"The results of this study demonstrate that dietary supplementation with dried plum has potent effects on bone metabolism and prevents the deterioration in bone mass… in a dose-dependent manner," wrote the researchers from Oklahoma State University and the University of Oklahoma.
Osteoporosis is estimated to affect about 75m people in Europe, the USA and Japan. According to the International Osteoporosis Foundation, the total direct cost of osteoporotic fractures is €31.7bn in Europe, and 17.5bn in the US (2002 figure). The total annual cost of osteoporosis in the UK alone is over £1.7bn (€2.5bn), equivalent to £5m (€7.3m) each day.
The study, led by Brenda Smith and published on-line ahead of print in the journal Bone (doi: 10.1016/j.bone.2006.05.024) divided 60 male Sprague-Dawley rats into five groups. One group was sham-operated (sham group) and four groups of rats had their testes removed (orchidectomised).
As men reach the age of 40 and beyond, the level of testosterone decreases by about one per cent per year, leading to weaker muscles and thinner bones. The researchers castrated the rats to model low testosterone levels on bone strength and the subsequent effects of dried plums.
The orchidectomised rats were further divided into four groups and assigned to eat one of four diets: a standard semi-purified diet (control), or the control diet supplemented with different amounts of dried plums (provided by the California Dried Plum Board) - five per cent (low-dose), 15 per cent (medium-dose) and 25 per cent (high-dose).
After 90 days the researchers found that both the medium-dose and high-dose dried plum supplementation totally prevented the orchidectomised-induced decrease in bone mineral density (BMD) in the whole body, femur, and lumbar vertebra.
"Even at the lower dose (i.e. 5 per cent), dried plum provided a modest degree of protection against femoral bone loss, suggesting that skeletal benefits may result from long-term use at lower doses as well," wrote Smith and her co-workers.
Measurement of bone resorption using the biomarker deoxypyridinoline (DPD) showed that total DPD rose by about 36 per cent in the castrated rats after 90 days, indicating that the bones were being weakened by resorption. However, dietary supplementation with dried plums appeared to decrease this resorption, said the researchers, and excretion of DPD in the urine was 57 per cent lower in the high-dose group than the control diet.
"Reasonable explanations to consider in future studies range from the general impact of a diet rich in fruit on the acid-base balance and to the specific nutrient and/or polyphenolic compound content (140 mg/100 g dried plum) of dried plum," they said.
The researchers said that, despite the promising findings, "a significant amount of work remains". The most burning questions were concerning the identity of the bioactive components responsible for the apparent benefits, and the specific mechanism behind the effects.
"We recognize that in the end only the whole dried fruit with multiple bioactive components may produce the very promising results observed in this and other studies; however, understanding how dried plums and what in dried plums alter bone metabolism may lead to the discovery of a novel mechanism and therapeutic option," concluded Smith and her co-workers.