Most people don’t need a cabinet full of supplements. What you actually need depends on your diet, your sex, your age, and whether you have a specific gap in your nutrition. Globally, more than 4 billion people fall short on key nutrients, with iron, riboflavin, folate, and vitamin C topping the list. But that doesn’t mean popping a multivitamin is the answer. The smarter approach is figuring out where your personal gaps are and filling those specifically.
The Nutrients Most People Fall Short On
A 2024 analysis published in The Lancet Global Health estimated that 65% of the world’s population doesn’t get enough iron, 55% falls short on riboflavin (vitamin B2), 54% on folate, and 53% on vitamin C. Vitamin D is another widespread gap, particularly for people who live in northern latitudes, work indoors, or have darker skin. B12 deficiency is common in older adults and anyone eating a plant-based diet.
These numbers don’t mean you’re deficient in all of them. They reflect broad dietary patterns across populations. Your actual needs depend heavily on what you eat every day. Someone who regularly eats leafy greens, citrus, nuts, and fatty fish may not need any supplementation at all. Someone skipping vegetables and rarely seeing sunlight probably has a few gaps worth addressing.
What Your Diet Type Tells You
If you eat a varied diet that includes animal products, fruits, vegetables, and whole grains, you’re likely covering most of your bases through food. Whole foods deliver nutrients alongside fiber, fats, and plant compounds that can improve how well your body absorbs them. Research on vitamin C, for example, shows that the version you get from fruits and vegetables often comes with plant compounds that enhance uptake into tissues, something a standalone supplement can’t replicate.
If you eat a vegan or mostly plant-based diet, your risk of specific gaps rises significantly. The German Nutrition Society identifies these as critical nutrients for vegans: vitamin B12, vitamin D, riboflavin, calcium, iron, iodine, zinc, selenium, and long-chain omega-3 fatty acids (the type found in fish, not flaxseed). Vegans tend to get more fiber, vitamin E, vitamin K, and folate than omnivores, but their B12, vitamin D, and iodine intake is often very low. B12 supplementation is essentially non-negotiable on a vegan diet because no plant food reliably provides it in adequate amounts.
How Needs Differ by Sex and Life Stage
Premenopausal women need significantly more iron than men because of monthly blood loss during menstruation. The recommended intake for women of reproductive age is roughly double that of men. After menopause, that gap closes. Interestingly, during pregnancy, the extra iron demand for fetal growth is partially offset by the fact that menstruation stops, though most pregnant women still benefit from an iron supplement.
Women also tend to have higher rates of inadequate iodine, B12, and selenium intake compared to men in the same age group. Men, on the other hand, are more likely to fall short on magnesium, vitamin B6, zinc, and vitamin C. These differences are driven partly by body size and partly by typical eating patterns.
Older adults face their own challenges. The ability to absorb B12 from food declines with age, making supplementation more important after 50. Vitamin D needs also increase: the recommended intake jumps from 600 IU daily for adults under 70 to 800 IU daily for those over 70.
The Supplements Worth Considering
Vitamin D
This is the supplement with the broadest relevance. Your body makes vitamin D from sunlight, but most people don’t get consistent, adequate sun exposure year-round. The recommended daily intake is 600 IU for adults up to age 70 and 800 IU for those older. The safe upper limit is 4,000 IU per day. Many people with confirmed deficiency take 1,000 to 2,000 IU daily, which sits comfortably within the safe range. Vitamin D is fat-soluble, so take it with a meal that contains some fat for better absorption.
Magnesium
Magnesium supports hundreds of processes in your body, from muscle function to sleep regulation, and many people don’t get enough from food alone. If you supplement, the form matters. Organic forms like magnesium citrate and magnesium glycinate are more bioavailable than inorganic forms like magnesium oxide. Glycinate is absorbed through a different pathway (the same one your body uses for amino acids), which may make it gentler on the stomach. All forms can maintain healthy levels in people without a prior deficit, but if you’re already low, the organic forms are a better bet.
Vitamin B12
Essential if you’re vegan, vegetarian, or over 50. Deficiency develops slowly and can cause fatigue, nerve tingling, and cognitive changes before it shows up on routine bloodwork.
Iron
Important for premenopausal women and anyone with diagnosed deficiency. Don’t supplement iron without reason, though. Excess iron accumulates in the body and can cause harm. Get tested first.
Omega-3 Fatty Acids
If you don’t eat fatty fish at least twice a week, a fish oil or algae-based omega-3 supplement can fill the gap. This is especially important for vegans, since the omega-3s in flaxseed and walnuts (ALA) don’t convert efficiently into the forms your body uses most (EPA and DHA).
When to Take Them
Fat-soluble vitamins, including A, D, E, and K, need dietary fat to be absorbed properly. Take them with a meal. If you take a multivitamin, the same rule applies since most contain a mix of fat-soluble and water-soluble vitamins. Taking a multivitamin on an empty stomach with just water means your body can’t properly absorb the fat-soluble components.
Iron is best absorbed on an empty stomach, but it commonly causes nausea, so many people take it with a small amount of food. Vitamin C taken alongside iron enhances its absorption. Magnesium is often taken in the evening, partly because some forms have a mild calming effect, though timing is less critical than consistency.
Why Testing Beats Guessing
A simple blood test can tell you exactly where you stand. The most commonly ordered nutrient test is for vitamin D, which measures a form called 25-hydroxyvitamin D in your blood. Results typically come back categorized as deficient, insufficient, or sufficient. Similar blood tests exist for B12, iron (usually measured alongside ferritin, which reflects your iron stores), and folate. If you’re experiencing unexplained fatigue, hair loss, muscle cramps, or mood changes, these tests can point toward a specific deficiency rather than leaving you to guess with a handful of supplements.
Testing is particularly valuable because some nutrients are harmful in excess. Iron and vitamin D both have well-defined upper limits, and chronic over-supplementation can cause real problems. Knowing your baseline turns supplementation from a guessing game into a targeted fix.
Interactions With Medications
Some supplements interfere with prescription medications in serious ways. St. John’s wort, a popular herbal supplement for mood, reduces the effectiveness of birth control pills, HIV medications, heart disease drugs, antidepressants, and anti-rejection drugs used after organ transplants. Vitamin E, ginkgo biloba, and fish oil all thin the blood. If you’re taking a blood thinner like warfarin, combining it with any of these increases the risk of internal bleeding or stroke.
Calcium can interfere with the absorption of thyroid medication and certain antibiotics. Even something as routine as a multivitamin can reduce how well your body absorbs specific prescriptions if taken at the same time. If you’re on any regular medication, check for interactions before adding a supplement to your routine.
What You Probably Don’t Need
The supplement industry is enormous, and much of what it sells solves problems most people don’t have. Biotin supplements are heavily marketed for hair and nails, but true biotin deficiency is rare. Vitamin C megadoses beyond what you’d get from a couple of servings of fruit offer no proven additional benefit for most people. Greens powders, collagen peptides, and adaptogenic blends may have some supporting evidence for narrow uses, but they aren’t filling nutritional gaps for the average person.
A shorter, targeted list of supplements based on your actual diet, bloodwork, and life stage will do more for your health than a dozen bottles based on marketing. Start with the likely gaps: vitamin D if you don’t get regular sun, B12 if you avoid animal products, magnesium if your diet is low in nuts and leafy greens, and iron only if you’ve confirmed a need through testing.