Adequate fibre in the diet is essential to prevent constipation, keep your digestive system regular and healthy. Together with increasing your fibre intake, you need to increase you intake of fluid.
Drink lots of water daily, at least 2 litres or more depending on how much you are exercising. Check with your doctor for your specific fluid allowance.
Your doctor should also advise you on taking a good probiotic and optimizing fluid intake will help you digest and tolerate fibre in your diet.