What is sugar?
Sugar (or sucrose) is a carbohydrate made up of 1 glucose and 1 fructose molecule.
Are all calories equal?
The short answer is no.
But this is what the sugar sweetened beverage companies want you to think. Why do you think companies like Coca-Cola have been funding research to try and show that exercise is more important than diet in preventing obesity? Or why the managing director of Coca-Cola in Australia recently said that “If you consume one can like that a week, no, I don't think that's unhealthy”
They want you to believe that all calories are equal, and losing weight is all about exercise. That way you can drink as much soft drink or other sugar-sweetened beverages as you like, as long as you exercise.
But the calories available actually depend on the type of food. For example, the fibre in fruit and vegetables delays their absorption into the bloodstream. And the bacteria in the gut also consumes some calories as well. That’s why the juice from an apple is not as good for you as eating the whole apple.
1 gram fat ≈ 9 calories (38kJ)
1 gram sugar ≈ 4 calories (17kJ)
So it’s easy to see why people think we should avoid fats, because they have more calories per gram.
We need to understand how sugar is metabolised in the body.
How is sugar metabolised?
Sugar (sucrose) is split by an enzyme in the small intestine into glucose and fructose. The glucose and fructose are then absorbed into the bloodstream, where they make their way towards the liver.
Along the way, glucose is pulled out of the bloodstream to be used by any cells requiring energy. Fructose cannot be used directly by cells for energy.
In the liver, the glucose is converted to glycogen, and is stored for later use.
There is another enzyme in the liver called fruktokinase, which pulls fructose out of the bloodstream and converts it to glycogen for storage. The problem with fructose is that fruktokinase is not regulated, so it will always pull fructose from the bloodstream, whether the liver needs to store glycogen or not. If the glycogen storage is full, then excess fructose is converted to fat in the liver, in the form of fatty acids and triglycerides.
This is how excess sugar consumption makes you fat.
It also creates a problem, because the increase in fat in the liver causes insulin resistance, which in turn leads to more glucose in the bloodstream. The body then secretes more insulin to try and control the blood sugar.
Is sugar an appetite suppressant?
According to Dr Robert Lustig, insulin may play a pivotal role in blocking the hormone leptin, which is responsible for sending signals to the hypothalamus that we are full.
What this means is that even though we have lots of sugar in our system, our brain still thinks we are hungry, so we keep eating. Overconsumption is a problem, because the excess sugar is converted to fat. That’s why we can drink a large glass of fruit juice or soft drink with dinner, but it doesn’t affect our appetite.
Dr Lustig believes that fructose is responsible for blocking the leptin signal.
The hypothalamus is the appetite control centre of the brain, and responds to the hormone leptin, which is sent from fat cells to the brain to tell us we are full, and should stop eating.