Which food cannot be digested by the body?

 The human digestive system is designed to break down most types of food into nutrients that can be absorbed by the body. However, there are some foods and substances that cannot be fully digested. One notable example is cellulose, a type of fiber found in plant cell walls. Cellulose is composed of glucose molecules linked together in a way that human digestive enzymes cannot break down. Although cellulose passes through the digestive tract largely intact, it plays a crucial role in aiding digestion by adding bulk to stool and promoting healthy bowel movements.

Another substance that the body struggles to digest is lactose, the sugar found in milk and dairy products. Many people lack sufficient levels of lactase, the enzyme required to break down lactose into simpler sugars. This condition, known as lactose intolerance, leads to symptoms such as bloating, gas, and diarrhea when dairy products are consumed. While lactose itself isn't completely digested, lactase supplements and lactose-free products can help manage these symptoms.

Certain artificial sweeteners like sorbitol and xylitol can also be poorly digested. These sugar alcohols are used in many "sugar-free" products. While they can be absorbed to some extent, they are only partially broken down and can ferment in the gut, leading to digestive discomfort such as gas and diarrhea. This happens because the body absorbs them slowly or not at all, causing them to reach the large intestine where they are fermented by bacteria.

Lastly, inorganic substances like metals or minerals, such as lead or mercury, are not digestible. These substances do not interact with digestive enzymes and cannot be broken down into usable nutrients. They can, however, accumulate in the body and cause serious health issues. For example, lead poisoning can disrupt various bodily functions, including neurological and developmental systems.