Forty healthy adults eating a daily serving of the beans experienced an eight per cent reduction in cholesterol levels after 12 weeks, according to the report in this month's issue of the Journal of Nutrition .
"Taken alone, these data are intriguing, but put into the context of many previous reports of the lipid-modulating properties of dry bean consumption they show a pattern of substantial health benefits," wrote the authors, led by John Finley from Pennsylvania-based A.M. Todd.
The researchers related the benefits to changes in the chemical environment of the colon by increasing the formation of short-chain fatty acids.
However, these changes were not associated with any benefits for colon cancer risk.
Finley, in collaboration with researchers from Brainerd Veterans Administration Clinic and the USDA's Agricultural Research Service, randomly assigned the volunteers to consume either 130 grams of dried, cooked pinto beans, or chicken soup for 12 weeks.
In addition to the eight per cent reduction in total cholesterol levels, LDL (bad) cholesterol levels were also reduced.
No changes in triglycerides, VLDL cholesterol, or glucose were observed.
HDL (good) cholesterol levels were also reduced, a result that could not be explained by the authors.
"This study adds to a growing and convincing body of evidence that adding dry beans to the diet in quantities of at least 100 g/d changes lipid profiles in a manner associated with decreased risk of CVD," wrote Finley.
People with early symptoms of metabolic syndrome (MetS) could also benefit from the beans, report the scientists.
Forty volunteers with pre-metabolic syndrome (pre-MetS) also participated in the study, and a four per cent reduction in cholesterol levels was observed for this group.
Symptoms of metabolic syndrome include low serum levels of HDL-cholesterol (less than 50 mg/dL), high serum triglyceride levels (at least 150 mg/dL), elevated blood pressure (at least 130 over 85 mmHg), and glucose levels outside of tolerable levels (homeostasis).
The syndrome has been linked to increased risks of both type-2 diabetes and CVD.
The pre-MetS subjects participating in the new study were found to have four per cent reductions in cholesterol levels.
In terms of the chemistry of the colon, the researchers stated that propionic acid production increased when comparing levels at the end of intervention period to those measured at the start.
Propionic acid has well-characterised cholesterol-lowering properties, they said, and increases in levels of this short-chain fatty acid could account for the cholesterol drops observed in the volunteers.
"Although the change in propionate was modest, it may be physiologically significant, because the cholesterol-lowering properties of propionate are well characterised," they stated.
"Propionate, unlike acetate, is not a substrate for lipogenesis and increased propionate production has been reported to inhibit fatty acid synthesis." High cholesterol levels, hypercholesterolaemia, have 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.
Source: Journal of Nutrition November 2007, Volume 137, Pages 2391-2398 "Pinto Bean Consumption Changes SCFA Profiles in Fecal Fermentations, Bacterial Populations of the Lower Bowel, and Lipid Profiles in Blood of Humans" Authors: J.W. Finley, J.B. Burrell and P.G. Reeves