The symptoms of genetic metabolic disorders can be very similar to those of other disorders and diseases, making it difficult to pinpoint the exact cause.

See your doctor if you suspect you have a metabolic disorder. Some genetic disorders of metabolism include:. This page has been produced in consultation with and approved by:.

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The State of Victoria and the Department of Health shall not bear any liability for reliance by any user on the materials contained on this website. Skip to main content. Actions for this page Listen Print. Summary Read the full fact sheet. On this page. What is metabolism? Two processes of metabolism Metabolic rate Metabolism and age-related weight gain Hormonal disorders of metabolism Genetic disorders of metabolism Where to get help.

Two processes of metabolism Our metabolism is complex — put simply it has 2 parts, which are carefully regulated by the body to make sure they remain in balance. They are: Catabolism — the breakdown of food components such as carbohydrates , proteins and dietary fats into their simpler forms, which can then be used to provide energy and the basic building blocks needed for growth and repair.

Anabolism — the part of metabolism in which our body is built or repaired. Anabolism requires energy that ultimately comes from our food.

When we eat more than we need for daily anabolism, the excess nutrients are typically stored in our body as fat. Thermic effect of food also known as thermogenesis — your body uses energy to digest the foods and drinks you consume and also absorbs, transports and stores their nutrients.

Energy used during physical activity — this is the energy used by physical movement and it varies the most depending on how much energy you use each day.

Physical activity includes planned exercise like going for a run or playing sport but also includes all incidental activity such as hanging out the washing, playing with the dog or even fidgeting! Basal metabolic rate BMR The BMR refers to the amount of energy your body needs to maintain homeostasis.

Factors that affect our BMR Your BMR is influenced by multiple factors working in combination, including: Body size — larger adult bodies have more metabolising tissue and a larger BMR. Amount of lean muscle tissue — muscle burns kilojoules rapidly. Crash dieting, starving or fasting — eating too few kilojoules encourages the body to slow the metabolism to conserve energy.

Age — metabolism slows with age due to loss of muscle tissue, but also due to hormonal and neurological changes. Growth — infants and children have higher energy demands per unit of body weight due to the energy demands of growth and the extra energy needed to maintain their body temperature.

Gender — generally, men have faster metabolisms because they tend to be larger. Genetic predisposition — your metabolic rate may be partly decided by your genes. Hormonal and nervous controls — BMR is controlled by the nervous and hormonal systems. Hormonal imbalances can influence how quickly or slowly the body burns kilojoules.

Environmental temperature — if temperature is very low or very high, the body has to work harder to maintain its normal body temperature, which increases the BMR.

Infection or illness — BMR increases because the body has to work harder to build new tissues and to create an immune response. Amount of physical activity — hard-working muscles need plenty of energy to burn.

Regular exercise increases muscle mass and teaches the body to burn kilojoules at a faster rate, even when at rest. Drugs — like caffeine or nicotine , can increase the BMR. Dietary deficiencies — for example, a diet low in iodine reduces thyroid function and slows the metabolism.

Thermic effect of food Your BMR rises after you eat because you use energy to eat, digest and metabolise the food you have just eaten.

Hot spicy foods for example, foods containing chilli, horseradish and mustard can have a significant thermic effect. Energy used during physical activity During strenuous or vigorous physical activity, our muscles may burn through as much as 3, kJ per hour.

Metabolism and age-related weight gain Muscle tissue has a large appetite for kilojoules. Hormonal disorders of metabolism Hormones help regulate our metabolism. Thyroid disorders include: Hypothyroidism underactive thyroid — the metabolism slows because the thyroid gland does not release enough hormones.

Some of the symptoms of hypothyroidism include unusual weight gain, lethargy, depression and constipation. Hyperthyroidism overactive thyroid — the gland releases larger quantities of hormones than necessary and speeds the metabolism.

Some of the symptoms of hyperthyroidism include increased appetite, weight loss, nervousness and diarrhoea. Genetic disorders of metabolism Our genes are the blueprints for the proteins in our body, and our proteins are responsible for the digestion and metabolism of our food.

Some genetic disorders of metabolism include: Fructose intolerance — the inability to break down fructose, which is a type of sugar found in fruit, fruit juices, sugar for example, cane sugar , honey and certain vegetables. Galactosaemia — the inability to convert the carbohydrate galactose into glucose.

Galactose is not found by itself in nature. It is produced when lactose is broken down by the digestive system into glucose and galactose. Sources of lactose include milk and milk products, such as yoghurt and cheese. Phenylketonuria PKU — the inability to convert the amino acid phenylalanine into tyrosine.

High levels of phenylalanine in the blood can cause brain damage. High-protein foods and those containing the artificial sweetener aspartame must be avoided. It's the calories found in foods we eat and beverages we drink — energy that may be used right away or stored especially in the form of fat for use later.

How fast your body's "engine" runs on average, over time, determines how many calories you burn. If your metabolism is "high" or fast , you will burn more calories at rest and during activity.

A high metabolism means you'll need to take in more calories to maintain your weight. That's one reason why some people can eat more than others without gaining weight.

A person with a "low" or slow metabolism will burn fewer calories at rest and during activity and therefore has to eat less to avoid becoming overweight. Lean people tend to be more active during everyday activities than people who are overweight. They may "fidget" more — that is, they tend to be in motion even when engaged in non-exercise activities.

Whether this tendency to move more or less is genetically programmed or learned remains uncertain. But it can add or subtract hundreds of calories each day. Obese people expend more calories, on average, than lean people during most activities, in part because it takes more effort to move around.

But they tend to be more sedentary, which makes it harder to get rid of body fat. It's part truth and part myth that metabolism is the key to weight. The rising tide of obesity in this country cannot be blamed entirely on an inherited tendency to have a slow metabolism. Genes do not change that quickly.

Something environmental — particularly, changes in diet and exercising too little — are much more likely culprits. Age can be a factor, too, although new evidence suggests metabolism reaches a peak earlier in life and slows down much later than previously thought.

The reality is that for most people, excess weight is not all due to bad luck, thyroid trouble or some other unexplained, uncontrollable external factor. For most of us, calories in, calories out has a strong influence on changes in weight over a lifetime.

Regardless of whether your metabolism is fast or slow, our bodies are designed to store excess energy in fat cells. So, if you eat and drink more calories energy "intake" than your body expends energy "output" you will gain weight. On the other hand, if you eat and drink fewer calories than are burned through everyday activities including exercise, rest and sleep , you'll lose weight.

Our bodies are also programmed to sense a lack of food as starvation. In response, our BMR slows down, which means fewer calories burned over time. That's one reason why losing weight is often difficult. Perhaps the most remarkable thing about all of this is how little our weight tends to change from day to day.

In fact, only a few excess calories each day could lead to significant weight gain at the end of a year.