http://en.wikipedia.org/wiki/Intestine

In the small intestine (duodenum, jejunum and the ileum) is about 10 feet in length. Food coming from the stomach is continually mixed and stirred for further digestion. Absorption of nutrients, carbohydrates and fats then takes place all along the small intestines through finger-like structures called villi. The villi significantly increase the surface area of the intestines and therefore provide a large area for which all these “goodies” can then be absorbed. In addition to this, the small intestines may also perform other functions, for example, the ileum also has lymphoid tissue where white blood cells are available to destroy any harmful bacteria.

In the large intestines, water is reabsorbed and feces are formed. Some cells in the colon also secrete mucus to lubricate the feces and protect the intestinal lining.

If the intestines were short, it is unlikely that we’d be very efficient in absorbing all our nutrient and energy requirements from our food.

The length of our intestines does not rely on our diet, though the health of them does.