Lettuce is a plant, and as such contains fibre. Normally a large proportion of the water we get in our food is absorbed by the large intestine back into the bloodstream. When too much of this water is absorbed, the stool can become hard and constipation results. Fibre is an essential part of a balanced diet because it can hold water in the large intestine, making the stool softer and making it easier to have a daily bowel movement. Too much fibre, however, can cause too much water to be retained by the intestines and result in diarrhoea, which can be associated with stomach cramps. When too much iceberg lettuce is consumed, or when you are not used to eating much iceberg lettuce, this can lead to too much liquid in your intestines and the potential for stomach cramps increases. Other types of lettuce contain even more fibre than iceberg lettuce, so if you have difficulty tolerating iceberg lettuce, other types of lettuce may affect you more severely.
Iceberg lettuce can also cause stomach cramps by increasing the amount of gas in the intestines. Millions of bacterial cells live within our intestines. These bacteria are beneficial because they help digest the food we eat. Bacteria tend to be less efficient at digesting bacteria than other forms of food and can produce gas during their digestion process. This gas builds up within your intestines and can cause stomach cramping, belching and flatulence. Air can also enter your digestive system during the eating process.
Another possible reason for experiencing stomach cramps after eating iceberg lettuce is that the lettuce was contaminated with a disease-causing bacteria, such as E. coli. Because lettuce is grown in the dirt ,which can be fertilised with manure containing intestinal bacteria such as E. coli, if the lettuce is harvested improperly or not cleaned well, the bacteria can cause you to become ill. For this reason, it is important to thoroughly wash all the iceberg lettuce you eat, even if the label states it has been pre-washed.