Vitamins & Minerals

Single-nutrient supplements let you target a specific shortfall instead of taking a full multivitamin. The most popular are vitamin D, magnesium, vitamin B12, zinc, iron and vitamin C, and each has a clear job: vitamin D promotes calcium absorption for bone mineralization, magnesium is a cofactor in more than 300 enzyme systems, B12 is required for nervous system function and red blood cell formation, iron carries oxygen in hemoglobin, and zinc supports immune function.

They work best when they correct a real gap. NIH reports that 48% of Americans get less magnesium from food than the estimated average requirement, older adults and people who eat few animal foods are prone to low B12, and menstruating women have the highest iron needs. For people who already have adequate levels, results are more modest; in the VITAL trial, 2,000 IU of vitamin D3 a day did not reduce fractures or major cancers in mostly vitamin D-sufficient adults.

Because single nutrients come in higher doses, the form, the dose relative to the tolerable upper intake level, interactions with medicines and independent testing matter more here than in almost any other category. Our reviews weigh all four.