You are not what you eat. You are what you can digest and absorb.

The fundamental design of the human body is a tube - a doughnut with a hole in the middle.

We, like other animals, spend our physical lives processing organic matter for waste. How good you are at this determines your energy level, longevity and state of body and mind, as well as your digestion.

Over a lifetime, no less than 100 tonnes of food passes along the digestive tract and 300,000 litres of digestive juices are produced by the body to break it down.

Our 'inside skin', a 9m-tract with a surface area the size of a small football pitch, is only the thickness of a quarter of a sheet of paper. Amazingly, most of the billions of cells that make up this barrier between us and the inside world are renewed every four days.

If you suffer from indigestion, bloating, abdominal pain or feeling sleepy after meals, or often get stomach upsets, diarrhoea or constipation, there are four simple steps you can take to tune up your digestion.

Your 'inner skin' gets easily damaged - alcohol, antibiotics, food allergens and painkillers (the average person takes 300 a year) - are the most common culprits.

The result is that the digestive tract becomes more permeable, and whole food proteins which are not on the guest list, so to speak, get through into the bloodstream instead of being broken down into amino acids.

Then your immune system attacks. That is the basis of food allergy.

You do not have to be allergic to milk for life.

By avoiding your current food allergens - wheat, milk and yeast being the most common - you give your digestive system a break.

The good news is that most food allergies are not for life. If you remove the offending item strictly for four months, then heal the gut, you can lose your sensitivity to foods. (There is a more severe and immediate 'IgE' allergy which lasts for life, but these are less common.)

Another way to lessen the load on your digestive system is to take a digestive enzyme with each meal.

These enzymes, called protease, amylase and lipase, literally help digest your food. If you instantly feel better, you know you have got a problem with your digestion.

If you get bloated after lentils or beans, choose an enzyme that contains glucoamylase. Next, you can help rebuild your digestive tract by feeding it glutamine. While the rest of your body runs on glucose, these rapidly repairing cells can run on this amino acid.

Having a heaped teaspoon (5g) last thing at night in a glass of water is like a visit to the health farm. Do this every day for a month, or after any kind of infection, alcoholic excess or course of antibiotics.

There are more bacteria than living cells inside your body. They flourish in a healthy digestive tract and die off in an unhealthy one.

So, once you have improved your digestion, 're-inoculating' your digestive tract with exactly the right strains of bacteria makes a big difference.

These are called 'human strain acidophilus and bifido bacteria' and work much better than dairy-derived strains found in yogurt.

Again, having a capsule or powder for up to 30 days is all you need to get your inner flora flourishing.

Then there is maintenance. What damages your digestive tract the most is too much alcohol, deep-fried food, burnt meat, coffee and wheat.

Wheat contains something called gliadin, not in oats or rice, which irritates many people's insides. Fresh fruit, vegetables and soluble fibres found in oats and vegetables, plus plenty of water helps digestion, as do chewing and not eating when you are stressed.

DISCLAIMER: Any information or advice given in this column is not intended to replace the advice or services of your physician. The writer's views are his own and do not represent that of the publication.

30-day action plan for healthy digestion

• Avoid wheat, milk or yeast (in beer but not in spirits, in bread but not in pasta)
• Take digestive enzymes, available in any health food store, with each main meal.
• Take a heaped teaspoon of glutamine powder last thing at night to improve the integrity of your digestive tract.
• Re-inoculate your gut with beneficial bacteria by taking a capsule or powder of human strain acidophilus and bifido bacteria.
• Eat lots of vegetables, fruit and fish, less deep-fried food, alcohol, coffee and wheat.
• Drink eight glasses of water every day. You really need it. Dehydration is the most common cause of constipation.

This 30-day tune-up is like a trip to the health farm for your insides.

As one Harvard professor of gastroenterology once said: 'Having a strong stomach and good set of bowels is more important to human happiness than a large amount of brains.'

Patrick Holford is the founder of the Institute for Optimum Nutrition in London and heads the Food of the Brain Foundation, pioneering nutritional approaches to mental health. He has written more than 20 health books, including the worldwide bestseller, the Optimum Nutrition Bible.

This story was first published in the Mind Your Body supplement on Jan 23, 2008.