Asked by: Holly Whelan, Crewe
Answered by: Dr Jeya Henry, Professor of Human Nutrition and Director of the Functional Food Centre, Oxford Brookes University
Sugar is a catch-all term that covers a number of chemically distinct substances. The three main sugars are lactose, fructose and sucrose. The latter is the stuff we put in our tea and is what we usually mean when we talk about sugar. Sucrose is a disaccharide molecule, containing both glucose and fructose molecules which are fused together by glycosidic bonds. It is these bonds that give sugar its shiny, crystalline appearance.
We need a constant supply of sugar in our diets. Our cells rely on sugar as their primary source of energy, and function at a laggardly pace without it. This is especially the case for the brain, where a lack of glucose makes it harder for us to concentrate and gives that light-headed feeling we get when we skip lunch. And while the body can synthesise a sugar replacement for short periods, exertion – mental or physical – becomes difficult.
Highs and lows
Sugar "highs" come about because the brain suddenly has access to a new and larger energy source, so it starts to function at top speed. Ten minutes after drinking a sugary cup of tea, the glucose hits the bloodstream and flies round the body, being absorbed by the cells most in need of energy. When the remaining blood glucose reaches the brain, we feel intense buzzing energy. However, the burst is short-lived and a "crash" soon follows. This low is an evolutionary response. The brain uses blood sugar as a metric by which to determine our need for food. A high quantity of sugar in the blood leads to a reduced appetite, so if we remained on a sugar high we wouldn't feel the need to eat. So the pancreas, detecting a high quantity of glucose in the blood, produces the hormone insulin. This stimulates cells in the muscles and liver to stop using fat as an energy source and take up glucose instead – thus reducing the quantity available to the brain and ending the high.
There's nothing like low blood sugar to aggravate feelings of irritation and frustration. The brain requires a constant and steady supply of glucose, without which our neurons struggle to function and we become more prone to fits of pique. This was confirmed by a recent American study, which suggested that people with higher blood-sugar levels are better able to control their anger than those whose sugar levels are depleted. There has even been a suggestion that there is an indirect link between diabetes and loss of self-control.
Despite what some commentators say, sugar is not the enemy. It is an unrivalled source of energy. If you reduce your intake of sugars you will need to replace those calories with another energy source– more often than not, this means fat. And high fat intake is by no means ideal. The key is balance. If you load up on sugar-rich carbohydrates, at the expense of other foods, you do risk serious health problems. Type 2 diabetes is the most pernicious effect of sugar-loading. If your blood-sugar level spikes too often, your pancreas loses its ability to produce insulin effectively. And you end up with too many glucose molecules ricocheting around in your blood, which irreparably damages the cells of your kidneys and nerves.
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