Vitamin D plays a crucial role in calcium homeostasis, bone metabolism, and balance and risk of falling. Low vitamin D levels are linked to impaired calcium absorption and an increase in parathyroid hormone (PTH) which can result in excessive bone resorption. Without sufficient vitamin D, calcium absorption fails to satisfy the body’s requirements, even when calcium intake is adequate.

Serum 25(OH)D levels, reflecting vitamin D levels produced cutaneously as well as those obtained from food, are the most effective measure of vitamin D. The principal sources of vitamin D are sunlight, food, and supplements. The skin synthesizes vitamin D from the ultraviolet rays (UVB) of the sun which vary depending on time of day, season, skin pigmentation, and other factors; in some areas, vitamin D production may not occur at all in winter. In addition, the use of sunscreen can severely limit the skin’s ability to make vitamin D [12]. The pathway of absorption of vitamin D from sunlight and dietary sources is illustrated in Figs. 1 and 2. Sources of vitamin D in naturally occurring foods are extremely limited and are primarily restricted to fatty fish (salmon, swordfish, tuna), fish liver oil, and egg yolks. Most of the vitamin D in American diets comes from fortified foods such as fluid milk (400 IU per quart), ready-to-eat breakfast cereals, yogurt, cheese, and juices [13]. Since it is difficult to obtain the recommended level of vitamin D from sunlight and food, supplements in the form of vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) may be recommended. Whereas the two forms were once regarded as equivalent, recent studies indicate that vitamin D3 is more effective at raising serum 25(OH)D concentrations than is vitamin D2 [14]. Manufacturers appear to be producing more vitamin D3 than vitamin D2 as well as increasing the levels in multivitamin supplements from the former 400 IU per daily dose to as much as 1000–1500 IU/day. Adequate oral intake of vitamin D is also a factor in preventing falls because it addresses several components of the fall-fracture construct including strength, balance, bone density, lower extremity function, and risk of hip and non-vertebral fractures [5, 6]. Table 2 summarizes the vitamin D content in common food group [15].

Calcium is the most abundant mineral in the body, and 99 % of it is stored in bones and teeth. A number of nutrients play a role in bone health, but calcium is accorded special attention not only because it is essential in bone composition but also because the average American’s intake of calcium is far below the amount recommended for optimal bone health, in part because food preferences such as soda have led to reduced consumption of dairy products [16]. Only 30 % of calcium intake is absorbed by the body; factors influencing calcium absorption include vitamin D intake, age, and, to an extent, the amount of phytic acid and oxalic acid in food. Some absorbed calcium is eliminated in the form of urine, feces, and sweat [17]. When calcium intake is insufficient, bone tissue is resorbed, bone mass is reduced, and bone strength is diminished. Bone resorption is controlled by PTH in response to extracellular fluid calcium ion homeostasis rather than to a structural need for bone mass. PTH is also implicated as a cause of bone fragility through its stimulation of bone resorption [18]. “Optimal” calcium intake is defined as the level of calcium consumption required to maximize peak adult bone mass, to maintain that mass, and to minimize bone loss later in life. In this sense, calcium serves as a “threshold” nutrient meaning that below a critical level, the effect of calcium on bone mass is limited by the amount of available calcium, whereas above that level, increased calcium intake provides no added benefit [19].

Calcium-rich foods include dairy products such as low-fat and nonfat milk, cheese, and yogurt; vegetables including kale and broccoli; fish such as sardines and salmon; and calcium fortified foods, particularly fruit juices, cereal, bread, and bottled water. Table 3 gives the calcium content of several of the most common foods [10]. To ensure bone health, the dairy group is most important, providing 20–75 % of recommended calcium, protein, phosphorus, magnesium, and potassium. As Weaver and Heaney point out, the rising consumption of soft drinks and the growing recognition of lactose intolerance have occurred concurrently with a significant decline in milk intake in the United States; alternative foods containing milk’s nutrients are not consumed in sufficient amounts to replace what milk can provide [18]. Recommended calcium levels for healthy individuals vary according to age group, as discussed above. If calcium intake through food is insufficient, two common forms of supplements may be advised. Calcium carbonate is inexpensive, available in several over-the-counter antacids, and absorbed most effectively when taken with food. Calcium citrate is easier to absorb, can be taken with or without food, and causes less constipation than the carbonate form [17]. Whether calcium is obtained from foods or from supplements, it is best absorbed in amounts of 500–600 mg or less. People with osteoporosis or those at risk are often referred to a registered dietician for nutrition counseling.