Kidney beans are rich in protein. One cup of boiled kidney beans (177 g) contains approximately 15 grams of protein, accounting for 27% of the total caloric content Although the nutritional quality of bean proteins is lower than animal proteins, beans are an affordable alternative for many people in developing countries. In fact, beans are one the richest plant-based sources of protein, sometimes referred to as "poor man's meat" The most widely studied protein in kidney beans is phaseolin, which may cause allergic reactions in susceptible individuals Kidney beans also contain proteins, such as lectins and protease inhibitors
Kidney beans are mainly composed of carbs. Carbs in kidney beans are known as starch, which accounts for approximately 72% of the total calorie content . Starch is predominantly made up of long chains of glucose, called amylose and amylopectin. Beans have a relatively high proportion of amylose (30-40%) compared to most other dietary sources of starch. Amylose is not as digestible as amylopectin. For this reason, bean starch is a so-called slow-release carbohydrate. Its digestion takes longer and it causes lower and more gradual rise in blood sugar than other types of starch, making kidney beans particularly beneficial for people with diabetes. Kidney beans rank very low on the glycemic index, which is a measure of how foods affect the rise in blood sugar after a meal. In fact, bean starch has a more beneficial effect on blood sugar balance than many other high-carb foods.