WHAT HAPPENS TO CARBOHYDRATE IN THE BODY?
Before examining the health and fat loss benefits of carbohydrates it is essential to take a brief look at how carbohydrates are digested and absorbed in the human body. First we need to start with the gastrointestinal tract (GIT). The GIT is a long tube stretching from the mouth to the anus. The digestion of carbohydrate starts in the mouth, where enzymes in saliva start to break some of the bonds between saccharide units. After swallowing, carbohydrate enters the stomach ready to be released into the small intestine in small quantities. In the small intestine various enzymes perform the major part of carbohydrate digestion by breaking the bonds and releasing monosaccharides for absorption. Once absorbed, carbohydrate, in the form of glucose or fructose, goes to the liver where it is stored or put into the bloodstream as blood glucose for energy production or for storage in the muscles as glycogen.
Dietary fibre meets a somewhat different fate. Most fibre resists the digestive enzymes of the intestine and passes through to the large bowel. Here, some of it is metabolised by bacteria that produce the specific enzymes necessary for breakdown. This process is known as ‘colonic fermentation’. The end products include the gases carbon dioxide, methane and hydrogen along with volatile fatty acids called short chain fatty acids (SCFAs). The SCFAs are absorbed from the large bowel, enter the bloodstream and are transported to the liver. The degree of fermentation in the large bowel depends on the type of fibre and varies between individuals. This explains why some people produce a lot of gas and others produce very little following consumption of fibre-rich foods. Soluble fibres are entirely digested by bacteria and produce most of the SCFAs. Insoluble fibres are digested to only a small degree and the amount depends on the time spent in the large bowel before excretion.
The term resistant starch is a relatively new addition to the fibre scene. This is used to describe polysaccharides which are resistant to normal enzymatic digestion. This means that a portion of the starch eaten in foods will pass through the stomach and small intestine to the large bowel where it is fermented in a similar way to fibre. For this reason most nutritionists currently regard resistant starch as a component of dietary fibre. Resistant starch can occur naturally, such as in raw potato or bananas, be formed in partly milled grains and seeds, or can form following heating and cooling of cooked potatoes, bread and cornflakes. The approximate resistant starch content of specific foods. Food manufacturers have been able to incorporate resistant starch into white bread and other foods without affecting the sensory qualities of texture and flavour. This is a major advantage as consumers can eat foods that are higher in this fibre-like substance, but not radically different from more accepted forms.
This new dimension in carbohydrate physiology has stimulated another broad classification of carbohydrates. Nutritional scientists now use the terms ‘available’ (sugars and most starches) and ‘unavailable’ (resistant starch and fibre) when referring to the nutritional implications of carbohydrates.
*103\186\4*