Effects of dietary fats on bone health in advanced age

Abstract

Evidence is accumulating that dietary lipids play an important role in bone health. Most of the data supporting the effects of lipids on bones have been collected in young adult and/or developing animals. Based upon this work, mechanisms have been proposed to explain how lipids act to enhance or inhibit bone resorption and deposition. Little work, however, has been done in older models. Since osteoporosis primarily afflicts the elderly, such work is needed in order to determine if mechanisms relevant to the young differ in advanced age, and to develop effective interventions for this especially vulnerable segment of the population. This article reviews evidence that dietary lipids are important to bone health in older individuals, and describes possible mechanisms that may be of particular relevance to the elderly. Specifically, studies supporting the influence of dietary lipids on calcium excretion, growth hormone secretion, fatty acid metabolism, and osteoblast formation are reviewed.

Introduction

Osteoporosis is a serious bone disorder that increases fracture risk and reduces quality of life for millions of people worldwide [1]. Although osteoporosis can affect men and women of all ages, nearly 75% of all hip fractures occur in older women [2]. The healthcare costs are enormous, with an estimated $10–15 billion being spent each year on osteoporosis-related care in the United States alone [2], [3]. These costs will only escalate as the size of the aged population grows unless increased efforts are directed toward reducing osteoporosis risk. Such efforts need to focus not only on the prevention of bone disease, but also on the treatment of persons already afflicted.

Epidemiological data indicate that high-fat diets, especially those rich in saturated fatty acids, may contribute to reduced bone density and increased fracture risk, in older as well as younger people. For instance, analysis of NHANES III data (n=14,850) indicated that total fat intake was negatively associated with bone mineral content and density in several sites in the hip across different ages. These effects were particularly strong for saturated fats and for men [4]. Among women, fat consumption was negatively associated with bone density in the lumbar spine and radius in 218 postmenopausal women and in the lumbar spine in 175 women, 60% of whom were postmenopausal [5], [6]. In premenopausal women, on the other hand, fat intake was negatively associated with bone density in the femoral trochanter (a part of the hip) [5]. This might suggest that certain bone sites are more sensitive to the effects of total fat intake than other sites in different age groups, although this cannot be definitely stated based upon the currently available reports.

That these effects are large enough to increase fracture risk among older individuals is indicated by a study assessing 6250 postmenopausal women. In this study, the risk of fracture increased with total fat intake [7]. Thus, the total amount and the type of fat consumed appear to be important dietary variables affecting bone health in younger as well as older individuals. These findings have potentially important public health implications because consumption of high fat diets, particularly those rich in saturated fatty acids, is of concern in many countries [8], [9], [10], [11], [12], [13], [14]. While data collected in the United States indicate that fat consumption profiles are similar in older and younger people, research is needed to determine if fats have different effects on skeletal outcomes in the elderly and in the young [8].

Although large amounts of fat, especially those containing primarily saturated fatty acids, may have negative effects on bone health, fats composed of other fatty acids may have positive effects. In particular, fatty acids of the n-3 series, as well as the specific n-6 fatty acid γ-linolenic acid (GLA), may prove beneficial when consumed in appropriate amounts. For example, in one study, elderly women with confirmed osteoporosis or osteopenia were given a supplement containing evening primrose oil (a good source of GLA as well as the n-6 fatty acid linoleic acid) and fish oil (a rich source of the n-3 fatty acids eicosapentaenoic acid and docosahexanoic acid). This supplement prevented loss of bone density in the lumbar spine, and increased bone density in the femur, relative to control women receiving coconut oil, which is rich in saturated fatty acids [15]. Since saturated fatty acids may have deleterious effects on bone health, the differences between the groups in this study may have been at least partially due to the use of a coconut oil placebo. Another report showed no effect of a combined evening primrose oil/fish oil supplement in younger post- and premenopausal women, relative to women receiving no supplemental fat [16]. Whether the different results obtained in these two studies were due to the different control manipulations, or to the different ages and/or health of the subjects will require further study. Since oils containing n-3 fatty acids and GLA have shown clinical promise in the treatment of inflammatory disorders often seen among the elderly such as arthritis, additional clinical trials designed to assess the effects of these oils on bone-related parameters are warranted [17], [18], [19], [20], [21], [22], [23].

Fats have also been reported to affect bone health in animals, indicating that animal models are useful for mechanistic studies. For instance, a high-fat/high-sucrose diet reduced mechanical properties in the metatarsus, tibia, sixth lumbar vertebral body (L6), and femoral neck of young rats [24], [25], [26], [27]. This diet also reduced cross-sectional area, length and volume of L6, and cortical shell size of the femoral neck [27]. That the fat in the diet contributed to these effects is supported by reports that high-fat diets can reduce bone mineral in several different species [28], [29], [30], [31], [32]. High-fat diets not only can affect bone health during development, but also in adult and aged animals. For instance, a high-fat diet reduced distal femur bone mineral content in old calorically restricted rats [30]. High-fat diets were also reported to reduce biomechanical properties in the tibia of old non-calorically restricted rats when fed for 20 weeks, but not if fed for only 5 weeks [33], [34]. Additionally, in adult roosters, a high-fat diet reduced bone mineral content and mechanical properties in cancellous bone sites [32]. Taken together, these reports indicate that high-fat diets have negative effects on bones in adult and/or aged animals, and support the use of animal models for mechanistic studies regarding the effects of total fat on bone health.

In addition to total fat, the fatty acid profile of the diet has also been reported to affect bones in animal studies. There is evidence to suggest that n-3 and n-6 fatty acids may be beneficial in appropriate amounts, but that diets with high concentrations of these fatty acids may be detrimental during development and in advanced age [29], [33], [35], [36], [37], [38], [39], [40]. For example, supplementation with docosahexanoic acid (n-3) resulted in oxidative DNA damage in the bone marrow of aged rats, and diets containing large amounts of borage oil (rich in GLA as well as linoleic acid, both n-6) reduced tibia biomechanical properties in aged rats [33], [40]. More research to determine amounts of individual fatty acids, ratios among the fatty acids, and interactions with other dietary constituents across the life span is needed before recommendations appropriate to different ages can be made.