Beans are the mature seeds of legumes. They have been cultivated for thousands of years and include a variety of different types such as kidney beans, soy beans, garbanzo beans (chickpeas), black beans, pinto beans, fava beans and white beans. Beans contain a lot of good stuff such as protein, fibers, vitamins and minerals. They also contain lectins. Though not all lectins are toxic, the ones found in beans can cause severe food poisoning. The presence of indigestible oligosaccharides has made beans notorious for their ability to cause the harmless but uncomfortable condition of intestinal gas. While ingesting raw beans can cause severe food poisoning the effect of lectins can be eliminated by certain preparation techniques that also serve to minimize the presence of gas-producing oligosaccharides. .
The Good Stuff
While beans vary considerably in shape, size, color and taste, their nutritional composition is somewhat similar. Typically high in fiber, vitamins and minerals, beans are also a great source of protein and make an excellent low cost food choice. The nutrient composition of selected beans is shown in table 1 and table 2.
Beans are the leading source of vegetable protein. They provide the full compliment of the 9 essential amino acids needed by our bodies, however they have relatively low amounts of the sulfur-containing amino acids methionine and cysteine [USDA Nutrient Database]. The protein content of most beans varies between 20-26%, while soybeans are exceptionally high with a whopping 36.5% protein (see table 1).
Dry beans are rich in fiber, which varies between 9-25% dependent on the type of bean (see table 1). Both soluble and insoluble fibers are present in beans [Slavin JL, 2013]. Soluble fibers dissolve in water and may bind to bile acids, which has been associated with lowering of LDL cholesterol (the bad type) [Anderson et al. 2009, Brown et al. 1999] and decreased risk of cardiovascular disease [Brown et al. 1999, Fernandez M-L, 2001].
Dry beans also provide substantial amounts of insoluble fibers. This type of fiber does not dissolve in water but helps to attract water to the stool resulting in increased stool bulk and shorter transit time in the gastrointestinal tract thereby helping prevent constipation.
Vitamins and Minerals
When it comes to vitamins, beans are an excellent source of the water-soluble B vitamins; B1 (thiamin), B2 (riboflavin), B6 and folate (see table 1). B1 and B2 play crucial roles in certain metabolic processes by participating in the conversion of carbohydrate into glucose, which is used by the cells of our body for energy [Powers HJ, 2003]. In its active form, B6 serves a co-factor in many enzymatic reactions related to the metabolism of proteins, carbohydrates and fats, and is also involved in the synthesis of neuro-transmitters [Selhub J, 2002]. Dietary folate intake has been associated with reduced risk of cardiovascular disease [Verhaar et al. 2002] and is very important during preconception or in early pregnancy to protect against neural-tube defects in infants [Bower and Stanley, 1989].
The micronutrient content of dry beans is somewhat similar with high amounts of the minerals; magnesium, phosphorous, potassium and zinc, and low amounts of sodium (see table 2). Compared to other beans, soybeans and white beans are especially high in iron. However, the iron bioavailability, that is the amount of iron that our bodies can absorb, is poor with a range of 1-3% [Petry et al. 2010]. This is caused by the presence of phytic acid and phenolic compounds, which can bind minerals such as nonheme iron (plane-based iron) in the gastrointestinal tract and form unabsorbable complexes thereby reducing their uptake [Brune et al. 1989]. In animals including humans, iron is usually attached to proteins called heme proteins, which have diverse biological functions such as transportation and delivery of oxygen to the cells in our body. In plants, iron is not attached to heme proteins and are called nonheme iron. Unfortunately, nonheme iron is absorbed at a lower rate than heme iron. The good news is that eating vitamin C in the same meal as nonheme iron significantly increases its absorption even in the presence of phytic acid [Hallberg L. 1981, Siegenberg et al. 1991]. Good sources of vitamin C include red peppers, parsley and broccoli [USDA Nutrient Database].
With the exception of soybeans, calcium absorption in beans is ≈ 22%, which is lower than that of cow’s milk (≈ 32%) and green leafy vegetables (≈ 60%) [Weaver et al. 1994, ]. However, calcium bioavailability from soybeans is quite good with ≈ 31% which is basically equal to that of cow’s milk [Weaver et al. 1993, Weaver et al. 1994].
In addition to protein, fiber, vitamins and minerals, beans contain several types of plant chemicals known as phytochemicals. Phytochemicals such as lignans and flavonoids are non-essential nutrients that have protective or disease preventing properties.
Lignans in beans are converted by the intestinal microbiota to bioactive compounds that acts as antioxidants by neutralizing free radicals. This in turn may reduce the risk of certain cancers and cardiovascular disease [Milder et al. 2004].
Flavonoids are water soluble pigments that are responsible for most of the red, blue and purple colors in plants. They represent an important class of antioxidants which may play a significant role in prevention of cardiovascular disease, cancer and atherosclerosis [Aparicio-Fernández et al. 2006, Takeoka et al. 1997]. Dark beans such as black beans and kidney beans are exceptionally rich in flavonoids mainly from from the anthocyanins group. .
The Bad Stuff
Packed with a lot of good stuff like protein, fiber, vitamins and minerals, dry beans sound like a pretty healthy food choice, so can’t they just be eaten raw? The answer is ‘NO!’ Never eat raw, soaked or undercooked beans. Raw beans contain toxic compounds known as lectins that depending of the type of bean can make you very sick. Only a few raw or undercooked kidney beans are enough to cause severe food poisoning.
Lectins, with the scientific name phytohaemagglutinins, are carbohydrate-binding proteins that can bind to cell membranes and cause clumping of the cells [Grant et al. 1983]. They occur naturally in raw legumes such as beans and lentils, and can also be found in nuts, grains and some fruits and vegetables [Nachbar and Oppenheim, 1980]. Lectins have different levels of toxicity, while most can cause harmful reactions in the body, not all are toxic. A high concentration of toxic lectins is found in raw, red kidney beans and ingesting as few as four or five beans can trigger severe toxicity symptoms [FDA]. The only thing that destroys lectins is cooking at temperature of minimum 100ºC [FDA].
Lectins seem to play an important role in the plant’s defense against insects and microorganisms [Lawley et al. 2012]. They are found at higher concentrations in those parts of the plant that are more susceptible to attack or parts that play key roles in survival of the plant such as the seeds [Peumans and Van Damme, 1995].
Ingested lectins from raw, undercooked or dry beans are able to resist the low pH in the stomach and enzymatic degradation, enabling them to survive digestion. In the gastrointestinal tract, lectins can interact with the gut epithelium causing damage to the cells. The result can be a series of harmful reactions such as severe gastroenteritis, interference with nutrient absorption, promotion of bacterial dysbiosis and immunological reactions [Lewis CA, 2014, FDA].
Onset of symptoms usually starts 1-3 hours after ingestion of raw, dry, soaked or undercooked beans and is usually marked by extreme nausea, followed by vomiting, abdominal pain, and diarrhea, which in some cases may require hospitalization [Lawley et al. 2012, FDA]. Symptoms usually clear 3-4 hours after onset and recovery is generally rapid and complete [Lewis CA, 2014, FDA]. .
The Ugly Stuff
The matter is subject to numerous jokes, nevertheless, it is a completely normal physiological process that everyone does on average 10 times a day – passing gas [Levitt et al. 1998]. It is commonly know, however, that some foods tend to increase people’s gas production. Beans in particular are notorious for their ability to cause this condition due to indigestible oligosaccharides found in members of the legume family.
Oligosaccharides are components of fibers made up of 3 or more sugar units (monosaccharides) that form long chains known as polymers. For the human body to digest oligosaccharides, the polymers have to be broken down into smaller monosaccharide units and absorbed in the small intestines. Unfortunately, our bodies lack the enzyme α-galactosidase capable of breaking down these types of polymers. This means that oligosaccharides pass undigested through the gastrointestinal tract to the the large intestines where they serve as food for the many bacteria living there. When bacteria digest the oligosaccharides several types of gases are produced including some odor-less ones such as carbon dioxide and hydrogen but also the rather foul-smelling gas methane. These gasses eventually built up in your large intestines and the only way to release them is by squeezing them out your behind, commonly known as passing gas.
For those who like eating beans but want to avoid the gaseous side effects, the commercial product “Beano” might do the trick. It contains the enzyme α-galactosidase derived from the fungus Aspergillus niger. When added to your beans, this enzyme will break down the oligosaccharides before they reach the bacteria in your large intestines thereby preventing the production of gas..
- Beans are usually soaked in cold water overnight i.e. 10-12 hours. Soaking beans prior to cooking reduces the cooking time, but it is not essential since it does not affect the lectins. Lectins are only destroyed by heat. If you soak your beans, it is important to store them in the fridge to prevent growth of harmful microorganisms.
- If you have soaked your beans, you need to discard the soaking water and replace it with fresh water. Indigestible oligosaccharides from the outer coating of beans are released into the water. Oligosaccharides can cause gas/flatulence, but this effect can be reduced by replacing the water [Han and Baik 2006]. The soaking water may also contain harmful dirt bacteria from the beans. Discarding the water, discards the bacteria.
- Boil in fresh water for approximately 45 minutes until the beans are tender. Cooking in water with a more alkaline pH (add some baking soda – not baking powder) may reduce the oligosaccharide content even further. Make sure the water is actually boiling (100°C) since only heating the beans to 80°C may increase the toxicity five-fold [FDA], making the beans more toxic than if eaten raw. Use of steam or slow cookers are not recommended for boiling beans since studies have shown that internal temperatures of slow cookers often does not exceed 75°C [FDA].
. Table 1. The nutritional composition of selected beans (values per 100 g raw beans), data adapted from [USDA Nutrient Database]. Bioavailability is not taken into account. *Daily values are developed by the Food and Drug Administration (FDA). They are standard values based on a energy intake of 8200 kJ (2000 calories), for adults and children four or more years of age. . . Table 2. The nutritional composition of selected beans (values per 100 g raw beans), data adapted from [USDA Nutrient Database]. Bioavailability is not taken into account. *Daily values are developed by the Food and Drug Administration (FDA). They are standard values based on a energy intake of 8200 kJ (2000 calories), for adults and children four or more years of age. Ca: calcium, Fe: iron, Mg: magnesium, P: phosphorous, K:potassium, Na: sodium, Zn: zinc.