Consumption of a soy-food based diet, providing soy protein and isoflavones in combination with 10 grams per day of oligofructose-enriched inulin led to significant reductions in levels of LDL-cholesterol, according to results of a small randomised controlled crossover study published in Metabolism.
The LDL reductions were only observed when soy and prebiotics were co-ingested, an observation that suggests “the provision of fermentable substrates may be one means to increase the effectiveness of soy foods as part of a dietary strategy for cardiovascular disease risk reduction”, wrote the researchers led by David Jenkins from the University of Toronto.
The association between soy protein and blood lipid levels led the Food and Drug Administration (FDA) to approve a cardiovascular disease (CVD) reduction claim for soybean protein in 1999.
Hypercholesterolaemia has a long association with many diseases, particularly cardiovascular disease (CVD), the cause of almost 50 per cent of deaths in Europe, and reported to cost the EU economy an estimated €169bn ($202bn) per year.
“The maintenance of soy's status as one of a few cholesterol-lowering foods recognized by the FDA, therefore, appears warranted,” they added.
The study was supported financially by the Heart and Stroke Foundation of Ontario and The Orafti Group, which provided the prebiotic (Synergy1) used in the study.
Twenty-three people with an average age if 58 and average blood LDL levels of 4.18 millimoles per litre were recruited and randomly assigned to one of three groups: One group received a soy food–containing diet, providing 30 grams per day of soy protein, and 61 milligrams per day of isoflavones plus maltodextrin (placebo); the second group received the soy food diet plus prebiotic; the final group received a low-fat dairy diet plus the prebiotic. Two weeks separated each dietary intervention and 23 people completed all three phases.
The results showed that the joint consumption of soy and prebiotic produced greater reductions in LDL-C of around 0.18 mmol/L and improved the ratio of LDL-C to HDL-cholesterol, compared with only the prebiotic phase.
HDL-cholesterol levels were also significantly increased following the soy plus prebiotic diet, compared with only prebiotic.
No significant changes were observed in gut microflora levels, however, noted Jenkins and his co-workers.
“These data support the lipid-lowering basis for the current FDA health claim for soy foods. They demonstrate how a non-significant (about 3 per cent) LDL-C reduction seen when soy was consumed alone can be converted to a significant (about 5 per cent) LDL-C reduction when soy was taken with a prebiotic,” wrote the researchers.
Implications for health claims
The FDA’s health claim for soy is currently being reconsidered following the “relatively poor performance of soy in recent studies”, said the researchers.
“We believe the present study therefore supports the value of soy as one of the few cholesterol-lowering foods, in the 5 per cent reduction range, especially when given with fermentable substrates such as would be naturally present in diets that also contained viscous fibres to lower serum cholesterol,” they added.
Source: Metabolism Clinical and Experimental
Published online ahead of press, doi: 10.1016/j.metabol.2009.12.017
“The effect on the blood lipid profile of soy foods combined with a prebiotic: a randomized controlled trial”
Authors: J.M.W. Wong, C.W.C. Kendall, R. de Souza, A. Emam, A. Marchie, E. Vidgen, C. Holmes, D.J.A. Jenkins