"This is the first controlled feeding study in humans to evaluate the effect of dietary plant-derived omega-3 PUFA on bone turnover, assessed by serum concentrations of [the markers of bone resorption and formation] N-telopeptides (NTx) and bone-specific alkaline phosphatase (BSAP)," wrote lead author Amy Griel from the Penn State Univerisity.
Previous studies have reported that diets with a low ratio of omega-6 fatty acids to omega-3 fatty acids may minimise bone loss, but the studies have mostly focussed on improving omega-3 using the longer-chain omega-3 fatty acids from fish, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).
Writing in the Nutrition Journal, Griel and her co-workers report the results of their randomised, double-blind, balanced order, three-period crossover study with 23 overweight, people (20 men, average age 49.9, average BMI 28.1 kg per sq. m) with moderately high cholesterol levels (5.85 millimoles per litre of serum).
The subjects were randomly assigned to one of three diet groups for six weeks, with three-week washout periods between diet interventions.
The first diet intervention saw the subjects consumed an average American diet (AAD) consisting of 34 per cent total fat, 13 per cent saturated fatty acids (SFA), 13 per cent monounsaturated fatty acids (MUFA), and 9 per cent polyunsaturated fatty acids (PUFA, of which 7.7 per cent was the omega-6 linoleic acid (LA) and 0.8 per cent was the plant omega-3 ALA).
The second diet intervention was the omega-6 rich linoleic acid diet, consisting of 37 per cent total fat, 9 per cent SFA, 12 per cent MUFA, 16 per cent PUFA, of which 12.6 per cent was LA, and 3.6 per cent was ALA).
The third diet intervention was the omega-3 ALA-rich diet, providing 38 per cent of energy from total fat, 8 per cent from SFA, 12 per cent from MUFA, and 17 per cent PUFA, of which 10.5 per cent was from LA, and 6.5 per cent from ALA.. Flaxseed oil and walnuts were the main sources of ALA.
At the end of all the three intervention diets, the researchers report that mean concentrations of the bone resorption marker NTx were significantly decreased following the ALA diet that the LA and AAD (13.2 versus 13.8 and 15.59 nanomoles bone collagen equivalents (BCE), respectively).
No changes in the marker for bone formation, BSAP, were observed.
Levels of tumour necrosis factor alpha (TNF-alpha), a marker for inflammation, was also found to decrease more after consuming the ALA-rich diet than the other two diets: 10.3 nanograms per litre for ALA, compared to 13.3 and 18.2 for the LA and AAD diets, respectively.
"The results indicate that plant sources of dietary n-3 PUFA may have a protective effect on bone metabolism via a decrease in bone resorption in the presence of consistent levels of bone formation," said the researchers.
The mechanism behind the benefits appears to be the ratio between omega-6 to omega-3 fatty acids, they said. Previous research has reported that omega-6 fatty acids are converted into pro-inflammatory prostaglandins, while omega-3 fatty acids are metabolised into anti-inflammatory prostaglandins.
Further studies are necessary to further clarify these relationships, they said.
"The present results suggest that incorporating plant sources of n-3 PUFA into the diet may provide health benefits not only to the cardiovascular system, but also to the skeletal system," concluded the researchers.
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.
Source: Nutrition Journal January 2007, Volume 6, doi:10.1186/1475-2891-6-2 "An increase in dietary n-3 fatty acids decreases a marker of bone resorption in humans" Authors: A.E. Griel, P.M. Kris-Etherton, K.F. Hilpert, G. Zhao, S.G. West and R.L. Corwin