Prunes are now “dried plums”. And whatever you call them, they may help bones. It’s an unlikely association, but as a person concerned about my own bone health, I was intrigued and started investigating. And I found that there may be something to it. Initial research showed that consumption of dried plums (prunes) improved bone density in animal models, and raised blood levels of certain molecules linked to bone strength in humans. The next essential step was to look for bone density changes in humans. The subjects were postmenopausal women who were not on hormone replacement or bone drugs, and did not have osteoporosis. Half were assigned to eat 100 grams of dried plums daily for a year; the other half ate dried apples. They were tested throughout the study for blood levels of molecules related to bone metabolism, and scanned for bone density.
I weighed out 100 grams of ‘dried plums’ from a typical pitted variety found in the grocery store. It came out to 14 plums. Imagine eating 100 grams each and every day for a year. That’s 365 days of 14 prunes a day, for a grand total of 5110 prunes. Despite this formidable number, 55 of the original 80 subjects completed the dried plum regimen. At the end of 12 months, the dried plum group showed improvement in certain bone mineral density scores. Bone metabolism markers shifted to a pattern related to bone strength and bone building. Conclusion: something about dried plums was good for bones.
But what? The researchers speculated that one benefit could be the high vitamin K content of dried fruit, particularly plums. Vitamin K plays a role in calcium metabolism and certain forms of vitamin K impact calcium in bones. Another possibility: fruits have anti-inflammatory properties, and inflammation is bad for bones. Another idea: the mineral boron, which is high in prunes, plays a role in bone metabolism.
The boron connection isn’t a new idea. Boron intake was linked to incidence of arthritis in the mid-20th Century. A review of bone nutrition from 1994 suggests an intake of 1-3 mg/day of boron from foods like prunes, raising, avocados and apricots. Clearly something is going on, but the exact reason boron would help bones is unclear. It is known to impact estrogen hormone levels, as well as magnesium, so an excessive intake from supplements is a really bad idea.
Here’s what the National Academy of Sciences had to say about boron in 2006:
Boron: A collective body of evidence has yet to establish a clear biological function for boron in humans. Although some evidence does suggest a role in the metabolism of vitamin D and estrogen, further research is necessary. (page 416)
If you use the USDA’s interactive nutrient recommendation calculator, set for “boron” it comes up zero; there is no recommended daily boron intake. The tolerable upper limit for daily intake is set at 20 mg/day. The National Library of Medicine notes that a high boron diet is 3.25 mg/day (per 2000 calories). There’s the contradictory observation that boron is used for “building strong bones”, but there’s insufficient evidence that boron supplements improve osteoporosis.
Accurate boron values for food are hard to find. The USDA food database doesn’t list it. In general, plant foods are good sources, although boron content of any plant food will vary according to boron content of soil. Animal foods are not good sources, so those high protein/low carb Paleo style diets are going to be low boron (as well as low calcium and low vitamin K and low magnesium and on and on).
So what should you think about the boron-bone-prune connection? Here’s what I think: it’s way too soon to put your faith in any dose of boron supplements as a way to build bone strength. Prunes apparently have some helpful impact on bone health. Maybe it’s boron, maybe it’s something else. The study did not investigate the “why” of the dried plum benefit; just that it exists. If eating a few prunes…. uh, dried plums… everyday can help bones, great. I’ll certainly be eating a few more at least in the short term, since I’ve got a box of them to use up.