Sucrose is the chemical name for table sugar. It is the white crystalline powder that most people put in their tea or coffee. Sucrose is chemically a disaccharide sugar, which means that it is composed of two monosaccharides joined together through a glycosidic (sugar) bond. The two monosaccharides that make up sucrose are glucose and fructose which are both hexose sugars (hexoses contain 6 carbons). When sugar is consumed, the enzyme sucrase, that is secreted from enterocytes, cleaves the glycosidic bond in a hydrolysis reaction, which means that water is added to the molecule to allow the two monosaccharides to split. The glucose is then absorbed to the blood where it contributes to blood sugar and is a source of energy for all cells. In contrast the fructose is sent to the liver to be processed, and here it can either be converted to glucose and subsequently used as energy or stored as liver glycogen, or alternatively it can enter the de novo lipogenesis pathway to be converted to fatty acids.
Evidence shows that the sugar intake of the developed nations has risen steadily since the industrial revolution, with a large increase in the last 50 years. Nutritionists and scientists now consider sugar consumption to be too high, and the available evidence suggests that this high intake is a major contributory factor in the development of Western lifestyle diseases including obesity and its associated secondary diseases. Sugar may lead to the development of Western lifestyle diseases because it can cause insulin resistance and the development of the metabolic syndrome. The metabolic syndrome is characterised by a number of metabolic changes to normal energy regulation, including changes to plasma lipoproteins. That sugar can increase plasma levels of triglycerides and cholesterol has been well reported. Feeding starch at 30 % of energy does not cause changes to plasma lipids, but exchanging that starch for an equal amount of sucrose causes increases in cholesterol and triglycerides in a just few weeks in human subjects1.
Further research has also revealed that feeding that 47 % of energy in the form of glucose to rats does not cause change to plasma lipid levels. However, when the 47 % of energy is fed in the form of fructose, there is a rapid and large increase in the plasma cholesterol levels of the animals. Therefore the detrimental effects of sugar are likely due to the fructose component2. If we have a depleted glycogen store because we have awoken from an overnight fast or have been performing physical activity, the fate of the fructose is likely to be a replenishment of that glycogen. However, if the glycogen store is full, or if we consume too much fructose such that it fills the glycogen stores, the excess must spill over to produce fatty acids. These fatty acids are then joined to a molecule of glycerol to form a triglyceride molecule, and this is exported to the plasma as a very low density lipoprotein (VLDL) particle, and this is subsequently converted to the low density lipoprotein (LDL) particle in the blood.
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