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Food composition, digestion, energy usage and body fat

Almost everything in the food we eat has some impact on our health. Food sustains the normal function of your body, promotes growth or creates energy.

Food is made up of a combination of the three macronutrients - carbohydrates, proteins and fats – which are needed in large amounts every day to keep your body healthy. You use each macronutrient in a different way.

Almost all food contains some carbohydrate, protein and fat, but in different proportions. For example, pasta is highest in carbohydrates so we consider pasta to be a carbohydrate food.

This section of the website explains the role of these three macronutrients and how we derive energy from them for digestion, growth and normal everyday functioning. When we eat too many macronutrients and do not use all the energy they produce, we store the excess as body fat.

A word about energy

The word ‘energy’ can mean different things to different people. We know how it feels to be ‘low on energy’ or to have ‘lots of energy’, but this is usually related to our physical and mental health. In nutrition, the word ‘energy’ is measured by the number of kilo- joules (or calories) in a food type. The more kilojoules, the more fuel (or energy) is available to run the body. Unfortunately, most of us eat more kilojoules than our body needs and the unused ones end up as stored fat.

Each macronutrient offers a different amount of energy:

1 gram of carbohydrates = 16 kJ (4 Calories)
1 gram of protein = 17 kJ (4 Calories)
1 gram of fat = 39 kJ (9 Calories)

How do the following impact our bodies and our weight?

Carbohydrates are the major macronutrient in the normal Australian diet – making up almost 50% of an average daily energy intake. They are built from units called monosaccharides, or sugars.

Glucose is the most common of these. Carbohydrates are used mostly to produce the energy to run the body, including muscle and brain functions.

Carbohydrate foods include:

While sugar is a carbohydrate food and can be used by the body for energy just like pasta or rice, it does not contain any other nutrients and is not a good source of food energy.

Lipids (fats and oils) have almost twice the energy of either carbohydrates or proteins, because they’re made up from fatty acids. The intake of fatty acids is now recognized as one of the most important factors influencing health.

Unhealthy fats (Saturated fats)
Saturated fatty acids are found most commonly in animal and dairy products. When eaten in excess they increase blood cholesterol levels.

Healthy fats (Unsaturated fats)
Monounsaturated fatty acids are found mainly in plants, particularly in olive and canola oil. The main monounsaturated fatty acid, oleic acid, is the major oil in the Mediterranean diet and not surprisingly is found in olive oil. Because oleic acid lowers blood cholesterol levels, people who follow a traditional Mediterranean diet have low rates of heart disease.

Substituting monounsaturated fatty acids for saturated fats can help reduce the risk of heart disease.

Polyunsaturated fatty acids (known as omega-3s) are found in plants, seafood and some animal foods. Such polyunsaturated fatty acids can also reduce the risk of heart attack and may help protect against a range of other illnesses such as depression, Alzheimer’s disease and arthritis.

Poly and mono unsaturated spreads and oils (canola, sunflower and olive)

Fish e.g. salmon, herring, mackrel



Hydrogenated trans fatty acids (baked products e.g. cakes, biscuits, pastries, some margerines)

Fast foods e.g. pies, pizza, hamburgers

Fatty processed meats
Protein helps build and repair the body and muscles. Protein also makes a range of vital enzymes and hormones. Proteins are made up of amino acids, 20 of which are absolutely essential to our health.

Think of amino acids as building blocks that the body can combine in thousands of different ways to make all the specialised proteins it needs. Eleven of these amino acids, our body makes itself – but the other nine (the ‘essential’ amino acids) come from food. Protein from animals is called ‘complete’ protein because it contains the nine essential amino acids.

Plant foods also contain protein, but they are ‘incomplete’ protein foods because they don’t have enough of the nine essential amino acids. However, eating two or more plant foods together can supply complete protein. Only soy provides complete protein. Some foods such as nuts and cheese can be high in both protein and fat.

Digestion is the process of breaking down foods into nutrients so they can be absorbed by the body.

The human digestive system (otherwise known as the gut or gastrointestinal tract)can be thought of as one continuous tube about 7 to 8 meters long that consists of the mouth, the throat (pharynx), the esophagus, the stomach, the small intestine, the large intestine and finally the rectum. Each part of the digestive system has a specialised role in breaking down and absorbing the foods and fluids we ingest each day and eliminating what we do not need.

Energy Creation
The majority of what we eat each day has one main purpose: to provide energy for the body. The energy in food that we can use comes from the chemical bonds between the atoms in carbohydrate, fat, protein and even alcohol. Each of these nutrients can be broken down (metabolised) into simpler molecules through different biochemical pathways to make energy. Every cell in our body has the ability to use nutrients to make energy; in fact, every cell in our body needs to make energy from nutrients or else it cannot function!

The main pathway where carbohydrate, fat and protein metabolism meet to produce energy is called the Kreb's cycle (also known as the citric acid cycle). The Kreb's cycle takes place inside the cell in a specialised structure called mitochondria. The mitochondria are quite rightly called 'the powerhouses' of the cell. The Kreb's cycle allows electrons to be taken from nutrients to help drive pumps that produce ATP (adenosine triphosphate). ATP is the main energy currency of all cells in the body and is used to pump molecules into and out of cell, allow enzymes to work and muscles to contract. Metabolism of nutrients to make ATP results in by-products such as carbon dioxide, water and heat. For a typical adult eating a normal diet, almost all of the ATP we make each day comes from the carbohydrate and fat we eat; only small amounts of ATP come from protein. When dieting (and typically eating less food), we rely on existing fat stores to make ATP and that is why we lose body fat. The body breaks down ATP every second of the day, so much of the 'energy' in food goes towards helping the body remake ATP.

Did you know...
The continual breaking down and resynthesising of ATP means each person makes their own body weight in ATP every day!

While the body can make some ATP from glucose without needing oxygen, almost all of the ATP made each day comes from metabolic pathways that require oxygen so this is given the name of aerobic metabolism. It should be no surprise that as someone increases the intensity of exercise, their breathing rate increases noticeably as the body needs more oxygen to 'burn' fuel (in this case a combination of glucose and fat) to meet the large energy needs of contracting muscle. Just like a fire that needs oxygen to burn wood, the same principle applies in the making of energy from food.

Many tissues in the body such as the brain, nervous system and contracting muscle, prefer to use glucose (carbohydrate) to make ATP rather than fat. In times of semi-starvation, fasting, or on a low-carbohydrate diet, the body cannot make enough glucose internally for its needs, so the brain and nervous system turn to an alternative fuel source - ketone bodies - to meet the shortfall. Ketone bodies are made from fat by the liver and can be used by the body in place of glucose to make energy. The term 'ketosis' is when a person has high levels of ketone bodies in their blood as they progressively build up during a fast or when following a low-carbohydrate diet. The process of ketosis does not slow or speed the effectiveness of a diet, it is just symptomatic of low levels of carbohydrates in the body.

What energy is used for
Most of the energy we make each day goes to support the normal functioning of our body that is essential for our survival. This includes all the physical and chemical processes that occur in the body that are necessary to allow our body to function such as breathing, regulating body temperature, maintaining a heart beat, repairing damage, circulating blood, and removing waste from the body. Even when resting, the body uses a tremendous amount of energy just to maintain and support itself and the term for this essential energy need is called the Basal Metabolic Rate (BMR). BMR can be thought of as the energy used to carry out necessary body activities not related to any type of physical activity. For an average adult, BMR can account for up to almost 70 percent of their daily energy needs. BMR is affected by many things.

Factors affecting BMR
-Amount of muscle
-Gender (males tend to have a higher BMR than females because of more muscle)
-Age (BMR decreases 2% per decade with age)
-Genetics (inherited tendency to certain conditions that affect metabolism)
-Thyroid hormone levels (an under active thyroid gland dramatically reduces BMR)
-Pregnancy and rapid growth (e.g. growth spurts, healing after illness or injury)
-Illness and stress (increases metabolism)
-Weight loss (lowers BMR)

Metabolism myth
There is a common myth that weight gain is caused by a 'slow metabolism'. In fact, BMR usually increases rather than decreases as people gain more weight. In other words, the more you weigh, the more kilojoules you need to maintain your body, as there is simply more of you to support. Only in rare situations where a person has an underlying medical condition (such as an under active thyroid gland), could weight gain be due to a ‘slow metabolism’.

For most people, physical activity accounts for just 20 to 30 percent of the body’s total energy output each day. The amount of energy (kilojoules) used for an activity depends on the intensity level and duration of the activity. The more intense and longer the activity, the greater the energy needs and that's why sportspeople can have energy needs for physical activity much greater than their BMR.

While the energy needs of adults can be divided between BMR and physical activity, the bodies of growing children, adolescents, and teenagers also use energy to produce new body tissues such as bone, muscle, and blood and that is why they can have higher energy needs based on their body weight than adults.

Body Fat

Fat, or adipose tissue, is found in several places in your body. Generally, fat is found underneath the skin (subcutaneous fat), but can be found in other locations depending on the sex of the person.
Females tend to store fat in the chest, hips, waist and buttocks creating a "pear" shape. Men tend to store body fat in their chest, abdomen and buttocks, producing an "apple" shape.

The main job of fat cells is to store fat. Fat cells have an incredible capacity to store fat with a 'full' cell containing almost 99% of its weight as fat! When people talk about weight gain, they are usually referring to an increase in body fat. Overeating results in increased fat storage as the body has more energy than it needs so it stores the excess nutrients (usually dietary fat) in fat cells where it can be called upon in times of food scarcity such as dieting. The simple explanation of why body fat is reduced when you go on a diet is that the body still needs to meet all its energy needs and if it is not getting it all from food, it will draw upon its largest reserves of energy - body fat.

As a person gains weight and the fat cells fill and expand, the cells can eventually multiply to allow more fat storage. This process should be fairly obvious as there appears no limit to how much fat humans can store. Having lots of fat cells though doesn't mean a person is overweight; that only occurs when the fat cells become filled with fat and for that to occur a person needs to be eating more food than what they need to fuel their body.

Why does the body store fat?
Our body is very complex and cleverly arranged to ensure survival. As humans evolved, storing body fat was very important as a means to survive food shortages, as the stored fat could be used by the body as an emergency ‘supply’ of energy. Our bodies haven’t changed much since those times although the way we live is very different; we are still genetically designed to store fat even though food shortages in Western societies are rare. We eat more today and more often than our ancestors did and we don’t have the same opportunities to burn off excess energy. Our bodies don’t know there isn’t a famine around the corner, and store that energy as fat just in case!

The reasons for overeating in the first place are more complex and are often a combination of the following:

-Genetics (e.g. an inherited medical condition, meaning that we would be overweight regardless of other factors).
-Environmental factors (e.g. the society we live in and the predominance of junk food)
-Psychological tendency (e.g. some people are ‘emotional eaters’ and overeat during times of stress or trauma). 
-Health consequences of carrying
too much weight

When people are or overweight or obese, they are more likely to develop health problems including:

-Type 2 diabetes
-Heart diseases
-High cholesterol
-High blood pressure (hypertension)
-Gallbladder disease
-Sleep apnoea and respiratory problems
-Some cancers (breast, colon, endometrium and oesophagus).

Carrying too much weight makes it harder to take part in regular exercise, which often means a vicious cycle making it not only harder to lose the weight, but also further increasing the risk of some diseases where exercise is known to be beneficial. The more overweight a person is, the more likely they are to have health problems. Studies have shown that even losing a small amount of weight, say 5 to 10 percent of body weight, combined with exercise can improve health and help reduce the chances of developing serious health problems.

How body fat is reduced
When dieting or in cases where we are eating much less than we usually would (such as during illness or when we are experiencing a lot of stress), the body still needs to meet its energy needs. As we lose weight we also lose a combination of water, carbohydrate (stored glycogen) and protein, but it is the body's fat stores that are lost the most. To use stored fat, special enzymes help liberate the fatty acids that are stored in a fat cell and these fatty acids can then be oxidised to make energy (ATP). As more fat is used, the amount of fat stored within each fat cell decreases and the fat cell simply shrinks in volume.

While it may seem that eating as little as possible is a good way to lose the most weight, research has shown that reducing energy intake too much causes the body to burn more protein from muscles in order to supply energy and essential amino acids (if very little protein is eaten in the diet) and this can severely lower the metabolic rate, making it harder to lose weight. Reducing kilojoules on a gradual and steady basis helps limit a decrease in metabolism and loss of body protein and ensures you are still eating enough food to provide the body with essential proteins, vitamins and minerals for good health.

As a we lose weight, even when eating well, we will be lose both fat and muscle. Being active helps you lose more fat than muscle, as the body will preserve its much needed muscle. Having more muscle helps you burn more energy during the day which further aids weight loss. Muscle tissue, unlike fat, is metabolically active, i.e. muscle burns kilojoules even when it isn’t being used. A person who has a high proportion of muscle will burn more kilojoules over the course of the day than someone of the same weight with less muscle.

The spot reducing myth
There is no 'magic' diet or exercise that can target the removal of fat from certain body areas. Fat loss follows the principle of 'first on, last off', meaning the first place you store fat when you gain weight will be the last place you lose it. With regular exercise and correct dietary changes you’ll decrease overall body fat and this will also help reduce fat in the most abundant parts of the body or those problem areas.

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