Dietary Considerations to Prevent Loss of Bone and Renal Function

At the beginning of the 20th century, the average life expectancy in the United States was less than 50 y, but after a century of technical and medical advances, the life expectancy of the average American has increased by more than 25 y. A longer life expectancy creates a compelling need to initiate early lifestyle changes that will maintain bone and renal health to ensure a good quality of life as we age. Two costly and debilitating diseases dramatically reduce the independence and quality of life of older Americans. These diseases are the brittle bone disease, osteoporosis,1,2 which is associated with an estimated 1.5 million bone fractures annually at costs in excess of $10 billion; and endstage renal disease,3 which requires more than 200 000 patients to undergo dialysis at an annual cost of $15.6 billion. Both diseases have different etiologies, and both are considered modifiable or their progression slowed through dietary intervention. Bone and kidney are two organ sytems inexorably linked by common nutritional factors and their key roles in the endocrine regulation of calcium, phosphorus, and vitamin-D homeostasis. Given the changing nature of the US food supply and changes in food-consumption patterns during the past century, two dietary recommendations for the 21st century come to mind that could help to optimize bone and kidney health. The first is to maintain adequate calcium and vitamin-D intakes from food sources at all stages of life, and the second is to consume less phosphorus and sodium especially in the form of high-fat fast foods and processed snacks. An adequate intake of calcium throughout life to prevent or delay the onset of osteoporotic fracture has been the focus of several national and international conferences.2,4 It also serves as the basis for which the US Food and Drug Administration allows a health claim describing the relation between calcium and osteoporosis on the labels of qualified foods and dietary supplements.5 The need for adequate intakes of vitamin D is not as well studied as calcium intake alone. Nevertheless, a growing number of studies support the importance of adequate intakes of both vitamin D and calcium, particularly in the elderly, to maintain bone mass. These findings stimulated the recent increase in the recommended dietary intake guideline for vitamin D in elderly men and women.6–8 Very recent evidence suggests that vitamin-D supplementation of breast-fed infants is associated with higher bone mass in white adolescent girls. These findings in both the elderly and adolescent girls underscore the importance of adequate intakes of vitamin D throughout life to optimize bone mass.9 The major source for vitamin D is through exposure of skin to sunlight. However, a variety of factors, from living at latitudes above 40° N to race, excessive sunscreen use, or air pollution, create a dependency on dietary sources of vitamin D for all ages.8 In the United States, almost all of our dietary intake of vitamin D comes from fortified-milk products and, to a much lesser extent, other fortified foods such as breakfast cereals or fatty fish that are eaten less frequently.8 It is important to note that, although all fluid milk in the United States and Canada is fortified with 10 mg (400 IU)/quart, or 9.6 mg (385 IU)/L,8 other dairy products such as cheese and yogurt are not usually fortified with vitamin D. Milk and dairy products, in general, are the best food source of calcium in the United States,4 and foods are generally encouraged over dietary supplements whenever possible because of greater nutrient bioavailability.2 Milk, unlike yogurt and cheese, is the richest food source of both vitamin D and calcium. Milk also contains several other nutrients needed by bone including a highquality protein, vitamin A, and zinc. Thus, it is only logical to encourage increased consumption of low-fat milk to people of all ages. For people who are lactose intolerant, lactose-free milk is commonly available in the United States. The benefit of enhanced bone mass accretion through the incorporation of even small amounts of milk in the diet of pubescent girls has been demonstrated recently by Cadogan et al. in the United Kingdom.10 This dietary-intervention study is one of many that reinforces the concept that milk consumption benefits bone. In contrast, the potential benefit of milk consumption to maintenance of renal health is less obvious. Appel et al.11 demonstrated significant reduction in diastolic and systolic blood pressure by changing the dietary pattern of adults to incorporate low-fat milk and other dairy products in combination with more fruits and vegetables and lower total and saturated fat. The mechanism through which this change in dietary pattern lowered blood pressure is unclear, but higher levels of potassium and calcium are suggested factors. Several meta-analyses of randomized calciumintervention trials that examined the effect of calcium supplements and dietary calcium concluded that higher calcium intake can lower blood pressure.12,13 High blood pressure14 is a key factor contributing to the progression of renal failure. The magnitude of this reduction in blood pressure by calcium is arguably small; however, even a small reduction in blood pressure may ultimately benefit renal function. Dietary strategies such as incorporating fat-free or low-fat milk into the diet to help maintain blood pressure and renal function merit consideration. Despite these health benefits, the overall trend in food consumption in the United States is to drink less milk and more carbonated soft drinks.15 In the past two decades, the percentage of people in the United States consuming soft drinks increased by 32%, whereas the percentage of those consuming fluid milk decreased by 18%.16 Mean daily intake of fluid milk decreased 24%, whereas mean daily intake of soft drinks increased 130% from 1977 to 1996.16 For both young and old, milk has been replaced by soft drinks, some of which contain phosphorus and sodium but no calcium or vitamin D or other nutrients that milk contains.16,17 This rise in soft-drink consumption is thought to contribute to the high phosphorus intake relative to a low calcium intake in the United States, largely because the most popular cola soft drinks use phosphoric acid as an acidulant to maintain carbonation.15 With respect to skeletal and renal health, concern has been raised This publication represents the views of the author and does not necessarily reflect the views or policies of the US Food and Drug Administration, nor does mention of trade names, commercial products, or organizations imply endorsement by the US Government.