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How Does Metabolism Affect Weight?
Author: Jamie Braithwaite
June 10, 2022
Metabolism plays an important role in our body and affects our weight management, energy expenditure and balance. In this blog we look at what metabolism is, how it can affect how we lose and gain weight, and if it’s possible to ‘boost’ metabolism.
What is metabolism?
Metabolism describes all the chemical processes that keep our bodies going, such as breathing, digesting food and moving. To put it simply, metabolism is the term used to describe how your body breaks down the food you eat turning it into energy, and how that energy is processed by cells to power the body.
Basal metabolic rate
Your basal metabolic rate (BMR) refers to the amount of calories (energy) that is used to fuel basic functions in the body. Your BMR accounts for approximately 60-75% of the body’s daily energy expenditure. The basic rate at which your body burns calories to fuel your body’s functions is therefore hard to change.
What are the factors that affect metabolism?
The factors that affect your metabolism include your body mass, age, height and gender, genetics. For women, the active form of oestrogen, oestradiol, plays a key role in the regulation of metabolism and body weight. As levels of oestradiol decline during perimenopause and menopause, this can cause some women to gain weight especially around their mid-section. As people get older, they begin to lose muscle mass which also impacts metabolism causing it to slow down.
‘Slow’ VS ‘fast’ metabolism
Metabolism plays a key role in weight regulation with a ‘slow’ metabolism burning fewer calories resulting in more fat being stored in the body which can lead to weight gain. In contrast a ‘fast’ metabolism burns calories at a quicker rate, meaning the body stores less fat, which can lead to weight loss. If you are consuming more calories than your body expends through BMR and exercise, you will gain weight. Conversely, if you consume fewer calories you’ll lose weight. 
How do hormones affect metabolism?
There are a few different hormones at play when we think about metabolism:
The thyroid gland produces both triiodothyronine (T3) and thyroxine. (T4), which are essential to metabolic processes. Both of these hormones increase our basal metabolic rate. When the thyroid gland malfunctions, it can have two outcomes when it comes to weight:
- Hyperthyroidism This is when the thyroid gland over-produces more thyroid hormone than it should, a rise in your BMR, leading to weight loss.
- Hypothyroidism When the thyroid produces less thyroid hormone than it should it causes your BMR to slow, causing weight gain and increased appetite. 
Cortisol is a steroid hormone which helps our bodies convert sugar and fat into energy. When you experience stress, your cortisol levels are raised. This causes a stress response which mobilises the body’s energy stores in response. This can result in metabolic changes in the body and cause weight loss. Furthermore, experiencing consistently high levels of stress can lead to either under or over consumption of food, causing weight changes.
As mentioned earlier in this article, the active form of oestrogen, oestradiol, plays a key role in the regulation of metabolism particularly in women. Oestrogens regulate key features of metabolism such as food intake, body weight, glucose homeostasis/insulin sensitivity, body fat distribution and energy expenditure. Women going through perimenopause and post-menopausal women have an increase in visceral fat (that’s fat around their organs) due to low levels of oestradiol. This puts women at a greater risk of metabolic diseases such as obesity, diabetes and high blood pressure.
Can we ‘boost’ our metabolism?
The simple answer is no. Given that the basic rate at which our bodies burn calories to keep our bodies functioning makes up the largest proportion of energy expenditure in the body, it is hard to change. There is no clear scientific consensus on a definitive way to boost metabolism. When it comes to our BMR, many factors are out of our control, such as genetics, our gender and height. However, adopting a healthy lifestyle can help us improve our weight and overall wellbeing and reduce our risk of developing metabolic diseases such as diabetes.
What lifestyle factors impact metabolism?
Ensuring you do not consume more food than your body needs for energy is a good way to keep your weight in check assuming other factors are not present e.g. thyroid disorder. With food being the fuel that drives our bodies, it’s important to have a healthy balanced diet containing whole grains, nuts, seeds, lentils, plenty of fresh fruit and vegetables and good sources of protein. There is also increasing focus on gut microbiomes and the role they play in our overall physical and mental health. Having a varied diet will ensure you maintain a rich gut microbiome.
Exercising burns around 15-30% of the calories we eat. This doesn’t mean if you exercise you can binge on your favourite sweet treats! It’s about ensuring balance between the energy you take in (food) and the energy you expel through living and exercise. Adding exercise to your lifestyle will ensure you are keeping your muscles and bones strong, keeping your heart and lungs healthy, increasing blood flow and of course burning energy to help maintain weight. Exercise, combined with a healthy diet, will also help reduce the risk of developing cardiovascular disease, high blood pressure, diabetes and osteoporosis.
Metabolism plays a key role in regulating our weight, with around 60% of the energy we gain through food being used to keep our bodies functioning. Certain conditions such as thyroid disorders, as well as stress and hormone imbalance can all affect our weight. By maintaining a regular exercise routine, combined with a balanced diet that supports the body, we can ensure we stay healthy and control our weight, particularly as we get older.
- Metabolic Syndrome and Psychiatric Illness (2008) Basal Metabolic Rate. Available at: https://www.sciencedirect.com/topics/neuroscience/basal-metabolic-rate
- McCargar, LJ. (1996) Can Diet and Exercise Really Change Metabolism? Available at: https://pubmed.ncbi.nlm.nih.gov/9746638/
- Thyroid UK. 2021) How Your Thyroid Gland Works. Available at: https://thyroiduk.org/how-your-thyroid-gland-works/
- Current Opinion in Behavioural Sciences (2016) Impact of stress on metabolism and energy balance. pp 71-77. Available at: https://www.sciencedirect.com/science/article/pii/S2352154616300183
- Clegg, D. (2012) The Year in Review of Estrogen Regulation of Metabolism. Available at: https://academic.oup.com/mend/article/26/12/1957/2614784
This information has been medically reviewed by Dr Nicky Keay
Nicola has extensive clinical and research experience in the fields of endocrinology and sport and exercise medicine. Nicky is a member of the Royal College of Physicians, Honorary Fellow in the Department of Sport and Exercise Sciences at Durham University and former Research Fellow at St. Thomas' Hospital.
Dr Nicky Keay
BA, MA (Cantab), MB, BChir, MRCP.
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