Vitamin D can be obtained from the diet or from sunlight exposure, but these sources can be insufficient, especially in older people and vegans during the winter months.65 In healthy adult Western European populations, 6 to 14% percent have been reported to be vitamin D deficient,66 but good data are not available for the US. Vitamin D insufficiency is associated with bone loss and fractures in older people.67 68 Reduced bone loss from vitamin D supplements has been reported in some,69 70 though not all,71 studies. In one double-blind study, supplementation with 800 IU per day of vitamin D prevented bone loss more effectively than 200 IU per day in postmenopausal women.72 In addition, vitamin D supplementation has been shown to reduce the risk of falls in older people.73 While vitamin D is known to be toxic in very high amounts, up to 2000 IU per day is considered safe.74

Minerals

What is the potential importance of taking potassium?

While potassium is lower in modern diets compared with so-called primitive diets, true deficiencies are uncommon. 120 Some,121though not all, 122 research suggests that raising potassium intake may help prevent high blood pressure. Other research suggests higher potassium intake may help prevent stroke.123 However, the maximum amount of supplemental potassium allowed in one pill (99 mg) is far below the recommended amounts (at least 2,400 mg per day). Multiple potassium pills should not be taken in an attempt to get a higher amount, since they can irritate the stomach. The best way to get extra potassium is to eat several servings per day of fruits, vegetables, or their juices.

What is the potential importance of taking iron?

Iron deficiency is not uncommon among some groups of healthy people, including some vegetarians,124 menstruating girls and women,125 pregnant women,126 and female and adolescent athletes.127 Nonetheless, many people in these groups are not iron deficient 128 and excessive iron intake has been associated in some studies with heart disease,129 some cancers,130 diabetes,131increased risk of infection,132 and exacerbation of rheumatoid arthritis.133 While none of these links has yet been proven, people should avoid iron supplements unless they have been diagnosed with having, or being at high risk of, iron deficiency.

What is the potential importance of taking iodine?

Iodine deficiency is a concern in many developing countries,134 Until recently it was considered abundant in Western diets, due to the introduction of iodized salt and the iodine added to many foods.135 However, iodine intake has decreased considerably in recent years and may be low in as many as 12% of the total U.S. population, and 15% of women of childbearing age.136 These numbers may be greater in countries where iodized salt is not available. Still, most people have adequate iodine intake unless they avoid iodized salt, seafood, and sea vegetables. People with thyroid diseases should check with their doctor before using iodine supplements.