Black beans (Glycine max) is a type of soybeans native to East Asia usually fermented for cooking, soy sauce or roasted to eat as snacks. Its nutrition is similar to that of the yellow soybeans.
In the five elements, the kidney is subsumed to water and the colour is black. This means that the black beans are the "grains of the kidney" because of its black colour. It is believed that black beans can tonify the kidney and calm the mind, improve eye sight, clear heat, invigorate blood circulation, reduce swelling and detoxification. It can descend qi to eliminate water retention because of magnesium and potassium, which have been found to decrease blood pressure. In Chinese Medicine, a strong kidney qi is the key to healthy bones, which is why we say tonify the kidney in order to strengthen bones. Black beans contains iron, magnesium, copper manganese and vitamin k which are important for healthy bone growth and strength.
Although black beans can tonify the kidney, it is also a diuretic for ridding swelling and water retention. Too much diuretic can stress the kidney organ. Beans are generally high in protein but when it is broken down to waste it requires the kidneys to help eliminate it from the blood. Consuming too much black beans may not be a good option for people with chronic kidney disease.