Dietary fibre is the name we use for complex plant based carbohydrate molecules that our intestines are not able to digest. Since we can’t digest fibre, it goes straight through the intestine after we eat it, and comes out the other end. And since it fills up the intestine, it contributes to a feeling a fullness. It can also bind up certain substances, so that they travel through the intestine and come out the other end, rather than being absorbed by the body. In theory, these effects should mean that increased intake of dietary fiber results in weight loss, at least if you’re overweight.