At a cursory look, prunes appear to be pretty unremarkable. They are an indeterminate dark brown in colour, deeply wrinkled, mushy and rather sticky to the touch. But their modest exterior conceals some remarkable health benefits.
Prunes begin life as plums and then are dried to become prunes that have longer storage capabilities and more concentrated nutrients within the resulting smaller fruit. Prunes consist of about 40% carbohydrates and are high in fiber, containing about 4 grams of fiber in a 60-gram serving (5 or 6 prunes) with over half of the fiber being soluble. Though each prune contains about 3.5 gm sugar, they do not cause large spikes in blood sugar because their high fiber content releases sugars very slowly. One serving of prunes also offers 2 gram of protein and 134 calories. Prunes are very low in fat (1).
GENERAL HEALTH BENEFITS OF PRUNES
Prunes are a good source of Vitamin A, Vitamin K, B vitamins, magnesium, manganese, potassium, boron, copper, iron, phosphorus and antioxidants. They also contain some zinc (1).
Benefits on blood sugar, cholesterol and the microbiome
The bountiful levels of nutrients in prunes, as in all plant foods, play their parts in maintaining and boosting health. For example, potassium can help to keep blood pressure low by keeping sodium in balance (2). Soluble fiber contributes to cholesterol lowering (3), blood sugar lowering (4) and encouraging growth of friendly microorganisms in the gut (5,6).
Preventing and treating constipation
Prunes have been used to prevent and treat constipation for many decades. Their fiber content of course contributes to this laxative effect but prunes also contain significant amounts of phenolic antioxidants, specifically chlorogenic acid, as well as sorbitol that can increase the rate of movement of food along the digestive tract (7).
A study from the University of Liverpool found that eating prunes may contribute to significant weight loss and reduced waist circumference. The study recruited 100 overweight or obese men and women whose diets were low in fiber. They were divided randomly into two groups, one of which was given prunes to eat daily for 12 weeks in amounts of 140 gm for women and 171 gm for men. The other group, the control, was simply given advice on healthy snacks. The group eating daily prunes experienced average losses of 2 kg in weight and 2.5 cm in waist circumference. The control group lost 1.5 kg of weight and 1.7 cm from their waistline on average. In addition, the prune-eaters showed increased feelings of fullness and escalating weight loss over the last 4 weeks of the study (8).
Other studies show that prunes promote satisfaction when consumed as snacks and so can limit food intake (9). Prunes also act as prebiotics, promoting the growth of healthy microorganisms in the gut which are linked with changes in fat metabolism and prevention of weight gain (10,11,28).
Slowed aging and prevention of chronic diseases
A study from Tufts University in Boston ranked prunes at the top of the list for antioxidant capacity with prunes containing more than twice the level of antioxidants found in other high-scoring fruits such as blueberries and raisins. The main antioxidants in prunes are flavonoid phenolic compounds such as chlorogenic and neochlorogenic acids as well as lutein, zeaxanthin, cryptoxanthin. These bioactives protect against free radicals that play a role in aging and chronic disease processes (12).
A DISTINCTIVE HEALTH BENEFIT OF PRUNES – STRONGER BONES
The variety of nutrients contained in prunes include many that are key factors in bone formation and structure. Recent research has been focused in this direction and the results reveal some remarkable hidden benefits of prunes.
Over the last few decades, research has uncovered positive links between eating fruits and vegetables and improved bone health. For example, studies have observed increases in bone mineral density of up to 5% after a simple increase in fruit and vegetable intake, a very substantial improvement (13). How can this be? One reason may be the effects of fruits and vegetables on the balance between bone formation and bone resorption (disappearance). Bone cells are specialized to cause either formation of new bone (through bone cells called osteoblasts) or breakdown of existing bone (through bone cells called osteoclasts). Research suggests that osteoclasts employ free radicals to destroy bone and that too many free radicals in the system may result in excessive bone breakdown (14). Indeed studies have shown a link between increased oxidative stress (presence of free radicals) and reduced bone density, likely through increased production of osteoclasts, the cells that break down bone (24,25). All fruits and vegetables are prime sources of antioxidants that destroy free radicals.
Prunes specifically are rich in phenolic antioxidants, especially the chlorogenic acids such as neochlorogenic acid and cryptochlorigenic acid that are known to be bone-protective through boosting osteoblast formation (29). Studies of prunes have also shown reductions in markers of inflammation such as CRP (C-reactive protein) (16). High CRP is also linked to osteoporosis (26).
Prunes have been studied in postmenopausal women for bone strengthening because of their rich content of phenolic and flavonoid antioxidant compounds. In 2002 a study of 58 postmenopausal women randomly assigned the subjects to consume either 100 gm of prunes (about 10 to 12 prunes) or 75 grams of dried apples daily for 3 months. Both regimens were similar in levels of calories, fat, carbohydrate and fiber. Markers of bone status were measured before and after treatment. Only the prune-eaters showed significantly increased markers indicating increased bone formation. Neither fruit had any effect on markers of bone resorption. The researchers concluded that prunes could help with building new bone (15).
Almost a decade later, a longer study was performed. Over 200 postmenopausal women were recruited and randomly assigned to one of two treatment groups: 100 gm of prunes or 65 gm of dried apple daily for 12 months. Both treatments contained an equivalent amount of carbohydrates, fat, fiber and calories. In this study bone mineral density (BMD) was measured at the start and end of the study and levels of bone markers were measured at the start and the end of the study and also at the 3 and 6 month points. Though both groups showed positive changes, the prune group had significantly higher increases in BMD compared to the dried apple group. The prune group also saw significant decreases in markers of bone resorption (bone breakdown) while the dried apples did not.
The above two studies revealed a beneficial effect on bones but perhaps a little too much of a laxative effect. In 2016 a new study of 48 osteopenic women (women with some amount of bone loss) were randomized into three groups; one group ate 50 gm of prunes daily (equivalent to 5 or 6 prunes); a second group ate 100 gm of prunes daily; the third group ate no prunes. After six months the women in both prune-eating groups had experienced no further bone loss during the trial while the women in the control group had small continuing losses of bone. The prune-eating women also showed decreases in a blood marker for bone resorption indicating a slowing of bone breakdown. Researchers concluded that lower intakes of prunes can be successful in maintaining bone health without a strong laxative effect (17).
A 2017 comprehensive review of the role of prunes in bone strength concluded that prunes are linked to consistent improvements in BMD. The possible mechanism suggested was the actions of the antioxidants in prunes on cell signalling pathways that influence the formation of osteoblasts and osteoclasts. Phenolic antioxidant compounds such as those in prunes have a stronger association to bone health than that of general fruit and vegetable consumption. These researchers suggest that long-term prospective cohort studies are needed to confirm the effects of prunes on bone. New studies should include participants of all ages, gender and ethnicities and look at endpoints such as fracture risk and BMD changes (7).
Why is it that prunes have this bone conservation effect? Is it only the antioxidant content? Prunes also have a high content of other nutrients that are important in the formation and maintenance of strong bones.
Nutrients in prunes that are important in bone health
Prunes are rich in the trace mineral, boron. Boron is essential for the growth and maintenance of bone and it affects many processes of bone formation and breakdown. For example, boron can boost magnesium absorption, increase blood levels of Vitamin D, impact the body’s use of estrogen and testosterone and reduce inflammation. Perhaps most importantly boron supports the formation of osteoblasts, the bone component that forms new bone (18). There is no official “Adequate Intake” set for boron due to insufficient data. However, its beneficial role in humans is recognized and the effects of deficiency are well-known. It appears that daily amounts of 3 to 5 mg are enough boron to gain its benefits but still well below an upper tolerable limit. Most people acquire much less than this amount in their diet (19,20). Prunes are among the top contributors of boron to the diet, a list that also includes peanut butter and other nuts, beans and raisins. Ten to twelve prunes contain 3 mg of boron (21).
Prunes are also surprisingly high in Vitamin K, a vitamin known to improve calcium balance and promote stronger bones. Studies of Vitamin K have shown that higher levels of this vitamin are associated with decreased risk of bone fractures (20). The average daily recommended amount for Vitamin K is 90 mcg for adult women and 120 mcg for adult men. Ten prunes contain about 60 mcg of Vitamin K (27).
Magnesium, manganese, copper, zinc and potassium
Many of the minerals contained in prunes also contribute to bone health. Prunes are considered to be a good to excellent source of magnesium, manganese, copper and potassium. Prunes also contain zinc in lesser amounts.
Magnesium is a major component of bone with about 60% of all magnesium in the human body concentrated in the bones. Magnesium deficiency is known to contribute to osteoporosis through several mechanisms and high magnesium intake is positively associated with increased bone density (22).
Manganese, with or without calcium but especially in combination with zinc and copper, has been shown to reduce bone loss in postmenopausal women (23).
Potassium contributes to the maintenance of bone mineral density in men and women. Higher intakes of potassium have been found to reduce bone breakdown (29).
FINAL THOUGHTS ABOUT PRUNES
Behind their unappealing façade prunes are nutritional powerhouses. Whether it is their antioxidants or their vitamin and mineral content they appear to have very significant positive effects on bones. Prunes are sweet and tasty and make an easy snack. It is worthwhile to eat a few every day even if all you desire is regularity in your trips to the bathroom. But their link with bone strength is a very compelling reason to eat prunes regularly. At our house we include 5 or 6 prunes in our breakfast every morning and they help to make a satisfying start to the day.
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