It is very common to lose a lot of weight in the early stages of this treatment and that weight loss may extend for a long period of time. The reason for this is that the digestive system is very inter-related to the illness of IC and the digestion and assimilation of nutrients is a fundamental problem for many patients. When sugar and junk food is removed from the diet, there is no longer an excess of calories in the body to keep the weight on. 

I have seen many people reach very low body weights, some for quite long periods of time. Then, what happens, is almost suddenly, the body seems to shift, just when the bladder seems to be making better progress in symptomatic improvement, and weight seems to come back on without any effort. These patients tend to gain back every pound they lost unless they were very overweight to begin with. In those cases, generally the body will find a new healthy weight to stabilize upon.

During the time in which the body is in the state of severe weight loss, it often does not matter how much one consumes, the weight just will not increase. This is very common in these cases. Again, it is not until the body seems to be getting into a better balance that the weight starts to increase.

Over the years I have had many phone calls and letters from family members voicing concerns over their loved ones who have become too thin. And, this is understandable, of course. Thus far, I have not seen any cases where someone's health was truly in any danger due to the weight loss. But, of course this must be watched to make sure those kinds of things don't happen.
 
Helping your family member struggling with this disease to eat healthy well rounded and plentifully portioned meals is the best thing that you can do for them. This will help them get the most use out of the calories that they are consuming and the nutrients that they need. Should there be concerns that go beyond these kinds of general recommendations, and a family member should ever want to discuss these kinds of issues to help their loved ones, then, permission needs to be given to me to speak to that family member and  time can be set up to discuss the details of that particular case.
 
However, I have seen smaller women's weight reach into the 80's over the years. And, no, I dont like that, but, it has happened. In every one of those cases, the weight has returned to normal without any additional intervention.

This came up recently, so I thought I would post for the benefit of everyone.