Scientists at the US-government funded Agricultural Research Service studied the impact of both a meat protein and soy protein diet on 13 postmenopausal women. Protein made up 15 per cent of the daily calories, a 'very typical' amount in the average US diet.
The findings that "calcium was similarly retained during the control and soy diets" provides further evidence that the protein does not damage bone health, countering a commonly held belief that high-protein diets can leach calcium from bones.
However the results, published in the January issue of the Journal of Clinical Endocrinology & Metabolism (vol 90, issue 1, pp181-9), may also be disappointing for soy ingredient firms researching the potential benefits of isoflavones on bone health.
Soy isoflavones are currently under trial as a potential supplement for women trying to prevent osteoporosis, one of the leading global healthcare problems. But there have been conflicting findings on this area to date.
Lead author of the new study, Fariba Roughead, told NutraIngredients.com: "We used a high isoflavone soy protein, the type thought to have benefits on bone health. But we saw absolutely no evidence of benefits."
The study subjects followed either a meat protein or soy protein diet (25 grams of high-isoflavone soy protein was substituted for an equivalent amount of the meat protein) for seven weeks before crossing over to the other regime after a two-week break.
Each diet contained typical daily intakes of calcium and other nutrients.
The scientists measured biomarkers in blood and urine collected during each seven-week diet phase and found no indications of differences in calcium or bone metabolism after either diet.
Some researchers have theorized that the phytate found in soy protein can interrupt mineral absorption. But the soy-protein diet did not change the excretion nor absorption of calcium.
"If benefits previously seen on bone health come through another mechanism, we may not have had the power to see it," added Roughead.
"But we also tested other bone health markers and found no beneficial effect. It is hard to know where the benefit could come from."
A study on more than 200 Chinese women in 2003 found that soy isoflavones had a mild but significant effect on the maintenance of hip bone mineral content in subjects with low initial bone mass.
Recent evidence also suggests that high levels of soy isoflavones (around 80 mg per day) may be required to have any benefit on bone health.