Fuel Basics
How the food you eat turns to fuel to keep you running strongly

You have read plenty about the benefits of carbohydrates for energy, protein for strong muscles, unsaturated fats for a healthy heart, and calcium for strong bones and weight loss. Now let’s take a step back and look at how the body actually processes food, and how it uses the food we eat to fuel us. When you understand the complex marvel that your digestive system is, you may be even more motivated to eat well.

Down it goes
The process of swallowing food involves chewing and mixing with saliva, making the food small enough, and lubricating it to make the long journey through your gastrointestinal tract. First the food travels down through your oesophagus, the long narrow pipe that leads from the back of the throat to the stomach. In the stomach, the food is broken down and then enters the small intestine. This consists of the duodenum (22cm long), then the jejunum (90cm) and the ileum (168cm) – that’s about three metres of carefully laid plumbing, all with a special function.

Most of the carbohydrates, protein and fat you eat is absorbed into the body as it travels through the intestine. Iron, calcium and magnesium are absorbed in the duodenum, vitamin C and the B vitamins and folate (all water soluble) are absorbed from the jejunum, and the fat soluble vitamins (D, A, E and K) and B12 are absorbed from the ileum. Once at the end of the small intestine, broken down food enters the 110cm long colon (the large intestine), where water and electrolytes are absorbed, and the semi-solid waste arrives at the rectum. It is stored there until ready to be released.

Carbohydrates contribute a substantial proportion of energy to the daily diet – the brain relies on carbohydrate to function, and the other tissues, nerves, and muscles use this fuel source for energy. Carbohydrates are broken down to glucosefructose (commonly found in fruit) and galactose (occurring as part of the milk sugar lactose). Glucose that is not immediately needed in the blood stream is stored in the liver and muscle tissue for later use – a very important process for athletes.

Fats also have important functions. A moderate amount of dietary fat and body fat, helps immune function as well as keeping you warm. It is necessary for normal hormonal functioning; women whose body fat is too low stop menstruating. Some fat is burnt up when you exercise and even at rest – it all depends on your overall kilojoule intake. When you eat more energy than you burn up, excess dietary fat can become body fat. It is very easy for the body to store fat as fat – it requires little energy to do so. Some fat in the body is made from carbohydrates and protein, but this is a more difficult process. Unfortunately there is no limit to the amount of fat that can be stored in the body.

Proteins are made up of amino acids, joined together in different permutations and combinations to give each protein its unique features and functions. Some proteins are used to transport iron, some to help build muscle tissue; others are needed for healthy hair and nails. Proteins are constantly being made and broken down in the body. We need more protein than usual in times of growth – infancy, adolescence, pregnancy, muscle development in resistance training. Protein is lost from the body during times of starvation, and in people who suffer sever burns.

Vitamins are important for energy. They do not directly give us energy; rather they help us access the energy we get from fat, protein and carbohydrate.

Kilojoule values
For every gram of carbohydrate you eat, you take in 17kJ of energy (about four calories). Protein provides 16kJ for every gram, and fat 37kJ for each gram. You can see how a high-fat diet pushes kilojoule intake up far quicker than carbohydrate or protein do. Fifty grams of chocolate is more likely to cause weight gain than 50g of jelly lollies. Remember though, that a low fat diet does not mean a no-fat diet – some fat is essential. Alcohol gives us 29kJ of energy per gram, so cutting out alcohol is one simple way to cut down your kilojoule intake if you need to lose weight.

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