Your metabolism refers to the millions of chemical processes that keep your body alive and functioning.
It is related to weight because it influences the amount of energy your body needs at any given point. Take in more energy than you need, and the excess will be stored as fat.
Nonetheless many people are quick to blame a “slow metabolism” for their weight gain, when in fact they need to make better food choices and exercise choices.
The biggest component of your metabolism – accounting for 50 to 80 per cent of the energy used each day – is your basal metabolic rate (BMR), which is the energy your body burns just to maintain functioning at rest.
(Other influences include how much physical activity you do, and the 'thermic effect' of the food you eat - that is energy you use to digest and absorb your food.)
While there are many pills, supplements and foods that claim to boost metabolism and burn fat, most of these claims are unproven, says Tim Crowe, associate professor in nutrition at Deakin University.
Even if they did work, they might come with unintended side effects, such as increasing your heart rate, he says.
Nonetheless, it can be helpful to know what factors do affect your metabolism, as some of them are within your control. And even knowing you have factors you cannot control may nonetheless be useful as it can motivate you to take extra care to compensate for the issue, perhaps by being more vigilant about your diet and exercise.
Here are 10 factors that affect BMR and metabolism:
1. Muscle mass
That is, the amount of muscle tissue on your body. Muscle requires more energy to function than fat. So the more muscle tissue you carry, the more energy your body needs just to exist. (While most forms of exercise will help boost muscle, resistance or strength training is most effective: for example lifting weights and exercises that work against the resistance of your body weight such as push-ups, squats and lunges.)
As you get older, your metabolic rate generally slows. This is partly because of a loss of muscle tissue, and also because of hormonal and neurological changes. When babies and children go through periods of growth, their metabolism speeds up.
3. Body size
People with bigger bodies tend to have a larger BMR because they usually have larger internal organs and fluid volume to maintain. Taller people have a larger skin surface, which means their bodies may have to work harder to maintain a constant temperature.
As men are usually larger than women, they generally have faster metabolisms.
This can also play a role in whether you have a slower or faster metabolism, and some genetic disorders can also affect your metabolism.
6. Physical activity
Regular exercise increases muscle mass and encourages your body to burn kilojoules at a faster rate, even when at rest.
7. Hormonal factors
Hormonal imbalances caused by certain conditions, including hypo- and hyperthyroidism, can affect your metabolism.
8. Environmental factors
The weather can also have an effect on your metabolism; if it is very cold or very hot, your body has to work harder to maintain its normal temperature and that increases the metabolic rate.
Caffeine and nicotine can increase your metabolic rate, while medications including some antidepressants and anabolic steroids can contribute to weight gain regardless of what you eat.
Certain aspects of your diet can also affect metabolism. For instance if you don't have enough iodine for optimal thyroid function, it can slow down your metabolism.