If you have decreased production of lactase and then something else happens to compromise the integrity of the brush border, it cases further reduction of lactase production. If you continue to take in lactose, that causes more irritation and loss of lactase production. It becomes a vicious negative feedback cycle.
When you are suffering from celiac sprue, there is damage to your intestinal villi. This can make one temporarily lactase deficient to the point where lactose becomes a problem also. This happened in my case. Once I started on the gluten-free diet and my intestines had healed, lactose was no longer a problem for me. I can eat any diary product now with no problems.
Not everyone will be so lucky. A lot of people will remain lactase deficient. Yogurt and aged cheeses are more easily tolerated because some of the lactose has been converted to lactic acid. One rule of thumb is that the higher the fat content of the dairy product, the lower the lactose level. People who are still producing some lactase would then be able to eat a very rich ice cream but would be bothered by skim milk or ice milk.
I highly recommend the book "No Milk Today: How to Live With Lactose Intolerance" (Steve Carper, Simon & Schuster, Inc., 1986 ISBN 0-671-60301-0). I found it at my local library. It's an excellent book for explaining the process, describing hidden sources of lactose (like whey), and tips on eating out. In regards to the fat content and lactose level I quote from page 119 of the book:
>Foods with high milk fat tend to be lower in lactose than other milk
>products. Heavy cream is lower in lactose than light cream, which in
>turn is lower than whole milk. Butter is higher in fat than any of
>these, and in turn is the lowest in lactose.
It was mentioned that aged cheeses are lower in lactose than unaged cheeses because the lactose had been broken down during the aging process.
So, even though lactase deficiency and gluten intolerance can give the same symptoms, they are not caused by the same processes. Lactase is composed of two sugars. The problems arise when you are unable to break it into its two parts and absorb them.
Gluten is a protein. It seems to cause a problem due to an immunological response as far as I know, there is no similarities between them.
I hope this is helpful.