The small and large intestines.

There are great differences in the functions of these two organs.  Digestion of food and uptake of released nutrients is the major function of the small intestine.  Here food is processed and sent further to the blood and distributed to the rest of the body. 

The large intestine is concerned with cleaning up the leftovers, taking up water from digested food and getting wastes ready to send to the toilet.  A really important difference is that the large intestine is packed with bacteria.  These are usually kindly and helpful,  and aid in cleaning up leftovers from the food we eat. 

Let us look at an example of what can happen if sugars go into the large intestine.

(or "I get a tummy ache from milk").

Milk sugar or lactose is also a disaccharide, being made up of one glucose molecule coupled to a galactose molecule.  These are bound by a strong chemical bond.  However, The link between these two small sugars is unlike that we saw in table sugar.  It lies buried and partially hidden between the glucose and galactose molecules. We must have a dyed-in-the-wool specialized enzyme to split lactose and break that link.  The enzyme is called lactase.  It manages to release the glucose and galactose from lactose but does not attack other sugars.

Lactase is an enzyme that almost all of us are born with.  Production of this enzyme continues in the small intestine during our first three to five years of life.  After that, lactase production stops up.  Most of us continue through a long life without lactase production.  We still have sucrase (the table sugar splitting enzyme), but this cannot break the sugar link in milk sugar. 

What happens when we (I am one of them) milk-intolerant people drink milk?

If we look at the next figure we can see that, in the presence of lactase, sugar digestion and uptake goes just about as with sucrose and sucrase.  The disaccharide is split in the small intestine and the two small sugars are taken up there.  They do not reach the large intestine.  Everything is OK as long as you have lactase.

But, what about those of us who have little or no lactase? 

Those un-split lactose molecules continue to the large intestine.  Here they meet our kindly and very active bacteria.  They split the lactose and make acids and lots of GAS.   You can see from the drawing that the large intestine becomes swollen and full.  OU!!  That hurts!

The bottom line here is that normally, sugars do not reach the large intestine.  When they do, discomfort will result since bacteria  split and ferment all of the usual sugars.  This leads to accumulation of water and bulk in the large intestine.  At the same time there is a lot of gas produced   The result is pain and diarrhea. 

All sugars have the same effect if they come to the large intestine.  Kids who eat too much candy or drink a lot of cola when they have a "stomach ache" often experience diarrhea and an increased tummy ache.  Too much table sugar (sucrose) has the same effect as milk sugar (lactose) in people without lactase.

Some adults do drink milk.

Scandinavians and several tribes in Africa have experienced genetic mutations which keep the gene responsible for lactase production active throughout life.  Most adults in Norway, Sweden and Denmark can drink milk without problems.   In other areas, particularly Asia and most of Africa, grownups usually do not tolerate fresh milk.  World-wide, more than 50% of all adults experience milk-intolerance. 

This does not mean that older people cannot drink milk products.  We simply let yeast or bacteria digest the milk for us.  Culture milk, sour cream and yoghurt are examples of milk products that people who lack lactase can consume without problems.  Here the lactose is split during preparation of the milk products before we consume them.