Black Beans Help Fix Insulin Resistance and Gut Bacteria Balance

USDA ARS — Adding cooked black beans to a high-fat diet improved sensitivity to insulin and other measures often related to diabetes and restored gut bacteria balance in obese mice, according to a USDA Agricultural Research Service study.

As little as the mouse-size equivalent of a single serving a day of black beans—about a half cup for a human—lowered insulin resistance 87 percent in obese mice compared to obese mice eating the same high-fat diet without the black beans. Insulin resistance is when a body’s response to the hormone insulin is impaired so glucose in the blood cannot be used for energy, resulting in high blood sugar, a factor often leading to diabetes.

Mice on the high-fat plus black beans diet also decreased low density lipoprotein (LDL) cholesterol, the so-called bad cholesterol, 28 percent and triglyceride levels 37 percent compared to mice eating the high-fat diet without black beans. These are both risk factors for cardiovascular disease.

Other diabetes-related biomarkers such as the levels of leptin, glucagon, and a group of inflammatory biochemicals were all significantly better in the mice on the high-fat plus black beans diet.

The researchers also found that adding black beans to the high fat diet restored the balance of healthier bacteria in the gut, particularly decreasing the ratio of Firmicutes bacteria to Bacteroidetes bacteria in the gut by 64 percent compared to mice on the high fat diet without black beans  and mice on a low fat diet. High ratios of Firmicutes to Bacteroidetes are associated with obesity. Intestinal bacteria associated with inflammation such as Blautia and Clostridium all were significantly reduced in mice fed the high fat plus black beans diet compared to mice on the high fat diet without beans.

“This research suggests that eating even a small amount of black beans can have multiple health benefits,” said ARS research chemist Wallace Yokoyama with the Healthy Processed Foods Research Unit of the Western Regional Research Center in Albany, California. Yokoyama led the study, which was published in the scientific journal Foods.

“We also tested if supplementing the high fat diet with individual components from black beans would have the same beneficial impacts on the obese mice and didn’t find the same effects at all. It was only adding whole black beans, and cooked whole beans at that, which had the benefits,” Yokoyama said.

Perhaps the most interesting scientific information coming from this study, according to Yokoyama, is data to begin determining just how black beans improve insulin resistance. It appears that black beans may inhibit the JNK/c-Jun pathway, a key metabolic pathway that has many but not necessarily well-defined functions including regulating inflammatory responses. Chronic inflammation is believed to be the basis for insulin resistance and other metabolic diseases.

Black beans, or more precisely black turtle beans (Phaseolus vulgaris), are generally low in fat and high in fiber and protein. They are popular in Latin American, Mexican and Caribbean cuisines as well as in Cajun and Creole cooking. Like all common beans, black beans are native to the Americas. Today, they have been introduced around the world to become known as frijoles negros or poroto negro in Spanish, feijão preto in Portuguese, and karuppu kaaramani and kala ghevada in various regional cuisines of India.

The Agricultural Research Service is the U.S. Department of Agriculture’s chief scientific in-house research agency. Daily, ARS focuses on solutions to agricultural problems affecting America. Each dollar invested in agricultural research results in $17 of economic impact.

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