Understanding Vitamins And Minerals: A Beginner’s Guide

Supermarket shelves make vitamins and minerals look complicated – dozens of bottles, each promising something different. The basics, though, are more straightforward than the marketing suggests.

The two categories, and why the difference matters

Harvard’s Nutrition Source draws the distinction simply. Vitamins are organic substances, generally classified as either fat soluble or water soluble. Minerals are inorganic elements present in soil and water, which are absorbed by plants or consumed by animals.

That split has practical consequences. Fat-soluble vitamins – A, D, E and K – dissolve in fat and tend to accumulate in the body. Water-soluble vitamins, including vitamin C and the B-complex group, must dissolve in water before they can be absorbed and therefore cannot be stored, with unused amounts primarily lost through urine. It is a large part of why the nutrients most associated with excess intake are the fat-soluble ones.

Food first, supplements second

Health authorities are consistent on this point: supplements are designed to fill gaps, not replace a varied diet. Whole foods provide nutrients alongside fibre, water, and other compounds that work together – something a tablet or gummy can’t fully replicate.

Harvard puts it in similar terms, stating that a diet including plenty of fruits, vegetables, whole grains, good protein packages and healthful fats should provide most of the nutrients needed for good health, while noting that multivitamins can play an important role when nutritional requirements are not met through diet alone.

When a supplement might make sense

Some situations make it harder to get everything from food alone – restrictive diets, certain medical conditions, pregnancy, or specific nutrient gaps identified by a healthcare provider. In these cases, a targeted supplement – vitamin D and B12, or minerals like calcium and iron are among the most commonly used – may be reasonable, ideally chosen with professional guidance rather than guesswork.

What guidelines around the world say

Published intake figures are less settled than they look, and two countries can give different numbers for the same nutrient without either being wrong.

Vitamin D is the clearest example. The NIH Office of Dietary Supplements fact sheet lists a US recommended dietary allowance of 15 mcg (600 IU) a day for adults aged 19 to 70 and 20 mcg (800 IU) for those over 70. The UK’s NHS lists 10 micrograms a day for adults and children from the age of one, and separately advises that everyone should consider taking a daily supplement containing 10 micrograms during the autumn and winter.

The terminology differs as well. The European Food Safety Authority uses Average Requirement, Population Reference Intake and Adequate Intake, the last of these applied when there isn’t enough data to calculate an average requirement – so a European figure and an American one are not always describing the same quantity.

Understanding the risks

More isn’t automatically better. Taking high doses, combining many supplements, or using a supplement in place of a prescribed medicine can increase the risk of side effects. Some supplements interact with medications in ways that matter – vitamin K can reduce the effectiveness of blood thinners like warfarin, for example, and St John’s wort can speed up how the body breaks down certain medicines. Excess intake of some nutrients causes its own problems; too much vitamin A, for instance, has been linked to headaches and liver damage.

The NIH’s vitamin A fact sheet, updated in March 2025, sets a tolerable upper intake level for preformed vitamin A of 3,000 mcg a day for adults, and lists dry skin, painful muscles and joints, fatigue, depression and abnormal liver test results among the effects associated with chronic excess. Notably, no upper limit has been set for beta-carotene and other provitamin A carotenoids, which the body converts less readily.

Other upper limits vary by nutrient and by country. The NIH gives 45 mg a day for iron and 40 mg for zinc in adults, noting that zinc doses of 50 mg or more can interfere with copper absorption and lower HDL cholesterol. EFSA’s summary of European upper levels sets zinc at 25 mg a day, calcium at 2,500 mg, and vitamin D at 100 mcg – the same vitamin D ceiling the NHS states, describing amounts above 100 micrograms as potentially harmful. Some nutrients have no upper limit at all: the NIH did not establish one for vitamin B12, whose adult RDA is 2.4 mcg, because of its low potential for toxicity.

What the multivitamin trials found

Multivitamins are the most widely used supplements and among the most studied. The NIH’s fact sheet, updated in July 2024, reports that 31.2% of US adults aged 19 and over had taken one in the past 30 days, according to national survey data from 2011 to 2014.

Two large randomised trials dominate the evidence. The Physicians’ Health Study II followed 14,641 male physicians for a median of 11.2 years and found no reduction in cardiovascular events, heart attacks or strokes; it reported a modest 8% difference in total cancer incidence between groups, and 9% for cataracts. The COSMOS trial followed 21,442 older adults for a median of 3.6 years and found no reduction in total invasive cancers, no reduction in cardiovascular events or deaths, and no effect on all-cause mortality. The NIH’s summary of the randomised evidence is that use of multivitamins did not reduce the risk of any chronic disease.

The same fact sheet makes a point that is easy to miss: multivitamin products have no standard or regulatory definition covering which nutrients they must contain or in what amounts. Two bottles labelled the same way can differ substantially.

How supplements are regulated

Regulation is lighter than most people assume. Under the United States Dietary Supplement Health and Education Act of 1994, described on the FDA’s dietary supplements page last updated in October 2024, manufacturers and distributors are responsible for evaluating the safety and labelling of their own products before marketing. There is no pre-market approval step; the FDA acts against adulterated or misbranded products after they reach the market.

That is why US supplement labels carry the wording that the product “is not intended to diagnose, treat, cure, or prevent any disease”. The NIH also flags a risk that grows with the number of products someone takes: people combining supplements with fortified foods and beverages may end up over an upper limit without intending to.

Practical guidance

Tell your healthcare providers about every supplement you take, including herbal products
Keep a written record of what you take and the dose
Remember that “natural” doesn’t automatically mean “safe”
Report any adverse reaction to your doctor
Check whether the same nutrient appears in more than one product you take, or in a fortified food

The bottom line

Vitamins and minerals matter, but the honest starting point for most people is a varied diet, with supplements used deliberately and under guidance – not as a shortcut around it.

This article is general information only and is not medical advice. It does not diagnose, treat, or cure any condition. Speak with a qualified healthcare professional before starting any supplement.