Hypovitaminosis, the medical term for having lower-than-adequate levels of one or more vitamins, is far more common than most people assume. Vitamin D levels below optimal thresholds have been found in every region of the world studied, and vitamin C deficiency turns up not only in low- and middle-income countries but in wealthier nations too. The causes range from straightforward dietary gaps to medication side effects, surgical history, genetic conditions, and even the latitude where you live. Because the symptoms often creep in slowly and mimic other problems, a deficiency can go unnoticed for months or years before it causes real damage.
What Causes Vitamin Deficiencies
The simplest explanation is not eating enough of the right foods. But that story is rarely the whole picture. Vitamin levels depend on a chain of events: you have to consume a vitamin, absorb it through your gut, transport it into your bloodstream, and convert it into the active form your cells can use. A problem at any link in that chain can leave you deficient even if your diet looks fine on paper.
For vitamin D specifically, the biggest risk factors identified across global studies include older age, female sex, higher latitude, winter season, darker skin pigmentation, less sunlight exposure, and the absence of national fortification programs. Serum levels below the commonly cited sufficiency threshold are widespread, and levels low enough to cause outright disease are especially common in South Asia and the Middle East.
1PubMed. Global vitamin D status and determinants of hypovitaminosis DVitamin C hypovitaminosis follows a different pattern. A review of the global evidence found that deficiency is common in low- and middle-income countries and not uncommon in high-income settings, driven largely by poor fruit and vegetable intake, smoking, and chronic illness.
2PubMed Central. Global Vitamin C Status and Prevalence of Deficiency: A Cause for Concern?When Your Body Cannot Absorb What You Eat
Malabsorption is one of the more underappreciated causes of hypovitaminosis, and it shows up in several common medical scenarios. Weight-loss surgery is a major one. Procedures like Roux-en-Y gastric bypass and biliopancreatic diversion physically reroute the digestive tract, which changes how and where nutrients are taken up. After biliopancreatic diversion or duodenal switch, there is a progressive increase in the incidence and severity of deficiencies in vitamins A, D, and K over time, because these fat-soluble vitamins depend on fat absorption that the surgery deliberately limits.
3PubMed. Serum fat-soluble vitamin deficiency and abnormal calcium metabolism after malabsorptive bariatric surgeryEven less invasive bariatric procedures carry risk. All bariatric surgeries alter gastrointestinal anatomy and physiology to some degree, making patients more susceptible to deficiencies of both macronutrients and micronutrients. The downstream consequences can include anemia, osteoporosis, and protein malnutrition.
4PubMed Central. Bariatric surgery and long-term nutritional issues Deficiencies tend to be more severe after malabsorptive procedures, but they occur with restrictive procedures as well, making lifelong monitoring and supplementation a standard part of post-surgical care.5PubMed. Nutritional deficiencies following bariatric surgery: what have we learned?
You do not need to have had surgery to develop malabsorption-related deficiencies. Celiac disease, Crohn’s disease, chronic pancreatitis, and other gastrointestinal conditions can all impair nutrient uptake. Less obviously, your gut bacteria play a role as well. The human gut microbiota can synthesize several B-group vitamins and vitamin K, and the diversity and abundance of these biosynthetic pathways shift with age and geographic origin. When your microbiome is disrupted by illness, antibiotics, or dietary change, you may lose some of that background vitamin production.
6PubMed Central. Exploring the vitamin biosynthesis landscape of the human gut microbiotaMedications That Quietly Deplete Vitamins
Proton pump inhibitors, the drugs commonly prescribed for acid reflux and ulcers, are among the most widely used medications in the world. They work by suppressing stomach acid, which is exactly the problem from a vitamin standpoint. PPIs have been linked to an increased risk of deficiencies in vitamin B12, vitamin C, calcium, iron, and magnesium.
7PubMed Central. Proton pump inhibitors and risk of vitamin and mineral deficiency: evidence and clinical implicationsThe vitamin B12 connection is especially well documented. Normally, stomach acid and the enzyme pepsin separate B12 from the food proteins it is bound to, which is the essential first step before the vitamin can be absorbed further down the digestive tract. When acid production is chronically suppressed by PPIs or H2-receptor blockers, that separation step fails, and dietary B12 passes through without being taken up.
8PubMed. Vitamin B(12) deficiency associated with histamine(2)-receptor antagonists and a proton-pump inhibitor This does not usually cause problems in the first few months of use, but after a year or more on these drugs, B12 levels can start to drop meaningfully.
Alcohol and Thiamine
Chronic heavy drinking is one of the best-known pathways to severe vitamin deficiency, and thiamine (vitamin B1) is the nutrient most affected. Alcoholism causes thiamine deficiency through a double hit: people who drink heavily tend to eat poorly, and alcohol itself impairs the gut’s ability to absorb the thiamine that is consumed.
9Alcohol and Alcoholism. Effects of Thiamine Deficiency on Brain Metabolism: Implications for the Pathogenesis of the Wernicke-Korsakoff Syndrome The result can be Wernicke’s encephalopathy, a brain disorder involving confusion, abnormal eye movements, and unsteady gait. If untreated, it can progress to Korsakoff syndrome, which involves severe, often irreversible memory loss.
Genetic Conditions That Block Vitamin Use
Some people are born with mutations that interfere with how their body handles specific vitamins. These inherited disorders are rare individually but collectively represent an important fraction of inborn metabolic diseases. For vitamin B12 alone, researchers have identified eight distinct genetic conditions (called complementation groups), each caused by a different gene mutation that disrupts a different step in B12 processing.
10PubMed Central. Genetic disorders of vitamin B₁₂ metabolism: eight complementation groups–eight genesOver the past decade, additional inborn errors of metabolism have been found that affect vitamin absorption, transport, activation, or the recycling of active vitamin cofactors, potentially resulting in acute or chronic multisystem disease or disorders selectively affecting the nervous system.
11PubMed. Inherited disorders of vitamin metabolism For example, mutations in thiamine transport genes can cause thiamine-responsive megaloblastic anemia, characterized by anemia, early-onset diabetes, and hearing loss. Mutations in folate transport genes can cause hereditary folate malabsorption, leading to severe anemia and neurological problems.
12Life Metabolism. Transporters in vitamin uptake and cellular metabolism: impacts on health and diseaseHow Deficiencies Show Up in the Body
The symptoms of hypovitaminosis depend entirely on which vitamin is low and how severe the shortage is. Some deficiencies produce vague symptoms like fatigue and weakness that could be caused by dozens of things. Others cause very specific, recognizable patterns of disease. Here are some of the most clinically significant ones:
- Vitamin A: Deficiency is the leading cause of preventable childhood blindness globally. The eye-related effects, collectively called xerophthalmia, progress from night blindness to dry eyes, corneal damage, and eventually permanent blindness if untreated. 13PubMed Central. The eye signs of vitamin A deficiency
- Vitamin D: Prolonged deficiency weakens bones. In children it causes rickets; in adults the equivalent condition is osteomalacia, literally meaning “soft bone,” where impaired mineralization leaves the skeleton prone to pain, deformity, and fractures. 14PubMed Central. Osteomalacia and Vitamin D Status: A Clinical Update
- Vitamin B12: Because B12 is essential for making myelin (the insulation around nerves) and for synthesizing neurotransmitters, deficiency can cause a range of neuropsychiatric problems including numbness, tingling, difficulty walking, memory trouble, and depression. 15PubMed Central. Neuropsychiatric Disorders Associated With Vitamin B12 Deficiency: An Autobiographical Case Report
- B12 or folate: Either deficiency can cause megaloblastic anemia, where the body produces abnormally large, dysfunctional red blood cells because DNA synthesis in developing blood cells is disrupted. 16D Y Patil Journal of Health Sciences. Megaloblastic Anemia: An Updated Review
- Niacin (B3): Severe deficiency causes pellagra, classically described by the “three Ds”: dermatitis, diarrhea, and dementia. The skin lesions are distinctive enough that clinicians can often diagnose pellagra on appearance alone. Left untreated, it can be fatal.
17PubMed Central. Pellagra: a non-eradicated old disease
Vitamin Deficiencies and Cognitive Decline in Older Adults
A growing body of research connects low vitamin levels with accelerated brain aging. A scoping review of the evidence found a trend of significant associations between low blood levels of folate, B12, vitamin D, vitamin A, and vitamin E and a higher risk of mild cognitive impairment in older adults. Low vitamin D, in particular, also showed an association with physical frailty.
18PubMed Central. Current Evidence on the Association of Micronutrient Malnutrition with Mild Cognitive Impairment, Frailty, and Cognitive Frailty among Older Adults: A Scoping Review This does not prove that fixing the deficiency reverses or prevents cognitive decline, but it raises a practical concern: older adults are already more vulnerable to poor nutrient absorption, and the stakes of missing a deficiency are higher when the brain is involved.
Why a Simple Blood Test Can Miss a Deficiency
Diagnosing hypovitaminosis is not always as straightforward as ordering a blood test. For vitamin B12, the standard serum test measures how much of the vitamin is floating around in your blood, but that number does not always reflect what is happening inside your cells. Some people have technically normal blood levels while their tissues are functionally starving for the vitamin.
This is why clinicians sometimes rely on functional markers instead. Two metabolites, methylmalonic acid (MMA) and homocysteine, build up when B12 is not doing its job properly at the cellular level. In a study of over 400 confirmed episodes of B12 deficiency, MMA was elevated about 98% of the time and homocysteine about 96% of the time. Only one patient had normal levels of both markers.
19The American Journal of Medicine. Sensitivity of serum methylmalonic acid and total homocysteine determinations for diagnosing cobalamin and folate deficiencies These functional markers are better indicators of deficiency at the tissue level than serum vitamin levels alone.20Clinical Chemistry. Cobalamin and Folate Evaluation: Measurement of Methylmalonic Acid and Homocysteine vs Vitamin B12 and Folate
A separate study in cancer patients illustrated the gap well: using the standard serum B12 cutoff, fewer than 2% of patients appeared deficient, but when MMA and homocysteine were used instead, around 11% to 17% qualified as deficient.
21PubMed Central. Methylmalonic Acid and Homocysteine as Indicators of Vitamin B-12 Deficiency in Cancer The practical takeaway is that if you have symptoms consistent with B12 deficiency but your standard blood test comes back normal, it is worth asking about functional testing.
Treatment Is Not One-Size-Fits-All
For most vitamin deficiencies, the fix sounds obvious: replace what is missing. In practice, the route and dose matter a lot. Vitamin B12 replacement has traditionally been given as an intramuscular injection, especially when the deficiency is caused by poor absorption. But a Cochrane review of the evidence found that high-dose oral B12 (at least 1,000 micrograms per day) achieves comparable blood levels to injections, with lower costs and no meaningful difference in side effects.
22PubMed Central. Oral vitamin B 12 versus intramuscular vitamin B 12 for vitamin B 12 deficiency At very high oral doses of 2,000 micrograms per day, blood levels actually ended up higher with the oral route than with injections. This is relevant if you are someone who dislikes needles or has trouble getting to a clinic regularly.
For fat-soluble vitamins like D, how you take the supplement matters. A study found that taking a vitamin D3 supplement with a meal containing fat significantly enhanced absorption compared to taking it on an empty stomach or with a fat-free meal.
23PubMed. Dietary fat increases vitamin D-3 absorption The type of fat (monounsaturated versus polyunsaturated) did not make a difference; what mattered was simply having some fat present. This is the kind of small practical detail that can meaningfully change whether a supplement actually works for you.
The Danger of Refeeding After Starvation
One treatment scenario where vitamin deficiency becomes acutely dangerous is refeeding syndrome. When someone who has been starved or severely malnourished begins eating again, the sudden influx of carbohydrates triggers insulin release, which drives phosphate, potassium, and magnesium from the blood into cells. The resulting drops in these electrolytes can cause heart failure, seizures, and death. Thiamine deficiency is a central part of this picture: the metabolic machinery that processes carbohydrates requires thiamine as a cofactor, and if thiamine stores are already depleted, refeeding without first replacing thiamine can trigger Wernicke’s encephalopathy.
24PubMed Central. Refeeding syndrome: what it is, and how to prevent and treat it This is why protocols for refeeding malnourished patients emphasize giving thiamine and other vitamins before or alongside calories, not after.
Food Fortification as a Population-Level Fix
Individually, you can take supplements and adjust your diet. But at the population level, the most effective tool against specific vitamin deficiencies has been mandatory food fortification. The best-studied example is folic acid. National programs to fortify staple grains with folic acid have reduced the prevalence of neural tube defects, serious birth defects of the brain and spine, in countries around the world.
25PubMed Central. Folic acid food fortification-its history, effect, concerns, and future directionsIn South Africa, for instance, the birth prevalence of spina bifida and anencephaly dropped by about 30% after mandatory fortification of flour was introduced, and maternal folate blood levels improved measurably.
26PubMed Central. Status of prevention of neural tube defects post-folic acid fortification of cereal grains in South Africa Results in other regions have been more uneven. In the Eastern Mediterranean, countries with established fortification policies show potential reductions, but inconsistent implementation and monitoring make it hard to measure the full impact accurately.
27PubMed Central. Mandatory food fortification in the eastern Mediterranean region results in reduced prevalence of neural tube defects Fortification works best when it is enforced consistently and covers the staple foods people actually eat; when loopholes exist (South Africa, for example, exempts cake flour), the benefit shrinks.
When Supplements Themselves Cause Harm
The instinct when you suspect a vitamin deficiency is to start taking a supplement. Usually that is reasonable. But with vitamin B6, there is a genuine paradox that trips people up. Taking high-dose pyridoxine (the form of B6 found in most supplements) can actually cause the same nerve damage that B6 deficiency causes. Reports of sensory neuropathy, including numbness, tingling, and burning pain in the hands and feet, have increased as high-dose B6 supplements have become more popular.
28PubMed Central. Vitamin B-6-Induced Neuropathy: Exploring the Mechanisms of Pyridoxine ToxicityThe mechanism is counterintuitive. In cell studies, the inactive form of B6 (pyridoxine) at high concentrations actually competes with and displaces the active form (pyridoxal-5′-phosphate), effectively creating a functional B6 deficiency even though the person is taking extra B6.
29PubMed. The vitamin B6 paradox: Supplementation with high concentrations of pyridoxine leads to decreased vitamin B6 function A case series of 90 reports to a pharmacovigilance center found that neuropathy was plausible as a side effect of B6 supplements, especially with higher doses and prolonged use, though doses under 50 milligrams per day could not be excluded as a cause either.
30PubMed. Vitamin B(6) in Health Supplements and Neuropathy: Case Series Assessment of Spontaneously Reported Cases The lesson: more is not always better, and symptoms of vitamin excess can look identical to symptoms of deficiency.
Why Humans Need Vitamins From Food in the First Place
Most animals can manufacture vitamin C internally. Humans cannot, and the reason is essentially a broken gene. In every species studied that has lost the ability to make vitamin C, the cause is mutations in the gene for L-gulono-γ-lactone oxidase, the enzyme responsible for catalyzing the final step of vitamin C production.
31PubMed Central. The genetics of vitamin C loss in vertebrates Humans, other primates, guinea pigs, and certain bat and bird species all carry nonfunctional versions of this gene. The mutation stuck around in our evolutionary lineage because our ancestors ate enough fruit to get vitamin C from their diet, so there was no survival penalty for losing the ability to synthesize it. The penalty arrived when diets changed, and scurvy became one of history’s most devastating nutritional diseases. This evolutionary quirk is a useful reminder that the need for dietary vitamins is not a design feature but a biological accident, a broken metabolic pathway that happens to be patched over by eating the right foods.