Vitamins and Supplements: Who May Need Them and Who Should Be Careful

This article reviews what vitamins and minerals do in the body, which groups of people may need a supplement, and who should be careful with supplements. It is general information, not personal medical advice.

Vitamins and minerals are essential nutrients the body needs in small amounts for various physiological functions. While a balanced diet can typically provide these nutrients, individuals with specific conditions or dietary limitations may benefit from supplementation. This review explores what vitamins and minerals do, highlighting groups who may need additional support and the risks of taking too much. For healthy adults, the evidence is insufficient to say whether multivitamins or most single vitamins prevent heart disease or cancer, and some supplements can cause harm (see "Who Should Be Careful with Supplements" below).¹

Essential Role of Vitamins and Minerals:

Micronutrients, including vitamins and minerals, play crucial roles in various bodily processes, including:

Energy production

B vitamins (e.g., B1, B2, B3) act as coenzymes in many of the reactions that convert food into usable energy.² Here's how B vitamins contribute to energy production:

• Carbohydrate Metabolism: 
  • Vitamin B1 (thiamine) acts as a coenzyme in the breakdown of carbohydrates into glucose, which serves as the primary energy source for cells.
  • Vitamin B2 (riboflavin) and vitamin B3 (niacin) are involved in the Krebs cycle, a central metabolic pathway that generates energy-rich molecules called ATP.
• Amino Acid Metabolism:
  • Vitamin B6 (pyridoxine) participates in the metabolism of amino acids, which can be converted into energy through processes like gluconeogenesis.
• Fatty Acid Metabolism:
  • Vitamin B12 (cobalamin) is involved in the metabolism of certain fatty acids and amino acids.
• Cellular Respiration:
  • Vitamin B5 (pantothenic acid) serves as a coenzyme in the citric acid cycle, a key stage of cellular respiration where energy is released in the form of ATP.
• Red Blood Cell Production:
  • Vitamin B12 and folate (vitamin B9) are essential for the production of red blood cells, which carry oxygen to tissues, facilitating energy metabolism.
• Central Nervous System Function:
  • B vitamins are involved in the synthesis of neurotransmitters, which are chemical messengers that facilitate communication between nerve cells. Adequate levels of B vitamins are needed for normal nervous system function.²

Overall, B vitamins act as coenzymes and cofactors in numerous enzymatic reactions, enabling the efficient release and utilization of energy from macronutrients. They work together to ensure the body has the energy it needs to carry out its various functions.

Immune function

Vitamin C, vitamin D, and zinc contribute to a healthy immune system. A Cochrane review found that taking vitamin C every day modestly shortened colds (by about 8% in adults) but did not reduce how many colds people in the general community caught, and taking it only after symptoms began had no consistent effect.³ A meta-analysis found that oral zinc may shorten cold symptoms in adults, but results varied widely between trials, no significant effect was seen in children, and side effects such as bad taste and nausea were more common with zinc.⁴ A meta-analysis of individual participant data from 25 trials, published in the BMJ, found that vitamin D supplementation reduced the risk of acute respiratory tract infection overall (adjusted odds ratio 0.88, 95% CI 0.81 to 0.96). The protective effect was seen with daily or weekly doses rather than large single doses, and was greatest in people who were very deficient.⁵

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Bone health

Vitamin D and calcium have a key role in skeletal health.⁶ Food is the best source of calcium. Dairy products, such as milk, yogurt and cheese are high in calcium. Certain green vegetables and other foods contain calcium in smaller amounts. Some juices, breakfast foods, soy milk, almond milk, cereals, snacks, breads and bottled water have added calcium. The US Recommended Dietary Allowance (RDA) for calcium ranges from 700 to 1,300 mg a day depending on life stage, and for vitamin D it is 600 IU a day for ages 1 to 70 and 800 IU a day from age 71.⁶ Higher vitamin D values were not consistently associated with greater benefit, and for some outcomes there were risks at both low and high levels.⁶ In a meta-analysis of long-term trials, vitamin D supplementation increased the risk of high blood calcium (hypercalcemia; relative risk 1.54, 95% CI 1.09 to 2.18) and high urine calcium.⁷ Check with your healthcare provider before taking more than the recommended amounts. Your body makes Vitamin D from sunlight, but it may be difficult for some people to get enough from sunlight and food alone. 

Cell health and development

Vitamin A and folate are needed for normal cell division and development. Vitamin A, which includes retinol, is a fat-soluble vitamin with a key role in multiple physiological functions,⁸ including vision, cell growth, immune function, reproduction and bone metabolism. At high intakes, vitamin A can also cause harm (see below).

• Vision: Vitamin A is vital for maintaining healthy eyesight. It is a precursor to rhodopsin, a pigment found in the retina of the eye, which allows us to see in low-light conditions. Without sufficient vitamin A, rhodopsin cannot be produced, leading to a condition called night blindness, characterized by difficulty seeing in dim light.
• Cell Growth: Vitamin A is necessary for the growth and differentiation of cells throughout the body. It regulates gene expression, cell signaling, and cell division. Sufficient vitamin A intake is associated with healthy cell growth, tissue repair, and wound healing.
• Immune Function: Vitamin A plays a crucial role in supporting the immune system. It enhances the production and activity of immune cells, such as lymphocytes and antibodies, which help protect the body against infections and diseases.
• Reproduction: Vitamin A is essential for both male and female reproductive health. In men, it is involved in sperm production and motility. In women, it contributes to the development of the reproductive system and the regulation of the menstrual cycle. However, high doses of preformed vitamin A (retinol) from supplements can cause birth defects. In a study of 22,748 pregnancies, babies of women who took more than 10,000 IU a day of supplemental vitamin A had defects of tissues derived from the cranial neural crest about 4.8 times as often (95% CI 2.2 to 10.5) as babies of women who took 5,000 IU a day or less, with an apparent threshold near 10,000 IU a day.⁹ Women who are pregnant or planning a pregnancy should ask their healthcare provider before taking any supplement that contains vitamin A.
• Bone Health: Vitamin A is involved in bone metabolism, but more is not better. In a meta-analysis of prospective studies, high intakes of vitamin A and retinol were linked to a higher risk of hip fracture (adjusted relative risk 1.29 and 1.40), and both high and low blood retinol levels were linked to a higher risk.¹⁰ A second meta-analysis found a similar association for high dietary intake of total vitamin A (relative risk 1.29) and retinol (1.23), while high intake of some carotenoids was linked to a lower risk.¹¹

Metabolism

Iron is necessary for the transport of oxygen throughout the body and for the production of red blood cells.

• Oxygen Transport: Iron is an essential component of hemoglobin, a protein found in red blood cells that binds to oxygen molecules and carries them throughout the body. Without sufficient iron, the body cannot produce enough hemoglobin, leading to a condition called iron-deficiency anemia. In iron-deficiency anemia, the red blood cells are smaller and contain less hemoglobin, resulting in reduced oxygen-carrying capacity.
• Metabolic Functions: Iron is also involved in various metabolic processes in the body, including energy production, DNA synthesis, and immune function. It plays a role in the electron transport chain, which generates energy in cells. Iron is also required for the synthesis of certain hormones and neurotransmitters.
• Sources of Iron: Dietary sources of iron include red meat, poultry, fish, beans, lentils, spinach, and fortified cereals. The absorption of iron from plant-based sources (non-heme iron) is lower compared to animal-based sources (heme iron). Vitamin C enhances the absorption of non-heme iron, so it is recommended to consume iron-rich plant foods with vitamin C-rich foods like citrus fruits, tomatoes, or bell peppers.

Overall, iron is a vital mineral that plays a crucial role in the body's ability to transport oxygen, produce red blood cells, and support various metabolic functions. Maintaining healthy iron levels through a balanced diet and appropriate medical care is essential for overall health and well-being. Iron supplements should be taken only when a healthcare provider advises them, because too much iron can damage organs.¹²

Individuals Who May Benefit from Supplementation

While a balanced diet is generally preferred, specific groups may benefit from additional nutrient intake through supplements:

• Pregnant and lactating women: Taking folic acid before and in early pregnancy reduces the risk of neural tube defects (relative risk 0.31, 95% CI 0.17 to 0.58 in a Cochrane review), but it has no clear effect on other birth defects.¹³ The American Academy of Pediatrics (2008) recommends that all infants, including those who are exclusively breastfed, have a minimum daily vitamin D intake of 400 IU beginning soon after birth.¹⁴ Pregnant and breastfeeding women should ask their healthcare provider which supplements they need, and should not take vitamin A supplements unless advised.
• Individuals with restrictive diets: Vegans and vegetarians, especially those who don't consume fortified foods, may require vitamin B12 supplements due to its limited availability in plant-based sources. In a review of 40 studies, vitamin B12 deficiency was relatively common among vegetarians, and vegans who did not take B12 supplements were at especially high risk.¹⁵
• People with specific medical conditions: Individuals with malabsorption syndromes, celiac disease, inflammatory bowel disease, or chronic kidney disease (including those on dialysis) may need individualized advice from their care team about which vitamins to take. In chronic kidney disease, for example, a review found that routine vitamin A and E supplements are not supported because of potential toxicity, and that decisions about supplementing water-soluble vitamins should be individualized.¹⁶
• Adults over 50: Vitamin B12 deficiency becomes more common with age. In large US and UK surveys, about 6% of people aged 60 and older were deficient and closer to 20% had marginal status, and poor absorption of food-bound vitamin B12 becomes the main cause in older people.¹⁷ Atrophic gastritis and higher-dose proton pump inhibitor use were associated with lower B12 status in older adults.¹⁸ The US RDA for vitamin D rises to 800 IU a day from age 71.⁶ Whether to supplement is best decided with a healthcare provider, who can check blood levels.

Who Should Be Careful with Supplements

Supplements are not right for everyone, and taking more is not always better. Needs depend on diet, age, health conditions, medicines and pregnancy status, so a blanket recommendation that everyone should take a particular supplement is not supported. For community-dwelling adults who are not pregnant, the US Preventive Services Task Force found the evidence insufficient to determine whether multivitamins, or most single or paired nutrient supplements, prevent cardiovascular disease or cancer. It recommends against beta-carotene supplements (the harms outweigh the benefits) and against vitamin E supplements (no net benefit) for that purpose.¹

Vitamins A, D, E and K are fat-soluble, and high supplemental intakes have been linked to specific harms:

• Vitamin A: high-dose preformed vitamin A can cause birth defects⁹ and high intakes have been linked to a higher risk of hip fracture.^10,11 Pregnant women and women planning a pregnancy should ask their healthcare provider before taking any supplement that contains vitamin A.
• Beta-carotene: in a trial of 29,133 male smokers in Finland, those given beta-carotene (20 mg a day) had a higher incidence of lung cancer than those who were not (18% higher; 95% CI 3 to 36%).¹⁹ In a trial of 18,314 smokers, former smokers and workers exposed to asbestos, a combination of beta-carotene (30 mg a day) and vitamin A (25,000 IU a day) was linked to a higher incidence of lung cancer (relative risk 1.28; 95% CI 1.04 to 1.57).²⁰ People who smoke or used to smoke should ask their healthcare provider before taking supplements that contain beta-carotene or vitamin A.
• Vitamin D: long-term vitamin D supplementation increased the risk of high blood calcium and high urine calcium in a meta-analysis of randomized trials.⁷
• Vitamin E: a meta-analysis of 19 trials found that high-dosage vitamin E (400 IU a day or more) may increase all-cause mortality, although those trials were often small and done in people with chronic disease.²¹ In SELECT, a trial of 35,533 men, vitamin E (400 IU a day) increased the risk of prostate cancer (hazard ratio 1.17; 99% CI 1.004 to 1.36).²²
• Vitamin K: vitamin K can interfere with vitamin K antagonist blood thinners such as warfarin. In healthy volunteers taking the anticoagulant acenocoumarol, vitamin K2 (MK-7) at doses as low as 10 mcg a day lowered the INR, a blood-clotting test, in some people.²³ People who take a blood thinner should ask their doctor or pharmacist before taking any supplement that contains vitamin K.

Other cautions:

• Iron: systemic iron overload causes organ toxicity.¹² People with hereditary hemochromatosis are advised to avoid medicinal iron and mineral supplements that contain iron.²⁴
• Vitamin B6: a systematic review concluded that current evidence supports a neurotoxic role of vitamin B6 at high levels, usually from supplements, which may lead to a sensory neuropathy.²⁵
• Folic acid and vitamin B12: a 2024 review found that people with low B12 and high folate had lower cognitive test scores than those with low B12 and non-elevated folate. The evidence is mostly correlative, but the authors considered it sufficient to warrant checking B12 status in older adults and others with low B12 intake who take high-dose folic acid.²⁶
• Medicines: some supplements can interact with prescription medicines (vitamin K and warfarin, above), and higher-dose proton pump inhibitors (acid-reducing medicines) were associated with lower vitamin B12 status in older adults.¹⁸ Do not stop or change a prescribed medicine because of a supplement. Tell your doctor or pharmacist about everything you take.
• Kidney disease, liver disease, pregnancy, breastfeeding and children: check with a healthcare provider before starting any supplement. In chronic kidney disease, for example, routine vitamin A and E supplements are not supported because of potential toxicity.¹⁶

Many multivitamins, fortified foods and single-nutrient products contain some of the same nutrients, so the total from all sources adds up. Check labels and keep supplements out of the reach of children.

Conclusion:

Vitamins and minerals are essential for health. While a balanced diet is the primary source, specific individuals with dietary limitations, medical conditions, or increased nutrient demands may benefit from evidence-based supplementation under the guidance of a healthcare professional. Individualized assessments and personalized recommendations are crucial to ensure safe and effective use of supplements. Always consult your healthcare provider when taking vitamins and supplements. 

References:

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This article is for general information and is not medical advice. Talk to your doctor or pharmacist before starting a supplement, especially if you are pregnant, take prescription medicines, or have a medical condition.