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The science of nutrition and healthy eating
The science of nutrition and healthy eating

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4.1 Sugars

Like saturated fats, sugars are singled out for special mention on food labels. Again, as you might have guessed, this is for health reasons.

An image of two men at a roadside fair stall in India selling sugar cane juice in cups.
Figure 11 Sweet drinks are popular everywhere – this fair stall in India is selling sugar-cane juice

We are advised not to eat more than 90 g of sugar per day. There are several different sorts of sugar. So far, glucose has been mentioned as one type. The sugar that is obtained by extraction from sugar cane or sugar beet is another sort called sucrose (Figure 11). Different methods of purification and processing of the sugary liquid from the cane or beet give us the various sorts of sugar (sucrose) that we buy (granulated sugar, brown sugar, etc.). Other sugars also occur naturally in fruit (fructose) and milk (lactose).

We have taste buds on our tongues that are sensitive to sugar and most people like sweet things. However, foods that are high in sugar may have very few other nutrients. They are also high in energy, so eating too much sugar can contribute to obesity. The sucrose that we eat is digested in the body and broken down into glucose, which circulates in the blood.

When the level of blood glucose rises after a meal, or after eating a sugary snack, the hormone insulin (which you met in Protein sequencing) is released. This has two effects. One is to stimulate cells in the muscles and liver to take up the extra sugar and store it as glycogen, thus safely removing it from the bloodstream. The other function is to stimulate fat cells to take up fat from the bloodstream and store it, rather than breaking it down for energy. So, there’s the link – eat too much sugar, and you risk putting on weight by storing fat.

There is an even clearer link between the intake of sugar and tooth decay. Sugar in the mouth is an ideal source of food for the bacteria that normally live there, in the moist film of saliva covering the teeth and gums. These bacteria, which form a layer called plaque, produce lactic acid and enzymes that digest proteins. The plaque can keep the acid in contact with the tooth surface for up to two hours before it is neutralised by the saliva.

Over time, the acid gradually dissolves the calcium salts in the tooth enamel, and the enamel protein is destroyed by the bacterial enzymes. Despite the resistance of the enamel, eventually the surface is breached and the underlying softer dentine is dissolved and cavities form in the teeth. The problem is made worse by ‘snacking’ on sugary items between meals. This increases the amount of time that the teeth are surrounded by sugar which the plaque bacteria can feed on, leading to tooth decay.