Hi, I am no particular expert in this diet, only kept it for a week, but this problem is stated in their website:
" Constipation seems to be the most common side effect of being on a VLCD. Often toilet habits change for a number of reason: as you are eating a lot less food there is less to be excreted so you may not need to 'go' as often as usual. However if it is difficult to pass motions then you may be constipated, this could be because you have not drunk enough water regularly and have become dehydrated. Increasing your water intake can help."
Personally, I used to "go" twice daily since forever, and since I started this diet it's once every 2-3 days.. But I wouldn't call it constipation to be honest, feels more like they say, that there is nothing much to excreted when you only have liquid food, and that in 600-700 ml per day...