Most people use supplements because their diet alone doesn’t provide enough of certain nutrients. This isn’t a hypothetical concern: national survey data from 2005 to 2016 found that 95% of U.S. adults fall short of the estimated average requirement for vitamin D, 84% for vitamin E, 46% for vitamin C, and 45% for vitamin A. Even with the best intentions, closing those gaps through food alone can be difficult depending on your age, diet, health conditions, and life stage.
Supplements exist to fill those specific gaps, but they also serve other purposes. Some provide compounds with therapeutic benefits beyond basic nutrition. Others compensate for absorption problems your body develops over time. Understanding the different reasons people reach for supplements helps you figure out whether, and how, they might fit into your own routine.
Filling Nutrient Gaps in a Modern Diet
The most straightforward reason to use a supplement is that you’re not getting enough of something from food. Vitamin D is the clearest example. Your body produces it from sunlight, but most people spend their days indoors, wear sunscreen, or live at latitudes where winter sun isn’t strong enough to trigger production. Food sources like fatty fish, egg yolks, and fortified milk contribute relatively small amounts. That’s why nearly all American adults test below adequate levels.
Vitamin E is another common shortfall. It’s found mainly in nuts, seeds, and vegetable oils, and you’d need to eat generous portions daily to reach the recommended intake. Many people simply don’t. Vitamins A and C round out the list of widespread inadequacies, though those are more achievable through a produce-heavy diet.
Crop quality may also play a role. Researchers have documented declines in the mineral density of common crops over recent decades, largely because plant breeding has prioritized yield over nutrient content. Farms using regenerative practices have shown mineral levels comparable to what older crop varieties once contained, but most commercially grown produce comes from conventional systems. The practical takeaway: even a diet rich in fruits and vegetables may deliver fewer micronutrients per serving than the same foods did a generation ago.
Life Stages That Increase Demand
Certain periods of life create nutrient demands that food alone struggles to meet. Pregnancy is the most well-known example. The American College of Obstetricians and Gynecologists recommends that pregnant women take a prenatal vitamin containing at least 400 micrograms of folic acid (to reduce the risk of neural tube defects), 27 milligrams of iron, and 450 milligrams of choline. Choline is particularly hard to get from food. Most prenatal vitamins on the market don’t even meet the recommended amount, so some women need a separate choline supplement on top of their prenatal.
Aging creates its own challenges. After about age 50, the stomach gradually produces less acid and less of a protein called intrinsic factor, both of which are essential for absorbing vitamin B12 from food. B12 deficiency among older adults ranges from 5% to 40% depending on how it’s measured, and a condition called pernicious anemia, where the stomach can no longer produce intrinsic factor at all, accounts for 15% to 25% of those cases. The crystalline form of B12 found in supplements bypasses part of this absorption problem, which is why supplementation is commonly recommended for older adults even if their diet includes meat and dairy.
Benefits Beyond Basic Nutrition
Not every supplement is about preventing a deficiency. Some provide compounds that offer specific functional benefits, even in people whose diet is otherwise adequate.
Creatine is a good example. It’s naturally found in meat and fish, and your body produces some on its own. But supplemental creatine increases the amount of stored energy available for rapid use in both muscles and the brain. A systematic review of controlled trials found that creatine supplementation improved cognitive function in two groups in particular: older adults, who need more energy to complete mental tasks than younger people do, and individuals under physical or psychological stress. Vegetarians and vegans saw some of the largest gains. In one trial, short-term memory improved significantly in vegetarians taking creatine but not in meat-eaters, likely because vegetarians start with lower baseline levels since they get none from their diet.
Omega-3 fatty acids from fish oil are another widely used supplement with benefits that go beyond correcting a deficiency. While the body needs these fats for basic cell function, higher intakes have been studied for their effects on heart health, inflammation, and brain function. People who eat little or no fatty fish often use omega-3 supplements to reach intakes associated with cardiovascular benefits.
How Absorption Affects What You Get
Taking a supplement doesn’t guarantee your body absorbs all of it. Fat-soluble vitamins, specifically A, D, E, and K, need dietary fat present in your gut to be absorbed effectively. These vitamins dissolve into fat molecules, which then get packaged into tiny clusters called micelles that your intestinal lining can take up. Without fat in the meal, much of the vitamin passes through you unused. This is why you’ll often see advice to take these supplements with food that contains some oil, butter, nuts, or avocado.
Vitamin K absorption improves specifically with fattier meals, as more fat means more efficient micelle formation. If you’re taking a multivitamin on an empty stomach first thing in the morning, you may be getting significantly less of these four vitamins than the label suggests.
Nutrient interactions matter in the other direction too. Calcium and iron compete for absorption, so taking them together reduces how much of each you absorb. Vitamin C, on the other hand, enhances iron absorption. These kinds of pairings are worth knowing if you’re taking multiple supplements or trying to address a specific deficiency.
Dietary Restrictions and Special Diets
Restrictive diets reliably create gaps that supplements can close. Vegans and strict vegetarians are at higher risk for deficiencies in B12 (found almost exclusively in animal products), iron (the form in plants is harder to absorb), omega-3 fatty acids (the EPA and DHA forms come from marine sources), and vitamin D. A well-chosen set of supplements can make a plant-based diet nutritionally complete in ways that would be extremely difficult through food alone.
People with food allergies or intolerances face similar challenges. Avoiding dairy eliminates a major source of calcium and vitamin D. Celiac disease damages the intestinal lining and can impair absorption of multiple nutrients even when intake is adequate. Bariatric surgery permanently reduces the stomach’s capacity to break down and absorb nutrients, making lifelong supplementation a medical necessity.
What Supplements Can’t Do
Supplements work best when they’re compensating for something specific: a nutrient your diet lacks, an absorption problem your body has, or a compound with a defined benefit at higher doses. They don’t work as a substitute for an overall poor diet. Whole foods contain fiber, phytochemicals, and combinations of nutrients that interact in ways a pill can’t replicate. A multivitamin won’t offset the effects of eating mostly processed food.
There’s also a real risk of taking too much. Every nutrient has a tolerable upper intake level, the point above which side effects become likely. For preformed vitamin A, that ceiling is 3,000 micrograms per day for adults. Exceeding it chronically can cause liver damage, bone loss, and birth defects during pregnancy. Stacking multiple supplements that contain overlapping ingredients, like taking a multivitamin plus a separate vitamin A supplement, can push you past safe limits without realizing it.
Choosing a Reliable Product
Unlike prescription drugs, supplements don’t require FDA approval before they reach store shelves. Manufacturers are legally responsible for ensuring their products aren’t adulterated or mislabeled, and they must follow good manufacturing practices covering identity, purity, strength, and composition. But the FDA doesn’t routinely test products before sale. It steps in after problems are reported.
This means quality varies. Independent testing has repeatedly found supplements that contain less of the active ingredient than the label claims, or that carry contaminants like heavy metals, pesticides, or microbes. The most practical way to verify quality is to look for third-party certification seals. The USP Verified Mark, for instance, means the product has been lab-tested to confirm it contains the listed ingredients at the declared amounts and doesn’t contain harmful levels of specified contaminants. NSF International runs a similar program. Products without any third-party seal aren’t necessarily bad, but you have less assurance of what’s actually inside.
Labels are required to include a Supplement Facts panel listing every dietary ingredient, its amount per serving, and the number of servings per container. They must also provide a domestic phone number or address for reporting adverse events. Structure and function claims on the label (like “supports immune health”) must be truthful but don’t require FDA pre-approval, so treat marketing language with healthy skepticism and focus on the ingredient list and dosages instead.