We often hear people say, “I have a slow metabolism” or “they just have a fast metabolism,” But do we understand what that means?
What is your metabolism? Can it be fast or slow? Does having a slow metabolism make you more inclined to gain weight? Does your metabolism really slow down or break when you diet?
These are the questions we are going to provide some clarity on in this article.
Understanding Metabolism
Let’s first understand what exactly metabolism is
- Metabolism refers to the countless chemical processes going on continuously inside the body that allow life and normal functioning.
- The amount of kilojoules your body burns at any given time is affected by your metabolism.
- Your metabolic rate is influenced by many factors – including age, gender, muscle-to-fat ratio, amount of physical activity and hormone function.
Metabolism refers to all the chemical processes going on continuously inside your body that allow life and normal functioning (maintaining normal functioning in the body is called homeostasis). These processes include those that break down nutrients from our food, and those that build and repair our body.
Or simply put, your total energy expenditure. This means that your metabolism is the cumulation of all the energy your body expends to function also referred to as daily energy expenditure (TDEE).
The amount of energy, measured in kilojoules (kJ), that your body burns at any given time is affected by your metabolism.
Achieving or maintaining a healthy weight is a balancing act. If we regularly eat and drink more kilojoules than we need for our metabolism, we store it mostly as fat.
Most of the energy we use each day is used to keep all the systems in our body functioning properly. This is out of our control. However, we can make metabolism work for us when we exercise. When you are active, the body burns more energy (kilojoules).
Metabolic rate
Your body’s metabolic rate (or total energy expenditure) can be divided into three components :
- Basal metabolic rate (BMR) – even at rest, the body needs energy (kilojoules) to keep all its systems functioning correctly (such as breathing, keeping the heart beating to circulate blood, growing and repairing cells and adjusting hormone levels). The body’s BMR accounts for the largest amount of energy expended daily (50–80 per cent of your daily energy use).
- 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. Thermogenesis accounts for about 5–10 per cent of your energy use.
- 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.
Basal metabolic rate (BMR)
The BMR refers to the amount of energy your body needs to maintain homeostasis.
Your BMR is largely determined by your total lean mass, especially muscle mass, because lean mass requires a lot of energy to maintain. Anything that reduces lean mass will reduce your BMR.
As your BMR accounts for so much of your total energy consumption, it is important to preserve or even increase your lean muscle mass through exercise when trying to lose weight.
This means combining exercise (particularly weight-bearing and resistance exercises to boost muscle mass) with changes towards healthier eating patterns rather than dietary changes alone as eating too few kilojoules encourages the body to slow the metabolism to conserve energy.
Maintaining lean muscle mass also helps reduce the chance of injury when training, and exercise increases your daily energy expenditure.
An average man has a BMR of around 7,100 kJ per day, while an average woman has a BMR of around 5,900 kJ per day. Energy expenditure is continuous, but the rate varies throughout the day. The rate of energy expenditure is usually lowest in the early morning.
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.
- Amount of body fat – fat cells are ‘sluggish’ and burn far fewer kilojoules than most other tissues and organs of the body.
- Crash dieting, starving or fasting – eating too few kilojoules encourages the body to slow the metabolism to conserve energy. BMR can drop by up to 15 per cent and if lean muscle tissue is also lost, this further reduces BMR.
- 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
Different foods raise BMR by differing amounts. For example:
- Fats raise the BMR 0–5 per cent.
- Carbohydrates raise the BMR 5–10 per cent.
- Proteins raise the BMR 20–30 per cent.
- Hot spicy foods (for example, foods containing chilli, horseradish and mustard) can have a significant thermic effect.
Metabolism and age-related weight gain
Muscle tissue has a large appetite for kilojoules. The more muscle mass you have, the more kilojoules you will burn.
People tend to put on fat as they age, partly because the body slowly loses muscle. It is not clear whether muscle loss is a result of the ageing process or because many people are less active as they age. However, it probably has more to do with becoming less active. Research has shown that strength and resistance training can reduce or prevent this muscle loss.
If you are over 40 years of age, have a pre-existing medical condition or have not exercised in some time, see your doctor before starting a new fitness program.
Hormonal disorders of metabolism
Hormones help regulate our metabolism. Some of the more common hormonal disorders affect the thyroid. This gland secretes hormones to regulate many metabolic processes, including energy expenditure (the rate at which kilojoules are burned).
Thyroid disorders include:
- Hypothyroidism (underactive thyroid) – the metabolism slows because the thyroid gland does not release enough hormones. A common cause is the autoimmune condition Hashimoto’s disease. 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. The most common cause of this condition is Graves’ disease. 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.
Sometimes, a faulty gene means we produce a protein that is ineffective in dealing with our food, resulting in a metabolic disorder. In most cases, genetic metabolic disorders can be managed under medical supervision, with close attention to diet.
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:
- 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.
1. YOUR METABOLISM IS MORE THAN ONE THING
As we have seen from few points above, our metabolism is a collection of many things. It is the sum of all the metabolic processes in our body.
- Resting metabolism (what most of us call our metabolism)
- The energy it takes to process the food you eat
- Physical activity (more on that in a bit)
2. YOUR METABOLISM ADAPTS
In one of the more interesting studies of the 1990s, scientists tested to see what happens to people when they increase or decrease their calories. They found that when you increase people’s calories, something very interesting happens: they start to burn more calories.
Primarily, they increase their non-exercise physical activity; they started moving around more. Their resting metabolic rate also increased very slightly, with some of that coming from an increased thermic effect of food, but some of it also comes from having an increased body mass.
The same thing happened when they decreased their calories, but in the opposite direction. When people decreased their calorie intake, their physical activity decreased, as did their thermic effect of food and their resting metabolism from reduced body mass.
In short, their metabolisms adapted to the scenario their body was being exposed to.
3. YOUR NEAT IS MORE IMPORTANT THAN YOU PROBABLY THINK
Outside of your resting metabolism, your non-exercise activity thermogenesis (NEAT) is the most essential aspect of your metabolism.
One study found that a person’s NEAT was almost single-handedly the thing that determined why some people who “overeat” gain a lot of weight, and why other people do not
4. YOUR RESTING METABOLIC RATE ISN’T SUPER USEFUL FOR WEIGHT LOSS
First, we can’t manipulate the resting metabolic rate to a meaningful degree through diet or exercise. We can measure things like metabolic equivalents, which relies on metabolic rates, but the accuracy is still suspect.
Second, when we look at most research, resting metabolism doesn’t appear to matter very much for weight loss. Your food intake and your NEAT are far more critical for weight loss efforts than your resting metabolic rate.
Conclusion
We often think about our metabolism as one thing. In reality, it is the full collection of all the energy-producing and energy-consuming processes that occur in our body. It is made up of our resting metabolism, the energy it takes to process our food, and our physical activity.
Your metabolism adapts to calorie increases and decreases, with a large part of the adaptation coming from changes in physical activity. While metabolisms can decrease, they do not “break.” Lastly, lower resting metabolisms do not appear to be predictive of weight gain and by themselves are not overly helpful measures for most people.