The honest answer: it depends on your diet, your age, where you live, and what your blood work says. There is no single supplement stack that makes sense for every person.
What the evidence does support is a short list of nutrients that many people are deficient in regardless of how carefully they eat - either because food sources are limited, absorption is inconsistent, or daily requirements are hard to hit through diet alone. Here's how to think about it properly.
The Vitamins Most People Actually Need
Vitamin D: The Near-Universal Deficiency
Vitamin D is produced in the skin through sun exposure. It's almost impossible to get adequate amounts from food alone. And modern life - offices, sunscreen, northern latitudes, dark winter months - means most adults simply don't get enough sun for their skin to produce what they need.
Estimates vary by population, but the NHS estimates that around 1 in 5 adults in the UK have low vitamin D levels. US data from the NHANES survey suggests 35-42% of adults are deficient. In South Asia, the Middle East, and northern Europe, rates are higher.
Vitamin D deficiency symptoms include persistent fatigue, low mood, bone pain, frequent illness, and muscle weakness - most of which are easy to dismiss or attribute to something else.
The standard supplemental recommendation for adults is 1,000-2,000 IU (25-50 mcg) daily, taken with a fat-containing meal for absorption. The form matters: vitamin D3 is more effective than D2 at raising blood levels. If you live above 40 degrees latitude or spend most of your time indoors, supplement year-round.
Magnesium: Widely Deficient, Often Overlooked
Magnesium is involved in over 300 enzyme reactions in the body. It affects sleep quality, muscle function, blood pressure, blood sugar regulation, and bone health.
Around 45-50% of Americans don't meet the daily recommended intake through diet alone, according to NHANES data. The UK National Diet and Nutrition Survey found similar gaps in adults under 35 and over 65.
Food sources are real (leafy greens, legumes, nuts, seeds, whole grains) but dietary patterns consistently leave many people short. The signs of low magnesium - muscle cramps, poor sleep, anxiety, constipation, headaches - are common complaints that rarely get traced back to magnesium status.
Supplemental forms differ significantly in bioavailability. Magnesium glycinate and citrate absorb well; magnesium oxide (the cheapest, most common form) absorbs poorly. 200-400 mg daily of a well-absorbed form is a reasonable starting point for most adults.
Omega-3 Fatty Acids: If You Don't Eat Oily Fish
Omega-3s (specifically EPA and DHA) are essential for brain function, heart health, and managing inflammation. Your body can't make them in meaningful quantities - you have to get them from food or supplements.
If you eat two or more portions of oily fish (salmon, sardines, mackerel, herring) per week, you're probably fine. If you don't, a fish oil or algae-based omega-3 supplement (at least 500 mg combined EPA/DHA daily) is worth considering.
The algae-based version is nutritionally equivalent to fish oil and is the only option for vegans - since fish get their omega-3s from algae anyway.
Vitamin B12: Essential for Vegans and Vegetarians
B12 is found almost exclusively in animal products. The body's ability to absorb B12 from food also declines with age, making older adults another at-risk group even on omnivorous diets.
Vegans who don't supplement B12 will develop a deficiency - it's a matter of when, not if. The timeline varies depending on how much B12 was stored before the dietary change, but without supplementation, neurological symptoms typically appear within 2-5 years.
For vegans, a B12 supplement is non-negotiable. 250-1,000 mcg of cyanocobalamin or methylcobalamin daily, or a higher weekly dose, both work. Vegetarians who eat eggs and dairy regularly have lower risk but should still monitor levels.
Vitamins Worth Taking in Specific Situations
Folate: For Anyone Who Could Become Pregnant
The evidence on folate supplementation during early pregnancy is definitive. It reduces neural tube defects by up to 70%. Since neural tube formation happens in the first 4 weeks of pregnancy - often before someone knows they're pregnant - anyone who could become pregnant is advised to supplement with 400 mcg daily.
The folate vs folic acid distinction matters here: people with the MTHFR gene variant (around 40% of the population) metabolise folic acid less efficiently and may benefit from the active form, methylfolate.
Vitamin C: Probably Not Necessary If You Eat Fruit and Vegetables
Vitamin C is the supplement people instinctively reach for. It's also one of the easiest nutrients to get from food. A single orange provides 70 mg; a red pepper contains over 150 mg. The daily requirement for adults is 65-90 mg.
Most people who eat any amount of fruit and vegetables are meeting their vitamin C needs. Supplemental vitamin C doesn't prevent colds in most people (it may shorten duration by about half a day in consistent users, according to a large Cochrane review). For heavy smokers - who have higher vitamin C requirements - supplementation is more justified.
Iron: Only If You're Actually Deficient
Iron supplementation is only warranted if a blood test shows deficiency or you're at high risk (heavy periods, pregnancy, absorption disorders, plant-based diet with no fortified foods). Taking iron when you're not deficient causes gut discomfort and can interfere with zinc absorption.
If you suspect iron deficiency, get a blood test first. Symptoms overlap with B12 deficiency, vitamin D deficiency, and low magnesium - supplementing iron without testing doesn't make sense.
What You Almost Certainly Don't Need
Vitamin A: Severe deficiency is rare in countries with diverse food supplies. Excess vitamin A is toxic - it accumulates in the liver and at high doses increases fracture risk and may raise lung cancer risk in smokers. Get it from food (sweet potato, leafy greens, liver) rather than supplements unless prescribed.
Calcium supplements: Research has largely moved away from recommending calcium supplements for general bone health. A 2019 analysis in the British Medical Journal found no consistent reduction in fracture risk from calcium supplementation in healthy adults. High-dose calcium supplements are associated with increased cardiovascular risk in some studies. Get calcium from dairy, fortified plant milks, leafy greens, and sardines.
B complex vitamins: Most B vitamins are abundant in a varied diet. Supplemental B vitamins are expensive, largely excreted in urine, and unnecessary for most people. The exception is B12 (addressed above) and folate in pregnancy.
Multivitamins: These provide a little of many things, which suits nobody's specific deficiency perfectly. If you're deficient in vitamin D, you need a vitamin D supplement - not a multivitamin that contains 400 IU alongside 20 other nutrients you don't need. Targeted supplementation based on actual deficiency is almost always more cost-effective.
How to Actually Know What You Need
The most useful thing you can do is get a blood test. A standard panel should include 25-OH vitamin D, B12, folate, full blood count (for iron status), ferritin, and if relevant, magnesium.
Most GPs will run these if you ask - particularly if you mention symptoms. Private testing services offer home blood tests that cover the main micronutrients.
Once you know your levels, you can supplement what you're actually deficient in, at the dose you actually need, rather than guessing.
Frequently Asked Questions
Q: Is it safe to take vitamins every day? A: For water-soluble vitamins (vitamin C, B vitamins), excess is generally excreted in urine, making daily supplementation at standard doses safe. Fat-soluble vitamins (A, D, E, K) accumulate in the body, so it's possible to over-supplement. Vitamin D toxicity from supplements is rare but does occur at very high doses (typically above 10,000 IU daily over extended periods). Stick to recommended doses unless a healthcare provider advises otherwise.
Q: Should I take vitamins in the morning or at night? A: Fat-soluble vitamins (A, D, E, K) absorb better when taken with a fat-containing meal - breakfast or dinner both work. Magnesium taken at night may support sleep quality. B12 is fine any time. There's no strong evidence that precise timing makes a dramatic difference for most people.
Q: Do I need to take vitamins if I eat a healthy diet? A: A varied, whole-food diet covers most micronutrient needs. The exceptions are vitamin D (hard to get from food in adequate amounts), omega-3s (if you don't eat oily fish), and B12 (for vegans). Even people with genuinely good diets are often low on these three specifically.
Q: Can I get all my vitamins from food alone? A: For most nutrients, yes. For vitamin D in particular, no - at least not in northern latitudes with limited sun exposure. Food sources of vitamin D (oily fish, egg yolks, fortified foods) provide small amounts relative to what the body needs, and supplementation fills the gap for most people.
The Bottom Line
Three supplements have broad enough deficiency rates and strong enough evidence to be worth considering without individualised testing: vitamin D, magnesium, and omega-3s (if you don't eat oily fish). Vegans need B12. Anyone who could become pregnant needs folate.
Beyond that, supplement what a blood test tells you you're actually low in - not what an advert tells you you might be missing. Targeted supplementation at the right dose beats a stack of nutrients you don't need.
Sources & References
- NHANES data on vitamin D deficiency, CDC/USDA
- NHS guidance on vitamin D supplementation (2023)
- Cochrane review: vitamin C and the common cold (2013)
- British Medical Journal: calcium supplements and fracture risk (2019)
- ODS (NIH) factsheets: Vitamin D, Magnesium, Omega-3, B12