47: Vitamins

INTRODUCTION

Vitamins are essential for normal human growth and development.⁴⁹ By definition, a vitamin is a substance present in small amounts in natural foods, is necessary for normal metabolism, and whose lack in the diet causes a deficiency disease.⁴²

A standard North American diet is sufficient to prevent overt vitamin deficiency diseases.^68,77 However, suboptimal vitamin status is common in Western populations and can be a risk factor for chronic diseases such as cardiovascular disease, cancer, and osteoporosis.^68,77 Groups at particular risk include the elderly, the hospitalized, alcohol-dependent individuals, pregnant women, and others with poor nutritional status. The American Dietetic Association posits that the best strategy for promoting optimal health and reducing chronic disease is to choose a wide variety of nutrient-rich foods, while the use of supplements can help some people meet their nutritional needs.¹⁵⁴ Health care professionals should identify patients with poor nutrition or other reasons for increased vitamin needs and offer guidance on vitamin supplementation. Some suggest that since most people do not consume an optimal amount of vitamins by diet alone, it is prudent for all adults to take vitamin supplements with physician guidance.^68,77

Dietary supplement use has increased over time in the United States. National Health and Nutrition Examination Survey (NHANES) data collected from 2003 to 2006 demonstrate that dietary supplement use was reported by 49% of the population age 1 year and older, an increase of approximately 10% from NHANES data from 1988 to 1994, with 79% of users taking them daily for the past 30 days.¹⁵ The categories of dietary supplements included multivitamin, multiminerals (MVMMs), botanicals, and amino acids. Among adults and children, MVMMs were the most commonly reported dietary supplement, with 33% of the population reporting use.

Unfortunately, many individuals share the mistaken beliefs that vitamin preparations provide extra energy or promote muscle growth and regularly ingest quantities of vitamins in great excess of the recommended dietary allowances (RDAs) (Table 47–1). The most commonly used MVMM preparations generally do not exceed 100% of the RDA, but excessive vitamin intake is more likely to occur in MVMM users who also take single vitamin supplements.¹⁹⁹ Excess vitamin intake may also occur in individuals taking an MVMM who ate a healthy diet that included fortified foods and beverages.¹⁶⁸ Some vitamins are associated with consequential adverse effects when ingested in very large doses. Adverse events also may be associated with the use of some vitamins at the RDA or at amounts less than or approaching the established tolerable upper intake level (UL) in certain populations. Those who smoke should avoid products containing large amounts of vitamin A and β-carotene because studies have linked use of these vitamins to an increased risk of lung cancer in smokers.¹⁹⁰ In one study, smokers taking 20 mg/day of β-carotene (UL 36 mg/day) had an 18% higher ...