Increasing the intake of soy isoflavones may have little effect on either LDL or HDL cholesterol, concludes a review by Dutch researchers.
Soy protein is thought to lower cholesterol and just last week a study found that eating a diet high in soy protein, as well as viscous fibre, almonds and plant sterols is as effective in managing cholesterol as taking a starting dose of cholesterol-lowering drugs.However scientists have hypothesised that differences in the isoflavone content of soy protein may explain the lack of a dose-response relationship between soy protein intake and blood cholesterol concentrations.
In a review of 10 controlled trials, carried out from 1995-2002, a daily dose of 36g soy protein with 52mg of soy isoflavones on average only decreased low-density lipoprotein (LDL) cholesterol by 0.17mmol/l. High-density lipoprotein (HDL) cholesterol only saw a 0.03 mmol/l increase, reported researchers in this month's European Journal of Clinical Nutrition.
The team from Unilever's Health Institute selected controlled trials using Medline searches from 1995 - 6 June 2002. They only used studies where the experimental diets differed in the amounts of soy protein and isoflavones and where diets were fed for at least 14 days. The research produced 959 subjects (336 men and 623 women), aged from 41 to 67 years with baseline cholesterol concentration from 5.42 to 6.60 mmol/l.
While the team concluded that there was no dose-response relation between soy-associated isoflavones and changes in LDL or HDL cholesterol, another study in the same journal suggests that the benefits associated with soy may not be entirely related to isoflavone content.
Researchers in Chile report that soy protein improved endothelial function, a risk factor for atherosclerosis, despite no evidence of change in cholesterol, in a small group of postmenopausal women.
During the trial, all patients followed a low fat/low cholesterol diet and were randomly assigned to receive isolated soy protein or control caseinate for four weeks, and then the alternative treatment for another four weeks.
LDL cholesterol was significantly lower at the end of the study, but there were no differences in effect between caseinate or soy protein interventions. However the soy protein produced significantly higher FMD compared to before the study and compared to the results seen after caseinate intervention.
The results suggest that in postmenopausal women with raised cholesterol, soy protein improves endothelial function, regardless of changes in plasma lipoproteins, said the team from the Pontificia Universidad Católica in Santiago.