Why do beans and other vegetables produce gas in our digestive tract?
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Legumes are plants that have a single row of seeds in their pods. What we commonly call legumes, such as peas, green beans, lima beans, pinto beans, black-eyed peas, garbanzo beans, lentils, and soybeans, are often the seeds of legume plants. Relatively short carbohydrate chains (oligosaccharides) such as stachyose, raffinose, and verbacose are found in legumes as well as broccoli, Brussels sprouts, cabbage, asparagus and other vegetables as well as whole grains. These carbohydrates are unique because they contain the dissacharide sucrose linked to one or more galactose molecules. |
People (like pigs and chickens) don’t produce the enzymes (e.g. alpha-galactosidase) necessary to efficiently break down stachyose, raffinose, and verbacose. So, similar to lactose in lactose intolerant people, these carbohydrates remain intact in our small intestine and move into the colon. In the colon, gas producing bacteria breakdown (ferment) these carbohydrates producing gases methane (CH4), CO2 and H2 which lead to bloating, cramping, and flatulence. A product available in stores called Beano is an enzyme preparation (including alpha-galactosidase) that will digest these carbohydrates when it is ingested just prior to the legume-containing meal.
Once monosaccharides are absorbed, where do they go?
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Monosaccharides (glucose, fructose, and galactose) are absorbed into the body by crossing the wall of the small intestine and entering circulation via a special blood vessel called the portal vein. As the portal vein carries blood from the digestive tract directly to the liver, the liver gets the first shot at the absorbed monosaccharides. The liver is able to pull most of the galactose and fructose from our blood as well as a respectable portion of the glucose (see Monsaccharide Absorption Figure). However, much of the glucose continues past our liver and enters the general circulation where other tissue will have a shot at it. This increases the concentration of glucose in the blood from a normal or “fasting” level of 70 to 100 mg to 140 mg of glucose per 100 mL of blood or higher. |
How does our body respond to the rise in blood glucose?
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The concentration of glucose in the blood is very tightly regulated. When the level of circulating glucose climbs above the normal fasting level, the pancreas releases the hormone insulin (see Hormones/Metabolic State Figure). Insulin will interact with receptors on muscle cells and fat cells and promote the movement of glucose into these cells (see Monosaccharide Absorption Figure). Because skeletal muscle and fat cells together tend to make up more than half of our total body mass, the net effect is a fairly rapid lowering of the glucose concentration. Insulin increases the movement of glucose in these cells by increasing the number of glucose transport proteins on their plasma membranes. As the level of glucose returns to the normal fasting level the pancreas responds by releasing less insulin into circulation. |
All cells in our body will continuously take glucose from our blood throughout the day to help meet their need for energy. However, after a meal, the liver, muscle, and fat cells will take a lot more glucose out of the blood than they immediately need. This allows blood glucose levels to return to a normal fasting concentration.
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