and those bacteria in turn influence how much vitamin K is available.15PubMed Central. Vitamin K and Hallmarks of Ageing: Focus on Diet and Gut Microbiome
That said, colonic absorption of B vitamins like biotin and folate, while measurable, is much less efficient than absorption in the small intestine. The colon was not designed as a primary nutrient-uptake organ. Its main jobs remain water reabsorption and electrolyte balance. Consider microbially produced vitamins a bonus contribution rather than a reliable nutritional source, especially if small intestinal absorption is already compromised.
When Absorption Goes Wrong
Knowing where each vitamin is absorbed helps explain why specific diseases create specific deficiency patterns. Celiac disease, which damages the lining of the upper small intestine, tends to cause deficiencies in vitamins whose absorption depends on that region. A study of newly diagnosed celiac patients found that about 20% were deficient in folate, 19% in B12, roughly 15% in B6, and about 8% in vitamin A.16PubMed Central. Vitamin and Mineral Deficiencies Are Highly Prevalent in Newly Diagnosed Celiac Disease Patients Zinc deficiency was even more common, found in roughly two-thirds of tested patients. The pattern makes anatomical sense: the duodenum and jejunum, where these nutrients are absorbed, are the segments most damaged by celiac disease.
Crohn’s disease presents a different pattern depending on which part of the gut is inflamed. When the terminal ileum is involved, B12 deficiency is the hallmark. The same study mentioned earlier found that ileocecal resection raised the odds of vitamin D deficiency by about threefold as well, likely because removing a large portion of the ileum reduces overall absorptive surface area for fat-soluble vitamins too.13International Journal of Translational Medicine. Disease Localization and Bowel Resections as Predictors of Vitamin B12 and Vitamin D Status in Patients with Inflammatory Bowel Disease
Bariatric surgery deliberately rearranges the anatomy of the upper digestive tract, and the nutritional consequences follow directly from which segments are bypassed. Procedures that reroute food past the duodenum and upper jejunum can impair absorption of iron, calcium, folate, and fat-soluble vitamins. Interestingly, many patients already have nutritional deficits before surgery, with vitamin D and iron deficiencies being particularly common in obese individuals presenting for bariatric procedures.17PubMed Central. Bariatric surgery and long-term nutritional issues This means the surgery can deepen pre-existing shortfalls, making lifelong supplementation and monitoring a standard part of post-surgical care.
How Medications Interfere with Vitamin Absorption
Some of the most widely used medications in the world affect vitamin absorption by altering the gut environment. Proton pump inhibitors (PPIs) like omeprazole and H2-receptor antagonists like ranitidine suppress stomach acid, which directly reduces the body’s ability to free B12 from food proteins. Studies have shown that even a week or two of PPI treatment significantly decreases absorption of protein-bound B12, while absorption of unbound (free) B12 remains unaffected.18Advances in Nutrition. Proton Pump Inhibitors, H2-Receptor Antagonists, Metformin, and Vitamin B-12 Deficiency: Clinical Implications – Section: Mechanisms by which PPIs, H2RAs, and metformin affect serum total vitamin B-12 concentrations This distinction matters practically: someone on long-term PPIs who supplements with a standard B12 tablet (which contains free, crystalline B12) may absorb it just fine, since the supplement form does not need acid to be released from food proteins.
Metformin, the first-line drug for type 2 diabetes, has also been linked to reduced B12 levels through a different mechanism. Rather than suppressing acid, metformin appears to interfere with the calcium-dependent binding of the B12-intrinsic factor complex to cubam receptors in the ileum. The effect is dose-dependent and cumulative. People on high-dose metformin for years are at meaningfully elevated risk of B12 deficiency, which is why many guidelines now recommend periodic B12 screening for long-term metformin users.
Other drug-nutrient interactions follow the same anatomical logic. Cholestyramine, a bile acid binder used for cholesterol management, can impair fat-soluble vitamin absorption by depleting the bile salts needed to form micelles. Certain antiepileptic drugs interfere with folate metabolism. In each case, knowing where and how a vitamin is absorbed points directly to which medications are likely to cause trouble.
The Food Matrix and What You Eat Alongside Vitamins
Absorption is not just about having healthy intestinal tissue and the right transport proteins. What you eat together with vitamins changes how much of them you actually take up. The physical structure of food, sometimes called the food matrix, determines how quickly and completely vitamins are released during digestion and made available to the gut lining.
Dietary fiber provides a good example of this complexity. Fiber can bind to certain antioxidants and fat-soluble compounds in the gut, affecting how readily they reach the intestinal wall. Research into the interactions between fiber and fruit-derived antioxidants has shown that the food matrix modulates the release of these compounds from chyme during digestion, sometimes trapping them and reducing absorption, and sometimes slowing release in a way that actually improves uptake by spreading it over a longer stretch of intestine.19PubMed Central. The role of dietary fiber in the bioaccessibility and bioavailability of fruit and vegetable antioxidants The net effect depends on the type of fiber, the specific nutrient, and how the food has been processed. Cooking, blending, and fermenting can all alter the food matrix in ways that either increase or decrease vitamin accessibility.
For fat-soluble vitamins, the most straightforward dietary factor is simply the presence of fat. Adding olive oil to a salad rich in beta-carotene (the plant-based precursor to vitamin A) dramatically increases carotenoid absorption compared to eating the same salad with a fat-free dressing. The same principle applies to vitamin D and K supplements: taking them with a meal that contains some fat improves uptake. For water-soluble vitamins, the matrix effects are generally smaller, but they are not zero. Folate from leafy greens, for instance, may be somewhat less bioavailable than the folic acid used in supplements and fortified foods, because the food form requires enzymatic cleavage before absorption can occur.
These interactions are worth keeping in mind when thinking about supplements versus whole foods. A supplement delivers a vitamin in a form that bypasses many of the matrix constraints, which is an advantage for someone with absorption problems but not necessarily better for a healthy person eating a varied diet. Whole foods bring along cofactors, fiber, and other compounds that may influence how vitamins are used once absorbed, in ways supplements cannot replicate.
Why the Geography of Absorption Matters for Supplementation
The segmented nature of vitamin absorption has direct implications for how supplements are designed and when they should be taken. Sublingual B12 supplements, which dissolve under the tongue and enter the bloodstream through the oral mucosa, exist specifically because some people cannot absorb B12 through the normal ileal pathway. For them, bypassing the entire gastrointestinal tract is the only reliable option. High-dose oral B12 can also work even in people lacking intrinsic factor, because at very high concentrations, a small percentage of B12 can cross the intestinal wall through passive diffusion rather than the receptor-mediated route.
Enteric-coated supplements are designed to survive stomach acid and dissolve only in the more alkaline small intestine, which can be useful for nutrients that are degraded by acid or for people who experience stomach irritation from certain supplements. Iron supplements, on the other hand, are typically designed to dissolve in the stomach because iron absorption peaks in the acidic environment of the duodenum. Taking iron with vitamin C enhances absorption, while taking it with calcium or alongside a PPI can reduce it.
For anyone who has had gastric bypass, ileal resection, or lives with a condition that damages a specific stretch of intestine, the map of vitamin absorption becomes a practical guide to which deficiencies to watch for and how aggressively to supplement. A person missing their terminal ileum needs B12 injections or high-dose oral B12 for life. A person missing their duodenum needs careful monitoring of iron, calcium, and fat-soluble vitamins. The anatomy determines the risk.