Vitamins

Vitamins are organic compounds required in limited amounts for normal growth and metabolism, and are typically categorized as fat soluble or water soluble. Thirteen compounds are recognized as vitamins; while choline is not technically classified as a vitamin, it is an essential nutrient and will be included in this review (Mahan et al., 2012; Mueller, 2012; NIH, 2016).

Most vitamins are actually groups of structurally related compounds called vitamers that have similar activities. For example, vitamin D comprises the vitamers calcitriol, ergocalciferol, and cholecalciferol. Vitamins are necessary for a wide variety of biochemical functions; they function as antioxidants and as cofactors for many enzymatic reactions and serve as regulators of mineral metabolism. See Table 14.1 for a list of common functions of vitamins (Mahan et al., 2012; Mueller, 2012; NIH, 2016).

Minerals

The term mineral is a misnomer; dietary minerals are actually chemical elements. The four most common elements—hydrogen, carbon, nitrogen, and oxygen—serve organic functions. Other elements known to be necessary for humans include sodium, magnesium, phosphorus, sulfur, chloride, potassium, and calcium. Additional elements considered essential in much smaller amounts are called trace elements and include chromium, manganese, iron, cobalt, nickel, copper, zinc, selenium, bromine, molybdenum, and iodine. Some elements are suspected to play a role in human health, although there is insufficient evidence to clearly describe their biochemical functions or needed amounts. These include lithium, boron, fluorine, silicon, vanadium, and arsenic (Mahan et al., 2012; Mueller, 2012; NIH, 2016).

Some elements such as sodium and potassium function primarily as electrolytes; others such as magnesium and calcium serve a structural function in addition to acting as electrolytes. Many minerals are required for the production and function of a wide variety of enzymes necessary for normal metabolic reactions. Other mineral functions include hormone synthesis, antioxidant activity, cell signaling, and energy production. See Table 14.2 for a list of common mineral functions (Mahan et al., 2012; Mueller, 2012; NIH, 2016).

Insufficient Intake

Micronutrient intake among older adults, as with other populations, can vary significantly. However, in a recent systematic review of 37 studies involving more than 28,000 community-dwelling older adults in 20 Western countries, researchers concluded that a significant percentage of older men and women had inadequate intake of many micronutrients—especially thiamin, riboflavin, vitamin D, calcium, magnesium, and selenium—when compared to the EAR (ter Borg et al., 2015). Nutrient intake assessment methods varied, making study comparisons challenging, but these data are compelling and suggest adequate micronutrient intake is lacking in many older adults.

The most significant factor affecting micronutrient intake, in the United States and worldwide, is socioeconomic status. The elder population suffers more socioeconomic hardships than younger adults (DiMaria-Ghalili, 2014). Both limited income and access to transportation can reduce the ability to purchase and consume a variety of micronutrient-rich foods (Allen et al., 2006).

Older adults face physical barriers to consuming sufficient amounts of micronutrients. Normal or common physiological changes associated with aging often result in reduced intake of nutrient-rich foods, especially fruits, vegetables, and meats. Changes include poor dentition, altered or decreased taste and smell sensations, decreased appetite, and difficulty chewing and swallowing (DiMaria-Ghalili, 2014). Functional limitations leading to decreased ability to obtain and prepare food are also a concern in the aged population. Functional capacity often declines with age, and the cause is usually multifactorial. Medical conditions such as arthritis, pain, neuromuscular dysfunction, and age-related sarcopenia can all contribute to decreased mobility, physical activity, and manual dexterity.

Psychological problems, such as depression and isolation, and cognitive issues such as dementia are more common in elderly populations and contribute to significant reductions in micronutrient consumption. Isolation and depression are associated with reduced appetite, while individuals with dementia often refuse or forget to eat meals (DiFrancesco et al., 2007; DiMaria-Ghalili, 2014).

The incidence of many chronic health conditions such as cancer, diabetes, heart disease, and hypertension increases as people age. Specific medications necessary for the treatment of these conditions, as well as the total volume of medications taken, can contribute to diminished food intake via decreased appetite, early satiety, taste changes, and GI disturbances such as diarrhea, constipation, and nausea. These challenges can result in the reduction of both the quality and total volume of food consumed (DiMaria-Ghalili, 2014). See Table 14.7 for conditions that reduce intake and their potential causes.

Food Quality

Besides individual conditions that reduce the ability to procure or consume nutrient-rich foods, the other key factor influencing intake is the micronutrient content of the foods consumed. Nutrient content is influenced by food-industry practices, cooking methods, and the environment in which the food is grown.

Food-production practices, including harvesting, transportation, and storage, can subject foods to light and oxygen exposure and time delays between harvest and consumption. Time, light, and oxygen exposure accelerate the ripening process as well as nutrient degradation. Freezing results in minimal micronutrient loss; in addition, produce is typically frozen shortly after harvest, thus maximizing micronutrient content (USDA, 2007). Frozen products may be ideal for some older adults, especially if the frequency with which they can obtain food is limited. Canning may preserve micronutrients if canned shortly after harvest, but minerals can leach from food into surrounding liquid, which is often discarded. Furthermore, adding salt during the canning process may alter the solubility of organometallic compounds, which can affect their bioavailability (Wapnir, 1998).

Other food-industry practices affect micronutrient concentration. Food enrichment is the practice of adding micronutrients back to a food product that were lost during processing, while fortification adds additional micronutrients not present (or present in small amounts) prior to processing. Food fortification has been practiced in industrialized nations for many years; common fortification practices include B vitamins and folic acid in grains, vitamins A and D in milk, iodine in salt, calcium in orange juice and soy products, and a variety of micronutrients in cereals, bottled water, and other beverages. Although less commonly utilized in developing countries, fortification practices including the use of sugar fortified with vitamin A in Central America and iron-fortified fish and soy sauces in Asia have successfully reduced the prevalence of some micronutrient deficiencies (Allen et al., 2006).

Food-preparation methods that utilize higher temperatures, longer cooking times, and large amounts of water or oil (which is then discarded) will increase the loss of some vitamins and minerals, most notably vitamin C. Other factors affecting nutrient retention include pH and exposure to air and light (Bergstrom, 1994). Because of the short cooking time and limited use of water associated with steaming and microwaving, these methods are recommended to minimize micronutrient losses.

The mineral content of plants depends on the mineral concentration of the soil in which it is grown, thus impacting the micronutrient intake of those who consume the plant. Although data are limited for many minerals, research has shown that soil concentrations of zinc and selenium are low in many areas of the world, including China, Africa, India, and parts of North and South America. Low soil concentrations directly correlate with rates of human zinc and selenium deficiency in developing countries but not in industrialized nations. Scientists believe this is due to the wider dietary variety and use of supplements seen in industrialized countries (Udo de Haes et al., 2012). Thus, individuals in these geographic areas may be more likely to have insufficient intake of these minerals if they do not use supplements and lack variety in their diets.

Impaired Absorption

Absorptive issues render older individuals more vulnerable to micronutrient deficiency, even in those whose intakes are considered sufficient for most adults. Micronutrient absorption is affected by several factors, including age, diseases and conditions that affect the GI tract, medications, and the form and amount of the micronutrient consumed. Most micronutrients are largely absorbed in the duodenum and jejunum, with the exception of vitamin B₁₂, which is absorbed in the ileum; thus conditions affecting the proximal GI tract are more likely to result in micronutrient malabsorption.

Human genetic differences may also result in significant variations in micronutrient absorption, as well as nutrient needs and metabolism. Although growing rapidly, the field of nutrigenetics is still in its infancy and insufficient data preclude any changes to current clinical practice related to micronutrients (Baumler, 2012).

Fat-soluble vitamins are absorbed with dietary fat; malabsorption of these vitamins is rare except in conditions resulting in fat malabsorption. Even in cases of decreased absorption, fat-soluble vitamins, with the exception of vitamin D, are rarely deficient in residents of developed countries because of their long-term storage in fatty tissues (Mahan et al., 2012; Mueller, 2012; NIH, 2016).

Absorption of water-soluble vitamins can be influenced by more factors than that of fat-soluble vitamins, and deficiencies are generally more common as body stores of these vitamins are smaller than that of fat-soluble vitamins. Factors that increase the risk of water-soluble vitamin deficiency include alcohol abuse, malnutrition, poor quality or restrictive diets, and malabsorption (Mahan et al., 2012; Mueller, 2012; NIH, 2016).

Mineral absorption is similarly affected by a number of factors, including concentration, form, and concomitant intake of other specific nutrients and dietary components. Dietary minerals are often found in more than one form, and some forms are more bioavailable than others. For example, iron is more efficiently absorbed in the heme form found in animal-based foods than the nonheme iron found in plant-based foods (Mahan et al., 2012; Mueller, 2012; NIH, 2016).

Absorption can also be affected by the amount of mineral consumed, particularly in supplement form; e.g., the percentage of calcium absorbed decreases in doses of more than 500 mg, so it is recommended that daily supplementation be given in divided doses to maximize absorption (NIH, 2016). Zinc excretion in feces also increases when the amount of zinc absorbed increases, so divided doses of zinc supplements may be beneficial (Hambridge et al., 2010). Iron supplementation has been shown to reduce absorption of nonheme iron consumed in foods but not heme iron. Copper absorption also decreases with an increase in intake (Wapnir, 1998).

Excessive or prolonged supplementation of one mineral may result in deficiency of another due to reduced absorption; in particular, zinc, copper, and iron act competitively in the absorption process. Some dietary elements such as phytic and oxalic acid have been shown to reduce micronutrient absorption, while other dietary components such as vitamin C with iron and vitamin D with calcium actually improve mineral absorption (NIH, 2016).

Common physiological changes in aging can also alter micronutrient absorption. While gastric acid production does not necessarily decrease with normal aging, the negative consequences of chronic gastric conditions will manifest with advancing age. In particular, calcium and B₁₂ absorption may be impaired in part due to hypochlorhydria, a common result of atrophic gastritis and the prolonged use of proton pump inhibitors or H₂ antagonists. Gastric acid is needed to cleave vitamin B₁₂ from food, and intrinsic factor is needed for B₁₂ absorption, both of which are produced by gastric cells. Hypochlorhydria may also reduce folic acid absorption (Holt, 2007; Salles, 2007).

No marked changes have been noted in the structure or function of small intestinal villi and enterocytes in older adults. Likewise, digestive enzyme production generally does not decrease with age—with the exception of lactase. Lactase production is highest in infancy and childhood and decreases steadily in adulthood. Lactose intolerance itself does not inhibit calcium or other micronutrient absorption, but impaired tolerance of lactose may reduce consumption of dairy products, thus limiting the intake of foods rich in Ca, Mg, P, K, and vitamins D and A (Salles, 2007; Corleto et al., 2014).

Diseases or conditions that affect nutrient absorption include short bowel syndrome, which is caused by resection of a significant portion of the small bowel; pancreatic exocrine insufficiency, which is seen in pancreatic resections and cystic fibrosis; total or partial gastrectomy, which is most commonly performed to treat gastric cancer and weight reduction for those who are morbidly obese; untreated celiac disease; an acute phase response typically caused by systemic infections; and genetic disorders resulting in reduced or absent production of specific digestive enzymes. Micronutrient malabsorption is largely influenced by the portion of the GI tract that is dysfunctional or absent. See Tables 14.8 and 14.9 for more details regarding absorption, including physiological and dietary (Mahan et al., 2012; Mueller, 2012; NIH, 2016).

Altered Metabolism

Adequate amounts of micronutrients may be successfully consumed and absorbed, but deficiency can still occur if nutrient utilization is impaired or altered. Some disease states and medications can increase micronutrient losses in the urine or stool, reduce conversion to active forms, and reduce the body’s ability to store or transport micronutrients.

Liver disease in particular has a significant effect on micronutrient utilization, as the liver is a primary storage site for some vitamins, produces transport proteins, and serves as a site for such metabolic reactions as hydroxylation or phosphorylation, which are necessary to convert some vitamins to biologically active forms. For example, adequate liver function is necessary for vitamin D to undergo the first of two hydroxylation reactions needed to convert it to the active form. Many individuals with advanced liver disease already suffer from malnutrition and insufficient nutrient intake; impairment in storage, metabolism, and transport further compromises micronutrient status (Johnson et al., 2013).

The kidneys are also a site for metabolism of some micronutrients, particularly vitamin D. People with chronic kidney disease are unable to produce sufficient amounts of the biologically active form of vitamin D because the damaged kidney cannot sufficiently perform the second hydroxylation needed for this conversion. Vitamin D deficiency is thus more common in this population (Tsiaras and Weinstock, 2011).

Individuals who undergo regular renal replacement therapy such as hemodialysis risk water-soluble vitamin and mineral deficiencies due to increased losses during the dialysis process, increased needs, and inadequate intake. It is recommended that those undergoing dialysis receive water-soluble vitamin supplementation in the presence of inadequate intake or deficiency. Water-soluble vitamin supplements have been specifically formulated for this population (K/DOQI, 2000).

The skin is another organ necessary for maintenance of adequate vitamin D stores because skin cells produce the inactive form of vitamin D when exposed to ultraviolet radiation. With sufficient sunlight, those with limited intake or absorption of vitamin D from foods can still produce enough of the vitamin to maintain adequate serum levels. However, because of a greater awareness of cosmetic changes and skin cancer risks associated with ultraviolet radiation, as well as lifestyle changes such as increasing participation in indoor activities, exposure to sunlight has decreased significantly in industrialized nations. Scientists suspect this has contributed to a greater prevalence of vitamin D deficiency. Older populations are particularly at risk because the skin’s ability to synthesize vitamin D decreases with age. Furthermore, older individuals, especially those who are residents of long term care facilities, often receive little exposure to sunlight (Tsiaras and Weinstock, 2011).