Our digestive system evolved over many millions of years to match the lifestyle of our ancestors, who hunted for meat or fish, and gathered seeds, fruits, roots and berries.

The change in our ancestors’ way of life to an agricultural and then industrial lifestyle has been so quick that the power of evolution to match the way our body works to the way we live has not yet had time to operate. We still have a digestive system designed to match millions of years of a hunter-gatherer existence.

Our bodies are designed to work their best using a variety of different types of food.

Animals like the cow, which evolved to eat only vegetable food, have developed a series of pouches, or extra stomachs, to separate one meal from the next. As digestion progresses food is moved from one to the next, or even brought back to the mouth for a further chew.

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Our digestive system is designed to cope in two distinct ways according to whether the food is mostly protein or mostly starch.

The process of making food available for our cells to use starts with cooking. Starches in cereals, pulses and tubers need to be softened and gelatinised by a period of moist cooking before they can be digested. Cellulose cell walls need to be ruptured by the cooking process before the contents are accessible to digestion. Meat fibres also need to be softened to improve their digestibility.

Cooking also destroys the toxins that many plants contain. For example red kidney beans require over-night soaking and then boiling rapidly for ten minutes before they are safe to eat.

Food in the mouth is broken down mechanically and mixed with salivary amylase. The amylase will only start the digestion of starch in a neutral solution. If the food is acid it will not work, if the starch is coated in fat, then digestion of the starch will be limited.

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The stomach produces gastric juice in response to the food entering the stomach. About three litres of juice are produced each day. The strong muscular walls mix the food with the gastric juices. The gastric juice contains much more of the enzyme pepsin and more hydrochloric acid when protein enters the stomach. This is a 0.2% to 0.4% solution of acid which is much stronger than acid foods. The acid provides the conditions needed for the digestion of protein as well as destroying most bacteria eaten with the food. A protein meal will remain between 2-4 hours in the stomach.

Ach to a state starch and vegetable meal will pass through the stomach very quickly without stimulating the production of large amounts of acid or pepsin.

Fat can bring the digestion in the standstill. The contents remain acid for a prolonged period and this can give rise to discomfort.

Small amounts of water, minerals such as sodium chloride, most B vitamins, vitamin C and alcohol are all absorbed directly from the stomach. Simple sugars are also absorbed and if the food contains too much sugar, this can result in a dangerous surge in blood sugar levels.

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The semi-liquid partially digested food (chyme) is allowed to move slowly into the small intestine. Here, bile from the liver emulsifies the fats into tiny droplets. An alkaline liquid from the pancreas neutralizes the stomach acid. In the neutral solution, the enzyme lipase splits the fats into fatty acids and glycerol, the enzyme amylase splits starch into maltose. The enzymes trypsin and chymotripsin continue to split proteins into short chain peptides and amino acids.

The final stages of digestion take place when the food is in contact with the cells that line the walls of the small intestine. The food molecules are absorbed by the cells and any remaining peptides are split into amino acids, maltose is converted to glucose, sucrose into glucose and fructose, lactose into glucose and galactose. Most of the absorption of digested food takes place through the surface of the small intestine, whose surface area is greatly increased by tiny finger-like projections called villa. Laxatives and excessive fibre interfere with the absorption of food, and aphetic acid in whole meal breakfast cereals can interfere with the uptake of calcium, iron and zinc.

The remaining cellulose and other fibre and indigestible remains now get processed by bacteria in the large intestine. These bacteria form some B-vitamins as well as useful amounts of vitamin K which, are absorbed through the walls of the large intestine.

All the products of the digestion of starch and sugars form simple sugars which are carried by the bloodstream directly to the liver. The liver keeps a close control of the level of sugar in the blood, supplying the tissues with-a constant level of glucose and storing the surplus as glycogen in the liver and muscles. When the glycogen reserves are full, further surplus sugar is converted into fat for storage.

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Fatty acids absorbed by the small intestine do not pass directly into the bloodstream, but are converted back into fats and added to the lymph fluid. This rejoins the bloodstream but bypasses the liver. The fat circulates in the bloodstream and is available as an energy source for the body cells. Fat in the diet is more easily converted into body fat than carbohydrate in the diet.

A protein meal of meat, cheese or eggs, is broken up by the teeth and passed down to the stomach. In response to the arrival of the protein, the stomach lining produces a strong acid, and enzymes which digest protein in acid solution, and a thick sticky mucus to protect the lining of the stomach from attack by its own enzymes. These enzymes snip the long protein molecules in our food into shorter chains and then into individual amino acids.

A meal of protein may take several hours for complete digestion before passing on into the small intestine. Here the stomach acid is neutralized by bile salts from the liver. The pancreas now adds further protein digesting enzymes to complete the digestion of protein and the soup of amino acids is now absorbed through the villi that line the small testing and are passed on into the blood stream.

A meal of starch is partly digested in the mouth where chewing mixes enzymes in the alkaline saliva with the food, starch molecule are split into sugar molecules. A starch meal does not stimulate the stomach to produce acid, or protein digesting enzymes, and the food is passed on to the small intestine within an hour. In the small intestine, further enzymes are added in an alkaline solution that complete the breakdown of starches into sugars. These are also absorbed through the villi that line the small intestine and pass into the blood stream.

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This system only works efficiently if starch and protein appear as separate meals. The food combining lifestyle puts this scientific knowledge into practice.

If starch is mixed with protein, the enzymes in the stomach are diluted and the stomach feels full for longer because digestion of protein is inefficient. Acid is being produced for every mixed meal, which puts the stomach under stress. The acid has to be neutralized by the bile salts for every meal, which puts the small intestine under stress.

Whole grain cereals contain starch with a little protein. They are digested comparatively slowly, but efficiently by the starch digestion process, but there are enough protein digesting enzymes in the small intestine to ensure the proteins as well as the starches are digested. The sugars that result from the digestion of starch are absorbed by the villa, and passed into the bloodstream for several hours. The liver is able to convert sugars not immediately required into glycogen for storage. If more sugar is absorbed than can be converted into glycogen, the surplus will be changed into fat.

White flour is almost pure starch. It mixes easily with saliva in the mouth and breaks down rapidly into sugars. As soon as the digested starch reaches the small intestine, a huge surge of sugars passes into the bloodstream. Insulin is produced rapidly to control it and the liver is put under stress to process the surplus. The body works flat out to remove the surplus from the bloodstream, and usually overshoots, leaving the bloodstream with too low sugar levels, and the person feeling tired and lacking in energy. These fluctuating blood sugar levels can put the body under too much stress and the system controlling blood sugar starts to break down. The result can be diabetes.

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Refined sugar is even worse because it requires no further digestion but enters the bloodstream in a dangerous surge.

Protein is digested to amino acids which are then absorbed into the bloodstream. These amino acids are then absorbed by all cells that require them for growth or the repair of tissue and are built up again into human protein.

All amino acids not needed for growth and repair have to be destroyed. The liver removes the amino group and converts the rest into sugar and then glycogen or fat. The amino group has to be converted into urea which is then removed from the blood by the kidneys.

Both the amino group and urea form an acid solution. Potassium and calcium are needed to neutralize this acid and are lost to the body. Excess protein in the diet puts the liver under stress. The loss of calcium may be made up by removing calcium from the bones and loss of potassium makes it more difficult for the body to balance the acid and alkaline substances in solution. The kidney has to deal with the acid end product of excess protein in the diet and is put under stress.

One protein meal a day provides most of us with all the protein that we need.