Dried Plums, Prunes and Bone Health: A Comprehensive Review

The 2015–2020 Dietary Guidelines for Americans advocate for increasing fruit intake and replacing energy-dense foods with those that are nutrient-dense. Nutrition across the lifespan is pivotal for the healthy development and maintenance of bone. The National Osteoporosis Foundation estimates that over half of Americans age 50+ have either osteoporosis or low bone mass. Dried plums, also commonly referred to as prunes, have a unique nutrient and dietary bioactive profile and are suggested to exert beneficial effects on bone. To further elucidate and summarize the potential mechanisms and effects of dried plums on bone health, a comprehensive review of the scientific literature was conducted. The PubMed database was searched through 24 January 2017 for all cell, animal, population and clinical studies that examined the effects of dried plums and/or extracts of the former on markers of bone health. Twenty-four studies were included in the review and summarized in table form. The beneficial effects of dried plums on bone health may be in part due to the variety of phenolics present in the fruit. Animal and cell studies suggest that dried plums and/or their extracts enhance bone formation and inhibit bone resorption through their actions on cell signaling pathways that influence osteoblast and osteoclast differentiation. These studies are consistent with clinical studies that show that dried plums may exert beneficial effects on bone mineral density (BMD). Long-term prospective cohort studies using fractures and BMD as primary endpoints are needed to confirm the effects of smaller clinical, animal and mechanistic studies. Clinical and prospective cohort studies in men are also needed, since they represent roughly 29% of fractures, and likewise, diverse race and ethnic groups. No adverse effects were noted among any of the studies included in this comprehensive review. While the data are not completely consistent, this review suggests that postmenopausal women may safely consume dried plums as part of their fruit intake recommendations given their potential to have protective effects on bone loss.


Introduction
The 2015-2020 Dietary Guidelines for Americans (DGA) advocate for healthy eating patterns that include a variety of fruits. This includes all fresh, frozen, canned and dried fruits and fruit juices [1]. The recommended intake of fruit in the Healthy US-Style Eating Pattern at the 2000-kcal level is two cup-equivalents of fruit per day. Increasing the amount and variety of fruits Americans consume is a strategy that helps individuals meet a wide range of nutrient requirements. However, per the 2015-2020 DGA, average intake of fruit is well below recommendations for almost all age-sex groups, except in children ages 1-8 years [1]. Average intake of fruit is lowest among girls ages 14-18 years and in women age 51+ years [1], two critical time points in bone development and maintenance.
Osteoporosis is a rising public health concern, given the aging population and suboptimal dietary intakes of dairy, fruits, vegetables and whole grains, which provide a variety of essential nutrients that influence bone accretion and maintenance across the lifespan. The National Osteoporosis Foundation estimates that 10.3% of Americans over the age of 50 years have osteoporosis (t-score ≤ 2.5), and 43.9% have low bone mass (also commonly referred to as osteopenia; t-score ≤ 1.0), a risk factor for osteoporosis [2]. The risk of fractures increases with age among individuals age 50+, and differs by sex, race and ethnicity [2,3]. Although many factors contribute to this debilitating event, the most significant causes are reduction in bone mass, structural deterioration and increased frequency of falls. It 2005, it was estimated that the over two million incident osteoporotic fractures occurring annually in the U.S. had an economic burden of $16.9 billion, which is anticipated grow to three million fractures at a cost of $25.3 billion by 2025 [4]. Men account for 29% of these fractures and 25% of the cost burden [4]. Optimization of lifestyle factors known to influence bone mass and strength is an important strategy aimed at reducing the risk of fractures later in life.
Plums are a type of drupe fruit that belong to the subgenus Prunus (family Rosaceae). They differ from other subgenera of drupe fruits (cherries, peaches, etc.) since the shoots have a terminal bud and unclustered single side buds, flowers combine in groups of one to five on short stems, the fruit has a crease running down one side and a smooth seed. There are over 40 species of plums currently documented, although two species, the European plum (Prunus domestica) and Japanese plum (Prunus salicina and hybrids) are of commercial significance globally [5]. The origin of European plum is thought to have been near the Caspian Sea, while Japanese plums originated in China, but derived their name from the country where they were cultivated. European plums were introduced in the U.S. by pilgrims in the 17th century, while Japanese plums were introduced to the U.S. in the late 19th century. China, Serbia and Romania are the world's leading producers of plums. Worldwide, greater than 11.2 million metric tons of plums were harvested in 2014 per the Food and Agriculture Organization (FAO) of the United Nations [6]. While all prunes originate from fresh plums, not all plum varieties are considered prunes. Commercialized prunes, also commonly known as dried plums, are the dehydrated version of the cultivar Prunus domestica L. cv d'Agen. This specific variety has a naturally-occurring sugar content that enables it to be dried while still containing the pit, without being fermented. The State of California produces~99% of the plums in the U.S. and~40% of the world's dried plums [7].
Dried plums are widely known for their laxative effect, which is commonly attributed to their dietary fiber content [8], but is also likely influenced by the significant amounts of phenolics (e.g., chlorogenic acid) and sorbitol present in the fruit. Dried plums are not only a source of dietary fiber, but also a good source of potassium and vitamin K (Table 1). One serving or~4 dried plums is 92 kilocalories and provides 2.4 g of dietary fiber, 280 mg of potassium and 22.8 µg of vitamin K. Dried plums also contain several dietary bioactives, including phenolic compounds, such as 3-caffeoylquinic acid, 4-caffeoylquinic acid, 5-caffeoylquinic acid, 3-p-coumarolylquinic acid, caffeic acid, p-coumaric acid and quercetin-3-O-rutinoside [9], whose benefits may extend beyond the basic nutrition requirements of humans. There is an emerging body of evidence from laboratory, animal and human studies that suggests that dried plums may exert an effect on bone health. Hooshmand and others found that two servings (100 g) of dried plums per day slowed the rate of bone turnover and helped to improve bone mineral density (BMD) in a clinical study of 160 randomized postmenopausal women (100 completed the study) not receiving hormone replacement therapy [10]. A more recent clinical study by the same group confirmed the bone protective effects in postmenopausal women receiving one serving of dried plums per day [11]. However, a comprehensive review of dried plums and bone health is not currently present in the peer-reviewed scientific literature.

Literature Search
A comprehensive literature search was conducted as of 24 January 2017 using the PubMed database. The search methodology is outlined in Table 2. A systematic literature search was not conducted for lack of clinical and observational evidence and since the focus was to evaluate potential mechanisms from various types of data.
Included in the review were cell, animal, population and clinical studies in the English language that assessed the effects of dried plums or extracts of the former on markers of bone health. All articles were screened by title/abstract and, in some cases, full-text. A complete manual search of reference lists of original studies was also conducted. Excluded studies (n = 26) were those of any kind that did not assess dried plum (prune) or plum intake on one or more markers or clinical endpoints of bone health.

Data Extraction
Quantitative and qualitative data information from each study, including author and year of study, geographic study location, study design, product information, intervention, population, markers measured, duration and results, were extracted (Tables 3-5).

weeks
Mean BMC and BMA were significantly higher in the 25% DP group compared to the control. 15% DP group had a significantly higher plasma IGF-1 compared to the control. 15% and 25% DP groups significantly increased BV/TV compared to the control. 15% and 25% DP groups significantly decreased TbSp beyond that of the control group. 15% and 25% DP groups experienced a significant increase in vertebra TbTh compared to the control. 15% and 25% DP groups had significantly lower Von Mises stress distribution compared to the control. 15% and 25% DP groups had significantly higher vertebral connective density and tibia apparent mean/density, and lower vertebral SMI and OC expression and TNF-α. 25% DP group had significantly higher apparent mean/density and tibia connective density, and significantly lower tibia SMI. 25% DP group significantly increased TbN compared to the control. All doses of DP groups had significantly lower plasma PINP, NFATc and Runx2 compared to the control. This study randomized mice to 25% DP intervention vs. a control to protect from bone loss and then later exposed them to ionizing radiation.

7-21 days
Compared to the irradiated controls, levels of Nfe2l2, RANKL, MCP-1, OPG, and TNF-α in the DP group were not statistically different. After exposure to radiation, DP mice did not have any significant decrease in TbN, BV/TV, TbTh or TbSp indicating a radio-protective effects against cancellous bone loss compared to irradiated controls. DP fed mice had significantly higher BV/TV, TbTh and TbN after being exposed to simulated space radiation compared to control diet. DP fed mice had significantly lower: • Ctsk gene expression.
Within the proximal tibia and when compared to controls: TbSp decreased significantly in 15% and 25% DP groups. When compared to controls: cortical thickness increased significantly in 5%, 15%, and 25% DP groups. 25% of DP significantly suppressed increase in urinary DPD excretion. 5%, 15%, and 25% DP groups significantly suppressed serum P1NP.Compared to controls:  BV/TV (lumbar and distal femur), trabecular number (lumbar and distal femur), connectivity density (lumbar and distal femur), and femur cortical thickness, were significantly higher in the DP group. • Trabecular separation (lumbar and distal femur), SMI (lumbar and distal femur) were significantly lower in the DP group. At 4 and 12 weeks, serum P1NP of DP group were significantly reduced compared to controls. At 12 weeks, serum PYD was significantly lower in the DP group. The DP group had significantly higher glutathione peroxidase activity than the control at 12 weeks. At 4 weeks, the DP group had significantly lower OcS, ObS, MS, and BMR. At 4 weeks, the DP group had significantly higher Pparc; lower Osx, Bmp4, ALP, Col1a1, Bglap2, RANKL, and NFATc1. Female rats were either ovariectomized or sham operated. The ovariectomized groups were then fed either a 5% or 25% DP supplemented diet vs. a control diet.

days
Compared to the controls the 25% DP group had significantly higher trabecular BA. Unreported results (data not shown): • DP diets dose-dependently enhanced IGF-1.

Discussion
Dried plums are being increasingly recognized for their role in bone health. This comprehensive review supports that consumption of dried plums is safe and may be a bone healthy option for postmenopausal women wishing to satisfy daily requirement for fruit as outlined by the 2015-2020 DGA. It is important to note that dried plums contain a higher amount of vitamin K as compared to other commonly-consumed fruits, which may influence bone health by helping to improve calcium balance.
The quality of clinical studies included in this comprehensive review was acceptable, noting that none utilized a sample size based on a priori power calculation, nor were the treatment allocations able to be concealed from the participants and/or investigators. All four clinical studies identified were derived from the same laboratory group [10][11][12][13][14], meriting the need for replication by additional investigators. Clinical studies included in this review had several limitations, such as a short duration of 3-12 months, which is a narrow window to see significant changes in BMD. All four clinical studies were un-blinded, and only the Hooshmand et al. 2016 [11] had an inactive placebo group. The Hooshmand et al. 2011 [10] and Hooshmand et al. 2014 [13] manuscripts used dried apples as the control group and represent the same study population with additional biomarkers being measured for the latter publication post hoc (BMD data are presented twice). Participants in this study also received 500 mg of calcium and 400 IU of vitamin D during the intervention [10,13], even though administration of these supplements was equal across both arms. Simonavice et al., 2014 [14], assessed the effects of dried plums and resistance training vs. resistance training alone on blood markers of bone and inflammation in female breast cancer survivors. While this study found null effects, these results are likely not generalizable to healthy postmenopausal women experiencing normal hormone-related bone loss. Resistance training has been shown to have a larger effect on preventing bone loss as compared to most dietary interventions and could have masked the much smaller the effects, if present, exerted by dried plums. The Hooshmand et al., 2016 [11], found that dried plum consumption at 50-100 g/day for a period of six months prevented loss of total body BMD, but not spine, hip or ulna BMD, likely due to its shorter duration.
Consistent improvements in BMD at several sites were noted in animal studies designed to model conditions at or before peak bone mass, pregnancy, post-menopause, osteopenia and/or osteoporosis. Rat models of ovarian hormone deficiency have been used for over 25 years to simulate postmenopausal bone loss in humans. Ovarian hormone deficient rats and postmenopausal women have many similar characteristics when it comes to bone loss. These characteristics include increased rates of bone turnover with resorption exceeding formation, an initial rapid phase of bone loss followed by a slower phase due to the ovariectomy, greater loss of trabecular vs. cortical bone, decreased intestinal absorption of calcium and a similar response to drug (e.g., bisphosphonate therapy) and lifestyle interventions (e.g., physical activity) [41]. Indeed, animal studies show that dried plums and/or their polyphenol-rich extracts can beneficially affect both BMD and bone biomarkers. The animal and cell studies presented in this comprehensive review are consistent with and supportive of the theory that a diet high in phenolics and/or flavonoids may enhance bone formation and inhibit bone resorption through their actions on cell signaling pathways that influence osteoblast and osteoclast differentiation [62]. Total body BMD and BMD at specific sites, as well as several blood biomarkers, including AP, BAP, BSAP, OPG, RANKL and TRAP-5b, have been shown to be consistently and beneficially impacted across both clinical and animal studies. Animal studies also collectively support that dried plums may beneficially influence bone area and micro-architecture.
Several bone turnover markers seemed to be improved among clinical studies; however, there was a lack of consistency among many of the markers across and between both clinical and animal studies. For instance, Arjmandi et al., 2002 [12], found significant increases in bone alkaline phosphatase (BAP), but the latter larger study Hooshmand et al., 2011 [10], reported a decrease in BAP. Noting the abundance of bone turnover markers measured in both research and the clinical setting, the International Osteoporosis Foundation (IOF) and International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) Bone Markers Working group recently reviewed the scientific literature to determine the clinical potential of bone turnover markers, which includes the prediction of fracture risk and monitoring treatments for osteoporosis [63]. The IOF/IFCC working group identified one bone resorption marker (s-CTX, serum C-terminal telopeptide of type I collagen) and one bone formation marker (s-PINP, serum procollagen type 1 N propeptide) to be used as reference markers and measured by standardized assays in observational and intervention studies [63]. While only one animal study assessed the effects of dried plums on CTX [27] and two on PINP [27,28], collectively, the animal studies included in this review showed beneficial effects of dried plums and/or their polyphenol-rich extracts on most, but not all, markers of bone turnover. Nevertheless, bone turnover markers are likely too premature in their standardization and clinical utility to accurately predict small changes in bone, as expected in dietary interventions. Differences in study design, dose and duration may also contribute to the inconsistencies in the bone turnover markers measured across and between rodent and cell studies.

Future Research
Identification of the active components, particularly individual phenolics, and their potential modes of action are necessary to fully understand the overall effect of dried plums on bone health across the lifespan. While existing data indicate that consumption of dried plums may be beneficial in postmenopausal women with ongoing bone loss, future clinical and prospective cohort studies in premenopausal women, men and adolescents prior to peak bone mass accrual are necessary to confirm their effects across the population and to make generalizable dietary guidance statements.
Recent epidemiological studies show that phenolic compounds may have a stronger association with bone than general fruit and vegetable consumption [62]. Even though BMD is a validated biomarker of bone health, fractures represent the most significant clinical endpoint of bone health across the lifespan. Prospective cohort studies designed to assess potential associations of dried plum intake on both fracture risk and changes in BMD across the population and various subpopulations are greatly needed to confirm the findings of studies included in this comprehensive review.

Conclusions
Dried plums are an easy means to help individuals meet their daily recommendations for fruit intake. The beneficial effects of dried plums on bone health may be in part due to the unique variety of phenolics and nutrients present in the fruit. Animal and cell studies suggest that dried plums and/or their extracts enhance bone formation and inhibit bone resorption through their actions on cell signaling pathways that influence osteoblast and osteoclast differentiation; however, results on specific markers are not consistent across and between studies. Animal studies are somewhat consistent with small clinical interventions that show dried plums may exert beneficial effects on total body and site-specific BMD. Long-term prospective cohort studies using fractures and BMD as primary endpoints are needed to confirm the effects of smaller clinical, animal and mechanistic studies. No adverse effects were noted among any of the studies included in this comprehensive review. While the data are not completely consistent, this review suggests that postmenopausal women may safely consume dried plums as part of their fruit intake recommendations given their potential to have protective effects on bone loss.