Digestion: the system underneath everything else

You do not get the benefit of what you eat. You get the benefit of what you absorb. Those are different quantities, and the gap between them is digestion.

How the work is divided

Digestion starts in the mouth, where saliva begins on starch, and continues in the stomach with acid and enzymes. Most of the real work happens in the small intestine, where the pancreas supplies enzymes and the liver supplies bile. Bile matters especially for fat: without it, fat passes through largely unused, and the vitamins riding along with it go too.

Then the large intestine, where bacteria work on what is left - mostly fibre - and where water is reclaimed.

What helps

Fibre, from vegetables, fruit and whole grains, is what the large intestine runs on. Most people eat too little.

Water, because fibre without water makes things worse rather than better.

Regular timing. Digestion runs on a rhythm and does badly with erratic meals.

Chewing. Unglamorous, free, and skipped by almost everybody.

Movement. A ten-minute walk after dinner does more for digestion than most things sold for the purpose.

When it is not just digestion

Occasional indigestion is ordinary. Persistent pain, blood, unexplained weight loss, difficulty swallowing, or a change in bowel habit that lasts more than a couple of weeks are not - those need a doctor, not a dietary adjustment.