Like all legumes, red kidney beans are a nutritious high-protein food that has many health benefits associated with the vitamins, minerals and fiber it provides. However, if not cooked, as few as four or five kidney beans can trigger symptoms of severe toxicity and cause digestive distress.
Are Kidney Beans Poisonous?
Kidney beans, a variety of the species Phaseolus vulgaris L., contain high amounts of a protein compound called lectin, as noted in a small study of 20 young adult male rats in the December 2017 issue of Toxicology Reports. This substance performs various functions in both plants and animals, but some types of lectin, such as the phytohemagglutinin (PHA) found in beans, can be toxic at high levels.
Red kidney beans have the highest concentration of phytohemagglutinin (PHA) compared to other types of beans, according to the FDA's "Bad Bug Book." The digestive enzymes in your gut cannot break down PHA, so it travels intact through your digestive system. This allows it to bind to specific carbohydrates, which induces changes in the absorptive, secretory and protective functions of your digestive system.
The study featured in Toxicology Reports found that high intakes of PHA can damage the intestinal wall and affect internal organs, such as the small intestine, pancreas, liver and thymus. Furthermore, it may affect the immune system.
Illness from red kidney beans is related to eating raw beans, including fresh, soaked or sprouted forms. Kidney beans may also be toxic if used in flour or eaten after minimal processing, such as steamed, dehydrated or slow cooked.
How to Eat Beans Safely
Kidney beans can help you meet Health.gov's 2015-2020 Dietary Guidelines for Americans recommendations for daily intake of foods from both the protein and vegetable groups. So how can you eat beans and avoid the potential threat of toxicity they possess?
If you've ever prepared dry beans before, you know the drill: rinse, remove any small stones or debris, soak, cook and eat. It all seems pretty straightforward, right?
Well, it's the "cook" part that you need to take note of. There are right and wrong ways to cook these legumes.
Even after an overnight soak, it is important to boil kidney beans at a high temperature to destroy toxins. To deactivate PHA, boil them for at least to 10 to 30 minutes at 100 degrees Celsius, says the FDA. It also recommends an initial soak of at least five hours in water, which should then be discarded and replaced with fresh water for cooking.
If the beans are cooked at a lower temperature, such as in a slow cooker or combined with other ingredients in a casserole, the toxic effect is increased. Research from the Toxicology Reports has found that beans cooked at 80 degrees Celsius are five times as toxic as raw beans.
Symptoms of Kidney Bean Toxicity
When it comes to the danger of poisoning from kidney beans, it's good to know that canned beans are properly processed and safe to use immediately without causing unwanted symptoms.
However, inadequate cooking of as few as four or five raw kidney beans can constitute a toxic dose, as reported by the FDA. The onset of symptoms usually begins with upper and lower gastrointestinal illness and can occur within one to three hours of ingestion. These may include:
- Extreme nausea
- Abdominal pain
Symptoms are generally not severe enough to warrant a trip to the hospital. Recovery is usually rapid — within three to four hours after ingestion. In rare situations, severe cases require medical attention.
Red Beans Nutrition
As long as you cook them properly, you should take advantage of red kidney beans' health benefits and include these legumes in your diet. With very little fat and no cholesterol, 1/4 cup of red kidney beans provides 56 calories and 3 percent of the daily value for energy-producing carbohydrates, according to the USDA.
Red kidney beans are an excellent source of protein, with 3.8 grams per 1/4 cup. That's about 8 percent of the daily recommended intake for a 2,000-calorie diet. If you're a vegetarian or vegan, beans can be a good alternative to animal-based protein.
These legumes are packed with essential vitamins and excel in their vitamin B content, which your body needs for the proper functioning of muscles, nervous system, skin and brain. Per 1/4 cup, cooked red kidney beans offer the following B-complex vitamins:
- Folate (B9): 57.5 micrograms or 15 percent of the DV
- Thiamine (B1): 0.08 milligrams or 6 percent of the DV
- Vitamin B6: 0.05 milligrams or 3 percent of the DV
- Vitamin B5: 0.01 milligrams or 2 percent of the DV
- Riboflavin (B2): 0.03 milligrams or 2 percent of the DV
- Niacin (B3): 0.25 milligram or 2 percent of the DV
In addition to the B vitamins, kidney beans are a good source of vitamin K, which is needed for proper blood clotting. These legumes have 3.75 micrograms, or 3 percent of the daily value for vitamin K per serving.
They have a lot to offer in the mineral department as well. The essential minerals, per 1/4 cup of cooked kidney beans, include:
- Copper: 0.1 milligrams or 12 percent of the DV
- Manganese: 0.2 milligrams or 9 percent of the DV
- Iron: 1.3 milligrams or 7 percent of the DV
- Phosphorus: 62.8 milligrams or 5 percent of the DV
- Magnesium: 20 milligrams or 5 percent of the DV
- Zinc: 0.48 milligrams or 4 percent of the DV
- Potassium: 178.3 milligrams or 4 percent of the DV
Eat Beans for Fiber
An important health benefit of red kidney beans comes from their high-fiber content, with a 1/4-cup serving offering 13 percent of the DV. Fiber not only aids in digestion and normalizes bowel movements, but it may also help lower cholesterol levels, stabilize blood sugar and even reduce your risk of dying from heart disease and cancer, reports the Mayo Clinic.
If you are trying to manage your weight, eating kidney beans may help. Fiber, the indigestible part of food, helps slow down digestion, which makes you feel full longer. Being satiated after a meal of beans could help reduce snacking and decrease your overall daily calorie intake.
Evidence based on the assessment of 940 participants has found consuming beans reduced body fat and led to modest weight loss, even without restricting calories. The results of the meta-analysis, published in the American Journal of Clinical Nutrition in May 2016, noted that future studies are needed to determine the role of beans in long-term weight loss.
Read more: 10 Foods That Give You the Worst Gas
While it's recommended that you consume 25 to 33.6 grams of fiber a day, eating too many beans too fast — if you are not used to a high-fiber diet — may cause digestive discomfort. You may experience gas, abdominal bloating and cramping, among other symptoms. Introducing fiber into your diet gradually may help prevent these side effects.
- FDA: "Bad Bug Book: Handbook of Foodborne Pathogenic Microorganisms and Natural Toxins: Phytohaemagglutinin (Kidney Bean Lectin)"
- Toxicology Reports: "New Research Highlights: Impact of Chronic Ingestion of White Kidney Beans (Phaseolus vulgaris L. var. Beldia) on Small-intestinal Disaccharidase Activity in Wistar Rats"
- Health.gov.: "Dietary Guidelines for Americans, 2015-2020: Chapter 1. Key Elements of Healthy Eating Patterns: Vegetables: About Legumes (Beans and Peas)"
- MyFoodData: "Nutrition Facts for Cooked Red Kidney Beans"
- Mayo Clinic: "Dietary Fiber: Essential for a Healthy Diet"
- American Journal of Clinical Nutrition: "Effects of Dietary Pulse Consumption on Body Weight: A Systematic Review and Meta-Analysis of Randomized Controlled Trials"
- Health.gov: "Dietary Guidelines for Americans, 2015-2020: "Appendix 7. Daily Nutritional Goals for Age-Sex Groups Based on Dietary Reference Intakes and Dietary Guidelines Recommendations"
- "The Journal of the American Medical Association"; Cancer Incidence...; M. Ebbing, et al.; 2009
- University of Maryland Medical Center; Iron; June 2009
- The Vitamins and Nutriton Center; Too Much Iron; G. Obikoya