An uncle who is interested in health food once told me that they are not the same thing, and that your body needs both. So what exactly is the difference between dietary fiber and roughage?
Perhaps he was referring to soluble vs insoluble fibre?
First, dietary fibre can be classified into soluble and insoluble fibre. Fibres are polysaccharides (chains of sugar monomers) of varying lengths.
Soluble fibre are shorter chained polysaccharides (5-10 units) - gums and pectin. These are water soluble. They are digested, broken down into glucose (a monomer), by enzymes in the small intestine to give you slow release energy. Complex carbohydrates (the more branched the more complex) are also soluble fibre.
Resistant starches are longer still (1000’s of sugar units). They do not break down in the intestine, they are broken down in the colon. These carry smaller fibres along with them. There is good evidence that resistant starches, found in pasta, rice and potatoes, reduce the risk of colon cancer. They ferment to give short chain fatty acids and are great for making farts.
The longest - insoluble fibre (aka roughage) such as cellulose, which is many 1000’s of saccharide monomers long, are not digested in humans. These carry smaller fibre (resistant starches) along with them, make your shit stick together and keeps you regular. You can also make t-shirts out of it.
When your uncle talks of fibre he must be referring to soluble fibre, as ladydisco alluded.