Vitamins: Complete Guide

What is Vitamins?

Vitamins are essential nutrients required in small amounts to keep the body functioning properly. They are “essential” because your body cannot make enough of them on its own (or cannot make them at all), so you must get them from food, fortified foods, or supplements. Vitamins do not provide calories like carbohydrates, fats, and proteins, but they are critical for turning food into usable energy and for regulating thousands of cellular processes.

Vitamins are typically grouped into two categories:

• Fat-soluble vitamins: A, D, E, K. These dissolve in fat, are absorbed along with dietary fat, and can be stored in the liver and body fat. Storage is useful when intake varies, but it also increases the risk of toxicity from high-dose supplements.
• Water-soluble vitamins: C and the B-complex (B1, B2, B3, B5, B6, B7, B9, B12). These circulate in water and are generally not stored in large amounts (B12 is a key exception). Excess is often excreted in urine, which lowers toxicity risk but increases the importance of regular intake.

People often talk about “vitamin deficiency” as if it is rare, but suboptimal status is common. Modern patterns like ultra-processed diets, low sunlight exposure, limited seafood intake, low fruit and vegetable variety, restrictive dieting, and certain medications can all push vitamin status below optimal even without causing classic deficiency diseases.

> Important: Vitamins are not “optional performance enhancers.” They are foundational inputs for normal physiology. More is not always better, and the best target is usually adequacy, not extremes.

How Does Vitamins Work?

Vitamins work mainly as coenzymes, hormone-like signaling molecules, antioxidants, and gene expression regulators. Rather than acting as building blocks, many vitamins help enzymes do their jobs, like turning glucose into ATP, synthesizing neurotransmitters, or repairing DNA.

Coenzymes and energy metabolism (B vitamins)

Many B vitamins become coenzymes that drive metabolic pathways:

• B1 (thiamine) supports carbohydrate metabolism and nerve function.
• B2 (riboflavin) and B3 (niacin) form FAD and NAD, central to mitochondrial energy production.
• B5 (pantothenic acid) is part of Coenzyme A, crucial for fat metabolism.
• B6 (pyridoxine) supports amino acid metabolism and neurotransmitter synthesis.
• B7 (biotin) supports carboxylation reactions, including fat and glucose metabolism.
• B9 (folate) and B12 (cobalamin) support DNA synthesis and red blood cell formation.

If you feel “tired all the time,” vitamins are not the only explanation, but B vitamin inadequacy is a common contributor, especially when paired with low protein intake, alcohol use, gut issues, or certain medications.

Hormone-like signaling (vitamin D)

Vitamin D functions more like a hormone than a typical vitamin. After activation in the liver and kidneys, it binds to the vitamin D receptor and influences gene expression in many tissues, including bone, muscle, immune cells, and the brain. This explains why vitamin D status has been studied for outcomes ranging from fractures to respiratory infections.

Antioxidant and redox roles (vitamins C and E)

Oxidative stress is not inherently “bad” (it is part of normal signaling), but chronic oxidative stress can damage lipids, proteins, and DNA. Vitamins C and E help manage oxidative balance:

• Vitamin E protects cell membranes from lipid oxidation.
• Vitamin C supports collagen synthesis and helps regenerate vitamin E.

Vision, epithelial integrity, and immune function (vitamin A)

Vitamin A supports the retina (night vision), immune defenses, and the integrity of skin and mucosal barriers. It is obtained as preformed retinol (animal foods) or as provitamin A carotenoids (like beta-carotene in plants), which the body converts to retinol with variable efficiency.

Blood clotting and calcium handling (vitamin K)

Vitamin K enables activation of proteins involved in blood clotting and also supports proteins that help direct calcium to bones and away from arteries. Vitamin K exists mainly as:

• K1 (phylloquinone) from leafy greens
• K2 (menaquinones) from fermented foods and some animal foods

Absorption and the gut connection

Absorption depends on digestion and gut function. Fat-soluble vitamins require bile and dietary fat, so low-fat diets, gallbladder issues, pancreatic insufficiency, and fat-malabsorption conditions can reduce status. This is one reason gut health content matters: if digestion is impaired, “eating healthy” may not translate into nutrient adequacy.

If you are exploring gut-first strategies, it can be helpful to connect the dots between:

• foods that irritate the gut lining or alter the microbiome
• symptoms like bloating or irregular stools
• and downstream nutrient absorption challenges

(See: 10 Gut-Damaging Foods, Explained by Gut Biology and Milk Kefir: A Deep Dive Into Gut Health Benefits for practical gut context.)

Benefits of Vitamins

Vitamins have well-established benefits when they correct deficiency or inadequacy. In already well-nourished people, benefits from extra supplementation are often smaller and depend on the vitamin, baseline status, and the outcome measured.

1) Preventing deficiency diseases and maintaining baseline function

Classic examples include:

• Vitamin C preventing scurvy and supporting collagen formation
• Vitamin D preventing rickets/osteomalacia and supporting calcium absorption
• Folate (B9) preventing neural tube defects when adequate before and during early pregnancy
• B12 preventing megaloblastic anemia and neurologic injury
• Vitamin A preventing night blindness and supporting immune barriers

Even mild insufficiency can show up as fatigue, poor exercise recovery, mouth sores, impaired night vision, frequent infections, hair shedding, or slow wound healing. These symptoms are non-specific, but they are often the reason clinicians test vitamin status.

2) Bone health and fall risk reduction (D, K, C)

Vitamin D supports calcium absorption and muscle function, which can influence fall risk in older adults. Vitamin K participates in activating bone-related proteins, and vitamin C supports collagen in bone matrix.

The most consistent benefit is seen when vitamin D corrects low baseline levels, especially in older adults, people with limited sun exposure, and those with osteoporosis risk.

3) Red blood cell formation and oxygen delivery (B9, B12, B6)

Folate and B12 are essential for DNA synthesis in rapidly dividing cells, including bone marrow cells that become red blood cells. B6 supports hemoglobin synthesis. Low status can contribute to anemia, fatigue, shortness of breath on exertion, and reduced exercise tolerance.

4) Immune function support (A, D, C, B6)

Several vitamins help maintain immune barriers and immune cell signaling. Vitamin D has been studied extensively for respiratory infections. Results vary, but benefits are more likely in those who start out deficient or who use consistent, moderate dosing rather than infrequent megadoses.

5) Neurologic function, mood, and cognition (B vitamins, D)

B vitamins support neurotransmitter synthesis and methylation pathways. B12 deficiency can cause neuropathy, balance issues, and cognitive changes. Vitamin D receptors in the brain have driven interest in links with mood and cognition, but supplementation benefits are most reliable when correcting deficiency.

6) Cardiometabolic support via adequacy (B vitamins, D, antioxidant vitamins)

Vitamins are not a replacement for addressing insulin resistance, inflammation, and overall dietary quality. However, adequacy matters for metabolic pathways and markers. For example:

• Folate, B12, and B6 influence homocysteine metabolism (a marker often discussed in cardiometabolic risk conversations).
• Vitamin D status correlates with multiple cardiometabolic markers, though causality is complex.

If you are focused on cardiometabolic health, it is usually more productive to combine nutrient adequacy with better markers and habits (sleep, resistance training, protein intake, and reduced ultra-processed foods). (Related: The Dangerous Cholesterol Lie and What Matters More and 10 Subtle Signs Your Diet Is Harming Blood Sugar.)

Potential Risks and Side Effects

Vitamins are essential, but they are not risk-free. The biggest problems arise from high-dose supplementation, interactions with medications, and using supplements to compensate for a poor diet.

Fat-soluble vitamin toxicity (A, D, E, K)

Because these vitamins can be stored, chronic high intake can cause toxicity.

• Vitamin A toxicity (usually from supplements or excessive liver intake) can cause headaches, hair loss, dry skin, bone pain, liver injury, and is teratogenic in pregnancy.
• Vitamin D toxicity is uncommon but possible with very high supplemental doses over time, leading to high calcium levels (hypercalcemia), kidney stones, nausea, confusion, and kidney damage.
• Vitamin E high-dose supplementation can increase bleeding risk in some contexts.
• Vitamin K is generally low-toxicity, but it strongly interacts with warfarin.

> Callout: If you supplement fat-soluble vitamins, dose conservatively and consider periodic lab monitoring, especially for vitamin D.

Water-soluble adverse effects (mostly from high doses)

Water-soluble vitamins are less likely to cause toxicity, but high doses can still cause problems:

• Niacin (B3) can cause flushing and, at high therapeutic doses, liver injury and glucose dysregulation.
• B6 at high chronic doses can cause sensory neuropathy.
• Vitamin C can cause GI upset and may increase kidney stone risk in susceptible individuals at high doses.
• Folic acid can mask B12 deficiency by correcting anemia while neurologic damage progresses.

Interactions with medications

Common examples:

• Warfarin and vitamin K: inconsistent intake of vitamin K can destabilize INR.
• Metformin and B12: long-term use is linked to lower B12 status in many people.
• Proton pump inhibitors (PPIs) and H2 blockers: can reduce B12 absorption by lowering stomach acid.
• Orlistat and bile acid sequestrants: can reduce absorption of fat-soluble vitamins.
• Some anti-seizure medications can affect vitamin D metabolism.

“More is better” and the supplement trap

A frequent mistake is stacking multiple products (multivitamin, greens powder, pre-workout, immune formula) that duplicate ingredients. This can push intake above tolerable upper limits without you realizing it.

Another pitfall is relying on vitamins to offset ultra-processed diets. Highly processed patterns can be calorie-dense but micronutrient-poor, and they can worsen blood sugar swings that increase cravings and fatigue. Fixing the food pattern often improves vitamin status naturally.

Practical Guide: How to Get Vitamins (Food, Testing, and Supplement Basics)

A practical vitamin strategy has three steps: build a nutrient-dense base diet, identify higher-risk gaps, and supplement selectively.

1) Food-first: the most reliable foundation

A varied diet tends to cover most vitamins without needing high doses. Emphasize:

• Protein foods (eggs, fish, dairy, poultry, legumes): B vitamins, A (some), D (fatty fish), K2 (some dairy)
• Colorful produce (leafy greens, orange and red vegetables, berries, citrus): folate, vitamin C, carotenoids
• Healthy fats (olive oil, nuts, seeds, fatty fish): supports absorption of A, D, E, K
• Fermented foods (yogurt, kefir, some cheeses): may support gut function and provide some B vitamins and K2 depending on the food

If gut symptoms limit variety, address tolerance and digestion first rather than simply adding more supplements. (Related: Milk Kefir: A Deep Dive Into Gut Health Benefits.)

2) Practical “gap” vitamins to consider

These are common shortfalls in many adults:

• Vitamin D: limited sun exposure, darker skin, higher latitude, winter months, indoor lifestyle
• Folate: low intake of leafy greens and legumes (though fortified grains provide folic acid)
• B12: vegan diets, older adults, metformin users, acid-suppressing medication users
• Vitamin A and K: low vegetable intake, fat malabsorption
• Vitamin E: low intake of nuts, seeds, and plant oils (food sources are usually sufficient)

3) Testing: when it makes sense

Not everyone needs extensive testing. It is most useful when symptoms, diet pattern, life stage, or medications raise risk.

Common labs to discuss with a clinician:

• 25(OH)D (vitamin D status)
• B12 with methylmalonic acid (MMA) if deficiency is suspected
• Folate (context-dependent)
• CBC and iron studies if fatigue is present (vitamins are not the only cause)

4) Supplement basics (general ranges, not prescriptions)

Supplement needs depend on diet and labs. Typical over-the-counter approaches include:

• Multivitamin: can help cover small gaps, especially during calorie deficits. Choose one near 100% of daily values rather than megadose formulas.
• Vitamin D3: commonly 1,000 to 2,000 IU daily for maintenance in adults, with higher amounts sometimes used under clinician guidance when levels are low. Pairing D with magnesium and adequate calcium intake can matter for tolerance and effectiveness.
• B12: oral cyanocobalamin or methylcobalamin is commonly used; higher doses are often used because absorption is limited. Vegans often need a consistent B12 plan.
• Folate: typically best addressed with food unless pregnancy planning or deficiency risk exists. If supplementing, avoid masking B12 deficiency.

> Callout: Avoid “megadose” stacks unless there is a clear indication, a time-limited plan, and a way to monitor outcomes.

5) Special note on vitamin D and high-dose trends

High-dose vitamin D content has increased online, including discussions about muscle, leptin signaling, and body composition. Mechanistic and animal data are intriguing, but human outcomes are less definitive, and safety depends on dose, duration, baseline levels, calcium intake, kidney health, and monitoring.

If you are curious about higher targets, start with testing and a clinician-guided plan rather than guessing. (Related: High-Dose Vitamin D for Muscle, Leptin, and Fat.)

What the Research Says

Vitamin research is tricky because vitamins are essential, but supplementation trials often fail to show dramatic benefits in well-nourished populations. This does not mean vitamins “do nothing.” It often means that once adequacy is met, extra intake has diminishing returns, and outcomes depend heavily on baseline status.

Strongest evidence: correcting deficiency and supporting high-risk groups

Research is most consistent for:

• Folate before and during early pregnancy to reduce neural tube defects
• Vitamin D for bone-related outcomes in people with low baseline levels, particularly when combined with adequate calcium and resistance training
• B12 supplementation in deficiency, including in older adults and those with absorption issues
• Vitamin A in populations with deficiency risk (more relevant in low-resource settings)

Mixed evidence: general “immune boosting” and chronic disease prevention

Large trials of antioxidants and multivitamins have produced mixed results. Reasons include:

• participants already had adequate intake
• trials used single vitamins to solve multi-factor problems (diet quality, sleep, exercise, smoking)
• some antioxidants can blunt beneficial training adaptations when taken at high doses around workouts

For immune outcomes, vitamin D supplementation shows the most consistent signal when baseline levels are low and dosing is regular rather than intermittent megadoses.

Nutrients do not work in isolation

Modern nutrition science increasingly emphasizes patterns and interactions:

• Vitamins interact with minerals (vitamin D with calcium and magnesium; folate with B12)
• Absorption depends on fat intake, bile flow, and gut integrity
• Ultra-processed diets can be fortified yet still harmful because of energy density, poor satiety, and metabolic effects

This is why vitamin adequacy should sit inside a broader “metabolic health” approach. If blood sugar is unstable, cravings are high, sleep is poor, and protein intake is low, a supplement alone rarely fixes the problem. (Related: 10 Subtle Signs Your Diet Is Harming Blood Sugar.)

Evidence quality: what to trust

In 2025, the most reliable vitamin guidance typically comes from:

• randomized controlled trials for specific outcomes (fractures, deficiency correction)
• systematic reviews and meta-analyses that stratify by baseline status
• clinical guidelines that consider tolerable upper limits and known risks

Be cautious with:

• single influencer anecdotes
• “one vitamin cures everything” claims
• protocols that skip labs, skip diet assessment, and recommend high doses indefinitely

Who Should Consider Vitamins?

Most people benefit from focusing on vitamin adequacy through food. Supplementation is most useful for people with higher risk of deficiency, higher needs, or limited intake.

People more likely to benefit from targeted supplementation

• Older adults: reduced absorption (especially B12), lower sun exposure, higher fracture risk
• Pregnant people or those trying to conceive: folate needs increase; prenatal formulations are often appropriate
• Vegans and many vegetarians: B12 is the main concern; vitamin D, iron, and omega-3 may also be relevant (not vitamins, but commonly paired)
• People with limited sun exposure: vitamin D risk increases with indoor lifestyle, high latitude, winter seasons, and consistent sunscreen use
• People with GI conditions or malabsorption risk: celiac disease, IBD, bariatric surgery, pancreatic insufficiency, chronic diarrhea
• People on certain medications: metformin (B12), PPIs (B12), orlistat (A, D, E, K)
• Athletes in calorie deficits: risk of micronutrient gaps rises when calories drop, even with “clean” eating

People who should be more cautious

• Pregnancy: avoid high-dose vitamin A (retinol) unless specifically prescribed
• Kidney disease or history of kidney stones: be cautious with high-dose vitamin D and vitamin C
• People on anticoagulants (warfarin): vitamin K consistency matters
• People taking multiple fortified products: risk of accidental excess increases

Common Mistakes, Interactions, and Smarter Alternatives

Mistake 1: Using supplements to compensate for ultra-processed diets

Fortified processed foods can contain vitamins yet still worsen metabolic health due to refined carbs, added sugars, industrial fats, and low fiber. If you notice constant hunger, energy crashes, or cravings, the lever is often dietary pattern and protein quality, not another supplement.

A practical alternative is to rebuild meals around:

• a protein anchor (eggs, yogurt, fish, legumes)
• a high-fiber carbohydrate (beans, whole fruit, root vegetables if tolerated)
• colorful plants
• a fat source that improves satiety and vitamin absorption

(See: RFK Jr’s “Banned Foods” List, What It Means for Your Plate for label-reading and food environment context.)

Mistake 2: Ignoring absorption and timing

• Fat-soluble vitamins absorb better with a meal containing fat.
• Iron and calcium can compete with absorption of other nutrients (not vitamins directly, but relevant in supplement stacks).
• B12 absorption can be impaired by low stomach acid or GI issues.

If you suspect absorption issues, improving gut tolerance and digestion may raise vitamin status more effectively than increasing dose. (Related: 10 Gut-Damaging Foods, Explained by Gut Biology.)

Mistake 3: Megadosing antioxidants around training

High-dose vitamin C and E taken close to workouts may blunt some training adaptations in certain contexts by reducing exercise-induced signaling. This does not mean you should avoid fruits and vegetables. It means be cautious with high-dose antioxidant pills as a default.

Mistake 4: Treating “normal labs” as automatically optimal

Reference ranges are not always “optimal ranges,” and individual context matters. For example, a borderline B12 level with neurologic symptoms may still warrant deeper evaluation (MMA, homocysteine) and clinician guidance.

Interactions worth remembering

• Vitamin D works best when overall mineral balance is adequate (magnesium, calcium) and when paired with resistance training for musculoskeletal outcomes.
• Folate and B12 are linked; supplementing folate alone can hide B12 deficiency.
• Vitamin K interacts with warfarin, but stable intake is often better than avoidance.

Frequently Asked Questions

Are vitamins better from food or supplements?

Food is usually better because it provides vitamins in context with fiber, protein, minerals, and bioactive compounds that affect absorption and metabolism. Supplements are useful when intake is limited, needs are higher, or deficiency risk is present.

Do I need a multivitamin every day?

Not necessarily. A basic multivitamin can be a reasonable “insurance policy” for some people, especially during dieting or low variety eating, but it is not a substitute for a nutrient-dense diet. If your diet is strong, you may only need targeted supplements (often vitamin D or B12 depending on lifestyle).

Can I take too many vitamins?

Yes. Toxicity risk is highest with fat-soluble vitamins (A, D, E, K) and with certain B vitamins at high doses (notably B6 and niacin). Also watch for accidental duplication across multiple products.

What are the most common vitamin deficiencies today?

In many countries, common issues include low vitamin D status, low B12 in vegans and older adults, and folate inadequacy in low vegetable diets (though fortification reduces severe deficiency). Individual risk varies by diet, sun exposure, gut health, and medications.

Should I take vitamins if I feel tired?

Sometimes, but fatigue has many causes: sleep debt, low calories, iron deficiency, thyroid issues, depression, overtraining, infections, and blood sugar instability. If fatigue is persistent, consider labs and a clinical evaluation rather than guessing.

Is “high-dose vitamin D” a good idea for body composition?

The mechanistic rationale is interesting, but human evidence is not definitive, and safety depends on baseline levels and monitoring. If you want to explore higher targets, test 25(OH)D first and work with a clinician to avoid hypercalcemia and other risks.

Key Takeaways

• Vitamins are essential micronutrients that enable energy metabolism, immune function, bone health, blood formation, and tissue repair.
• Aim for adequacy, not extremes. More is not always better, especially for fat-soluble vitamins.
• Food-first patterns with protein, colorful plants, and healthy fats cover most needs and improve absorption.
• Common modern gaps include vitamin D (low sunlight) and B12 (vegan diets, older adults, metformin or acid-suppressing meds).
• The strongest evidence for supplementation is in deficiency correction and high-risk groups (pregnancy folate, low vitamin D, B12 deficiency).
• Biggest risks come from megadosing, product stacking, and medication interactions (notably warfarin with vitamin K).
• If symptoms persist or you are considering high-dose protocols, use testing and monitoring rather than trial-and-error.