Dietary supplements are used to fill nutritional gaps, support specific health goals, and address the needs of certain life stages or lifestyles. They include vitamins, minerals, herbs, amino acids, probiotics, and other concentrated ingredients taken in pill, powder, or liquid form. Nearly 58% of U.S. adults report using at least one supplement in any given month, a figure that has climbed steadily from about 48% a decade earlier.
Correcting Nutrient Deficiencies
The most straightforward use for supplements is replacing nutrients your diet doesn’t provide in sufficient amounts. In the U.S., the most common deficiencies affect vitamin B6, vitamin D, and iron, each showing up in roughly 10% of the general population. Other nutrients like folate, vitamin A, and vitamin E fall below adequate levels in less than 1% of people, largely because of food fortification programs.
These averages hide significant variation by age, sex, and ethnicity. Vitamin D deficiency, for example, affects about 31% of Black Americans and 12% of Mexican Americans compared to 3% of white Americans. Iron deficiency is particularly common among women of childbearing age and young children in certain populations. Young women aged 20 to 39 also have iodine levels that border on insufficient. For people in these groups, targeted supplementation can prevent real health consequences, from fatigue and weakened immunity to complications during pregnancy.
Supporting Pregnancy and Fetal Development
Folic acid is one of the clearest success stories in supplement science. The U.S. Public Health Service recommends that all women capable of becoming pregnant consume 400 micrograms of folic acid daily to reduce the risk of neural tube defects like spina bifida and anencephaly. Before widespread folic acid fortification of grain products began in 1998, roughly 12% of women of childbearing age were deficient in folate. That number has since dropped below 1%.
Prenatal supplements typically contain up to 800 micrograms of folic acid along with iron, calcium, and other nutrients that support both the mother and fetus. Because neural tube defects develop very early in pregnancy, often before a woman knows she’s pregnant, consistent daily intake matters more than starting supplementation after a positive test.
Exercise and Physical Performance
Creatine is the most widely studied performance supplement. Your muscles use a molecule called ATP as their immediate energy source during short, intense efforts like sprinting or lifting weights. Creatine helps regenerate ATP faster by donating a chemical group that converts spent energy molecules back into usable fuel. Your body produces about 2 grams of creatine per day on its own, and you get more from meat and fish, but supplementation raises the amount stored in muscle tissue.
Research using muscle biopsies and imaging has shown that five days of creatine supplementation produces a measurable, linear increase in energy stores within muscle fibers. This translates to better performance in repeated high-intensity efforts and, over time, greater training capacity that supports muscle growth. Creatine is one of the few supplements with a robust evidence base for its claimed benefits, which is why it remains popular decades after its initial research boom.
Heart Health and Chronic Disease
Many people take supplements hoping to prevent heart disease or other chronic conditions, but the evidence here is more nuanced. The VITAL trial, one of the largest randomized studies on the topic, tested both fish oil (omega-3 fatty acids) and vitamin D in over 25,000 adults. Vitamin D supplementation at 2,000 IU per day did not reduce heart attacks, strokes, or cardiovascular deaths compared to placebo.
Omega-3 fatty acids showed a more mixed picture. Overall, they didn’t significantly reduce the combined rate of major cardiovascular events. But the results weren’t uniform: people who ate fewer than one and a half servings of fish per week saw a 19% reduction in major heart events and a 40% reduction in heart attacks with supplementation. Black participants experienced an even larger benefit, with a 77% reduction in heart attack risk. For people who already eat fish regularly, the added benefit of a supplement appears minimal. Neither supplement affected overall mortality.
Stress and Mental Health
Ashwagandha, an herb with roots in traditional Indian medicine, has drawn attention for its effects on stress. A meta-analysis pooling 15 clinical trials with 873 participants found that ashwagandha supplementation for eight weeks significantly reduced both cortisol levels (the body’s primary stress hormone) and self-reported stress scores. Participants also showed improvements on a standardized anxiety scale.
These are real, measurable effects, but they’re modest. Ashwagandha won’t replace therapy or medication for clinical anxiety disorders. It fits better as a tool for people dealing with everyday stress who want an additional option alongside sleep, exercise, and other lifestyle strategies.
How Absorption Affects What You Get
Not all supplements are absorbed equally, and how you take them matters. Vitamins A, D, E, and K are fat-soluble, meaning they dissolve in fat rather than water. Your small intestine absorbs them alongside dietary fat, so taking these on an empty stomach significantly reduces how much actually enters your bloodstream. A meal containing some fat, even a handful of nuts or a splash of olive oil, makes a real difference.
Water-soluble vitamins like the B vitamins and vitamin C dissolve readily and are absorbed more easily, but your body doesn’t store them as efficiently. Excess amounts are excreted in urine rather than banked for later. This is why consistent daily intake matters more for water-soluble nutrients, while fat-soluble vitamins can accumulate in body tissue over time, which also means they carry a higher risk of toxicity at extreme doses.
Drug Interactions and Safety Risks
Supplements aren’t regulated the way prescription drugs are. Under U.S. law, they’re classified as a food category, not medications. Manufacturers don’t need to prove their products work before selling them, and the ingredients on the label aren’t independently verified unless the company voluntarily seeks certification.
Some supplements interact with medications in ways that can be dangerous. St. John’s wort, widely used for mild depression, is one of the most problematic. It activates a liver enzyme responsible for breaking down more than half of all commonly prescribed drugs. When this enzyme ramps up, medications like blood thinners, birth control pills, and immunosuppressants get cleared from the body faster than intended, potentially rendering them ineffective. The degree of interaction is directly related to the amount of the active compound, hyperforin, in a given product, which varies across brands.
What Third-Party Certification Tells You
Because the supplement industry is largely self-regulated, third-party testing programs fill an important gap. NSF International, one of the most recognized certifiers, evaluates supplements on three criteria: whether the ingredients on the label match what’s actually in the bottle, whether the formulation contains harmful contaminants, and whether any undeclared ingredients are present. They conduct annual audits and periodic retesting, not just a one-time check.
For athletes, the NSF Certified for Sport program goes further, screening for 280 substances banned by major athletic organizations, including stimulants, steroids, and masking agents. USP (United States Pharmacopeia) runs a similar verification program. A certification mark from either organization doesn’t mean a supplement works for its marketed purpose, but it does mean you’re getting what’s on the label without hidden contaminants. If you’re choosing between two similar products, a third-party certification seal is one of the most reliable quality signals available.
The Limits of Supplementation
Supplements work best when they address a specific, identified need: a documented deficiency, a life stage with increased nutrient demands, or a performance goal supported by strong research. They work least well as insurance policies against vague health concerns. The VITAL trial’s findings on vitamin D and heart disease illustrate this well. Millions of people take vitamin D hoping to prevent cardiovascular problems, but in a population without severe deficiency, it made no measurable difference.
The supplement market also increasingly features compounds marketed around cellular aging, such as precursors to a molecule called NAD+ that plays a role in energy metabolism and declines with age. While animal studies have been promising, human clinical trials so far have shown limited efficacy. The gap between what works in a mouse and what works in a person remains wide for many of these newer products.