The supplements worth taking depend entirely on your diet, age, and health status, but a few gaps are so common they apply to most people. National survey data shows that roughly 90% of U.S. adults fall short of recommended intake for vitamins D and E, 61% for magnesium, and about half for calcium and vitamin C. That doesn’t mean everyone needs a cabinet full of bottles. It means a few targeted supplements, ideally guided by a blood test, can fill the gaps that food alone often misses.
What Most Adults Should Consider
Three nutrients stand out as the most widely under-consumed in the general population: vitamin D, magnesium, and omega-3 fatty acids. These are the ones where diet alone frequently falls short, even for people eating well.
Vitamin D is the biggest gap. The standard recommendation for adults under 70 is 600 IU per day, rising to 800 IU after age 70. The 2024 Endocrine Society guidelines also support supplementation during pregnancy (600 to 5,000 IU daily) and for adults 75 and older (400 to 3,333 IU daily) to reduce mortality risk. Your body makes vitamin D from sunlight, but most people who live in northern latitudes, work indoors, or have darker skin don’t produce enough. A simple blood test measuring your 25-hydroxyvitamin D level will tell you exactly where you stand.
Magnesium supports muscle function, sleep, and hundreds of enzyme reactions. Since 61% of adults don’t hit the recommended intake, a supplement is reasonable for most people. The form matters: magnesium glycinate is gentle on the stomach and absorbs well, making it a good general choice. Magnesium citrate absorbs well too but has a mild laxative effect, which can be helpful or not depending on your situation. Magnesium oxide is the cheapest option but absorbs the least efficiently.
Omega-3 fatty acids (EPA and DHA) are the fats found in oily fish like salmon, sardines, and mackerel. If you eat fatty fish twice a week, you’re likely covered. If you don’t, a fish oil supplement providing around 1 gram of combined EPA and DHA daily is a reasonable target. The American Heart Association recommends roughly that amount for people with existing heart disease. During pregnancy, guidelines suggest at least 300 mg per day of combined DHA and EPA, with at least 200 mg coming from DHA, to support fetal brain development and reduce the risk of preterm birth.
Supplements for Adults Over 65
Aging changes how your body absorbs nutrients, which makes certain supplements more important in your later decades. Vitamin B12 is a key example. Your body needs it to maintain healthy red blood cells and nerve function, and the recommended intake stays at 2.4 mcg throughout adulthood. But many older adults gradually lose the ability to extract B12 from food because the stomach produces less acid over time. People taking acid reflux medication are especially affected. A B12 supplement or fortified foods bypasses this absorption problem.
Calcium becomes more critical for bone preservation. Women over 50 need 1,200 mg per day, and men need 1,000 mg until age 70, then 1,200 mg after that (with an upper limit of 2,000 mg). Getting some of this from food, like dairy, fortified plant milks, or leafy greens, is ideal. Supplementing only the gap between what you eat and what you need helps avoid taking more than necessary.
Vitamin D at 800 IU daily is recommended for everyone over 70 to protect bone density and reduce fall risk, with an upper limit of 4,000 IU per day.
Supplements for Plant-Based Diets
If you eat a vegan or mostly plant-based diet, vitamin B12 is non-negotiable. Plant foods contain almost no B12. In one study comparing vegans and omnivores, vegans consumed a median of just 0.29 mcg of B12 per day from food, compared to 5.22 mcg for meat eaters. The daily recommendation is 3.0 mcg. The good news is that most vegans already know this: B12 is by far the most commonly taken supplement among people eating plant-based, and those who supplement consistently maintain B12 levels comparable to omnivores.
Iodine is a less obvious but significant concern. In the same study, only 8% of vegans achieved adequate iodine levels in urine testing, compared to 25% of omnivores (which is also low). Dairy products and iodized salt are the main dietary sources, so if you avoid dairy and use sea salt or kosher salt instead of iodized table salt, you’re likely falling short. An iodine supplement or switching to iodized salt can close this gap.
Iron deficiency affected about 11% of vegan participants in cross-sectional studies, roughly similar to rates in the general population. Plant-based iron is harder for your body to absorb, but vegans often eat more total iron. Pairing iron-rich foods like lentils or spinach with vitamin C improves absorption significantly. Supplementing iron without a confirmed deficiency isn’t recommended because excess iron can cause problems.
Other nutrients to watch on a vegan diet include zinc, selenium, calcium, and omega-3s (specifically DHA and EPA, which come primarily from marine sources). An algae-based omega-3 supplement provides DHA and EPA without fish oil.
Supplements Before and During Pregnancy
Folic acid is the single most time-sensitive supplement for anyone who could become pregnant. It prevents neural tube defects, and the protective window is very early, often before a person knows they’re pregnant. The CDC recommends 400 mcg of folic acid every day for all women of childbearing age. If you’ve had a previous pregnancy affected by a neural tube defect, the recommendation jumps to 4,000 mcg daily, starting one month before conception and continuing through the first trimester.
A standard prenatal vitamin typically covers folic acid along with iron, DHA, and other key nutrients. For omega-3s specifically, women with low DHA intake (under 150 mg per day) are advised to supplement with 600 to 1,000 mg of DHA plus EPA starting by the second trimester.
How to Choose Quality Supplements
Supplements in the U.S. aren’t tested or approved by the FDA before they hit shelves. This means what’s on the label doesn’t always match what’s in the bottle. Third-party testing is the most reliable way to verify quality. Look for the USP Verified Mark, which confirms that the product contains exactly the ingredients listed at the declared amounts, doesn’t contain harmful levels of contaminants, will actually break down and release in your body within a specific time frame, and was manufactured under strict quality controls. NSF International offers a similar certification. Products without either seal aren’t necessarily bad, but you’re taking the manufacturer’s word for it.
Supplements That Can Cause Harm
More is not better, and some supplements interact dangerously with prescription medications. St. John’s wort, a popular herbal supplement for mood, reduces the effectiveness of HIV medications, heart drugs, antidepressants, organ transplant drugs, and birth control pills. Vitamin E and ginkgo biloba both thin the blood, which can become dangerous if you’re already taking a blood thinner like warfarin. Taking these together increases the risk of uncontrolled bleeding.
High-dose single-nutrient supplements carry their own risks. Excess calcium above 2,000 mg per day is linked to kidney stones and cardiovascular concerns. Too much vitamin A can cause liver damage. Iron supplements taken without a deficiency can lead to toxicity over time. The safest approach is to test before you supplement, fill specific gaps rather than taking everything, and let your healthcare provider know what you’re taking so they can flag any interactions with your medications.
Getting Tested First
A blood test is the most direct way to know what you actually need. The most commonly ordered panels check vitamin D (measured as 25-hydroxyvitamin D), vitamin B12, and ferritin (your body’s stored iron). These three tests catch the most prevalent deficiencies and give you a clear baseline. Some providers also test magnesium, zinc, and folate depending on your symptoms and risk factors. Testing removes the guesswork and helps you avoid spending money on supplements your body doesn’t need.