Most people don’t get enough essential nutrients from food alone, and that gap has real consequences for energy, immunity, and long-term health. A national survey of over 16,000 Americans found that 94.3% fall short of the daily requirement for vitamin D, 88.5% for vitamin E, and 52.2% for magnesium. These aren’t rare deficiencies affecting a small slice of the population. They’re the norm. Vitamins and supplements can close those gaps, but the reasons to take them depend on your body, your diet, and your stage of life.
Nutrient Gaps Are More Common Than You Think
The numbers above come from NHANES data collected between 2007 and 2010, one of the largest ongoing nutrition surveys in the United States. What makes them striking is that these shortfalls exist in a country with year-round access to a wide variety of foods. The issue isn’t starvation. It’s that modern diets, even ones that seem reasonably healthy, often miss the mark on specific micronutrients.
An inadequacy isn’t the same as a full-blown deficiency. You might not develop scurvy or rickets, but your intake is still below what your body needs to function optimally. Over months and years, running low on key vitamins affects everything from how well your immune system responds to infections to how efficiently your cells produce energy. A daily multivitamin or targeted supplement can bridge that distance between what you eat and what your body actually requires.
Your Immune System Depends on Certain Vitamins
Vitamin D plays a central role in immune function. Your immune cells carry receptors specifically designed to respond to it, and without adequate vitamin D, a critical type of white blood cell called the T cell can’t fully activate, multiply, or survive. T cells are the ones that identify and destroy infected cells, so when vitamin D is low, your body’s ability to mount a targeted defense slows down. Vitamin D also helps your innate immune system, the fast-acting first line of defense, clear bacteria more effectively.
Given that more than 9 out of 10 Americans don’t meet the daily requirement for vitamin D through food, supplementation is one of the most straightforward ways to support immune health. This is especially relevant during winter months, when sun exposure (your body’s primary way of producing vitamin D) drops significantly.
B Vitamins and Energy Production
If you’ve been feeling persistently tired despite getting enough sleep, a B vitamin shortfall could be part of the picture. Vitamin B12 is a required partner for an enzyme that feeds directly into your cells’ main energy-production cycle, the process inside mitochondria that converts food into usable fuel. Specifically, B12 helps convert a molecule into a form called succinyl-CoA, which enters the energy cycle and is also needed to build hemoglobin, the protein in red blood cells that carries oxygen throughout your body. Without enough B12, both energy production and oxygen delivery suffer.
Other B vitamins, including B6, thiamin, riboflavin, and niacin, serve as cofactors at different points in the same energy pathway. Think of them as keys that unlock sequential doors. If any one is missing, the whole process slows down. A B-complex supplement covers several of these at once, which is why it’s a common recommendation for people experiencing unexplained fatigue.
Aging Changes How Well You Absorb Nutrients
Your body doesn’t absorb vitamins at 65 the way it did at 25. Vitamin B12 is a clear example of why this matters. To absorb B12 from food, your stomach first needs to use hydrochloric acid to break the vitamin free from the protein it’s attached to. Then a second stomach protein, called intrinsic factor, binds to the freed B12 so your intestines can actually take it in. As you age, your stomach produces less acid, and this two-step process becomes less efficient. The result is that even people who eat plenty of meat and dairy can develop a B12 shortfall in their later years.
Supplemental B12 bypasses part of this problem because it’s already in a free form that doesn’t need stomach acid to be released. For people with significant absorption issues, options like sublingual tablets (dissolved under the tongue) or B12 injections skip the digestive system entirely. This is one of the clearest cases where a supplement does something food alone can’t reliably accomplish past a certain age.
Pregnancy and Fetal Development
Folic acid is one supplement with an unusually strong evidence base for a specific purpose: preventing neural tube defects, which are serious birth defects of the brain and spine that develop in the first few weeks of pregnancy, often before a woman even knows she’s pregnant. The CDC recommends that all women capable of becoming pregnant get 400 micrograms of folic acid every day. Women who have had a previous pregnancy affected by a neural tube defect are advised to take 4,000 micrograms daily.
The timing matters more than people realize. Because the neural tube forms so early in development, waiting until a pregnancy is confirmed means the critical window may have already passed. This is why the recommendation applies to all women of childbearing age, not just those actively trying to conceive.
Plant-Based Diets Create Specific Gaps
Vitamin B12 occurs naturally only in animal foods: meat, fish, eggs, and dairy. If you follow a vegan diet, you have essentially zero natural food sources of B12. Vegetarians who eat some dairy and eggs get partial coverage, but their risk of deficiency is still elevated. The adult recommended intake is 2.4 micrograms per day, and vegans can meet this through fortified foods like nutritional yeast or through supplements.
One important detail about B12 supplements: absorption rates drop sharply as the dose increases. At very low doses (1 to 2 micrograms), your body absorbs about 50%. At 500 micrograms, absorption falls to roughly 2%, and at 1,000 micrograms it’s about 1.3%. This doesn’t mean high-dose supplements are useless. Even 1.3% of 1,000 micrograms is 13 micrograms, well above the daily requirement. But it does explain why supplement doses seem so much higher than the RDA. The forms of B12 used in supplements, whether methylcobalamin or cyanocobalamin, appear to be absorbed at similar rates.
Brain Health and Omega-3 Fatty Acids
DHA, one of the omega-3 fatty acids found in fish oil supplements, is a major structural component of brain cell membranes. It influences how flexible and permeable those membranes are, which directly affects how well brain cells communicate with each other. At a molecular level, DHA loosens the packing of fat molecules in the membrane, changes how water interacts with the membrane surface, and alters the organization of specialized signaling zones within the membrane. These physical changes influence the localization and activity of proteins involved in signaling cascades, essentially fine-tuning how efficiently your neurons transmit information.
People who eat fatty fish two or more times per week typically get enough DHA from food. For everyone else, a fish oil or algae-based omega-3 supplement is the most reliable alternative. Algae-derived DHA is the go-to option for people on plant-based diets, since it provides the same fatty acid without the fish.
More Is Not Always Better
Vitamins have upper limits, and exceeding them can cause real harm. Fat-soluble vitamins (A, D, E, and K) are stored in your body rather than excreted in urine, which makes them easier to accumulate to toxic levels. The tolerable upper intake for zinc in adults, for example, is 40 milligrams per day. Going above that can cause nausea, vomiting, and, over time, copper deficiency, because excess zinc interferes with copper absorption.
Vitamin A toxicity can cause liver damage, headaches, and bone thinning. Excessive vitamin D can lead to dangerously high calcium levels in the blood. These problems almost never happen from food alone. They happen from supplements, particularly when people take multiple products with overlapping ingredients or assume that doubling the dose will double the benefit. Check labels for overlap if you take more than one supplement, and stick to doses at or below 100% of the daily value unless you have a confirmed deficiency and a reason to go higher.
Who Benefits Most From Supplementation
- Older adults: Declining stomach acid reduces absorption of B12, and less sun exposure plus changes in skin efficiency lower vitamin D production.
- Vegans and vegetarians: B12, iron, zinc, and omega-3 fatty acids are harder to get without animal products.
- Women of childbearing age: Folic acid at 400 micrograms daily protects against neural tube defects, ideally started before conception.
- People with limited sun exposure: If you live at a northern latitude, work indoors, or have darker skin (which produces less vitamin D from sunlight), supplemental vitamin D is worth considering.
- People on restricted diets: Calorie restriction, food allergies, or medical conditions that limit food variety increase the chance of multiple micronutrient gaps.
Supplements work best when they’re filling a specific gap rather than serving as nutritional insurance “just in case.” If you eat a varied diet rich in vegetables, fruits, whole grains, and protein sources, you may only need one or two targeted supplements rather than a full medicine cabinet. The goal isn’t to replace good eating habits. It’s to cover the spots where even a good diet falls short.