Your metabolism describes all the chemical processes that occur every second in your body, in order to keep you alive and functioning normally. This includes the processes in which your body converts food and drink into energy. Once food is broken down into tiny building blocks and absorbed primarily via your small bowel, it is further broken down into a chemical called pyruvate.
In each cell in your body, are mitochondria, commonly termed the ‘power houses of the cell’, since these power each cell in your body. Interestingly they have their own DNA, that is inherited directly from your mother. In the mitochondria, the pyruvate is converted into another chemical called acetyl CoA, and this is processed via the Krebs or Citric Acid Cycle, into adenosine triphosphate (ATP). ATP is the energy currency within the body and is fundamental to all active processes requiring energy. This process is the foundation of metabolism, converting food into energy.
Metabolism and Metabolic Rate
The word metabolism is often used interchangeably with basal metabolic rate, which is the number of calories you burn. Most people though when they think of metabolism, associate it with weight gain and weight loss.
Metabolism and Burning Energy
Every day we metabolise our food, creating energy that we use in different three ways:
1. Performing vital functions such as breathing to keep us alive.
2. Thermogenesis: this is the process of digesting, absorbing and transporting the food you eat.
3. Physical activity such as walking and exercise.
The basal metabolic rate (BMR) is the energy required for performing vital body functions at rest. Your brain, liver, heart and kidneys account for almost half of your BMR. An estimate of BMR can be calculated from your weight, age and gender.
Our BMR accounts for most of the energy we use, with thermogenesis only 10% and physical activity between 10-30%.
Metabolism and weight
Many people assume that boosting your metabolism will help with weight gain, and that people who are overweight, have a slower metabolism. In fact, people with a higher weight have a higher BMR (1) and use more energy to perform everyday activities as it is harder to move about. Lean body mass contributes most to your BMR, so the greater lean body mass you have, the higher your BMR.
Exercise and Metabolic Rate
In general, your resting BMR remains constant and proportional to your muscle mass (2). Exercise, particularly resistance training, does increase your BMR, but this is partly due to increased lean mass (3). Additionally, the effect of exercise on appetite is proportional to the intensity of exercise. An intense aerobic training session suppresses appetite by decreasing blood levels of the hormone ghrelin (2). Exercise therefore doesn’t just increase your BMR, it also suppresses your appetite, thereby decreasing your energy intake and increasing your energy expenditure. However, decreasing appetite and energy intake can lead to a decrease BMR (4), so consider exercise important for health in general, not just effects on your BMR.
Fundamentally what controls your weight is the balance of how much fuel you eat, compared with how much you consume. If you eat more than you need, you will store it as fat and gain weight. If you eat less than you need, you will lose weight. However, if you eat fewer calories than you need, this is sensed as starvation, and leads to a drop in BMR, meaning that you will lose less weight (4). However, a slow gradual reduction in food intake, and weight loss, has less effect on reducing BMR (4).
Only in rare cases is the metabolic rate the reason for weight gain, for example with hypothyroid disease (under active thyroid). As most of us know, it is much easier to gain weight than lose it, and there are ideas that this is probably an evolutionary adaptation to protect us in times of famine, the so called ‘thrifty gene hypothesis’. However, using this hypothesis you might predict that everyone with freely available food would become obese, and this isn’t the case.
Microbiota and Their Role in Metabolism
The gut microbiota contains trillions of single celled microorganisms, that play an important role in our health. Multiple associations between the gut microbiota and chemicals along metabolic pathways have been found, with suggestion that the gut microorganisms have a role in shaping metabolism (5). Recent research has found that changing the composition of the microbiota, using prebiotic supplements (compounds that are not digestible by the human body, but preferentially nourish beneficial strains of gut microorganisms), is associated with improvement of some of the parameters of the metabolic syndrome (a name given to a set of conditions seen together including increased blood pressure, high blood sugar, a large waistline, and abnormal cholesterol or triglyceride levels) (6).
In studies of special germ-free mice, who have no organisms living on or inside them, they found that faecal transplant from obese humans, was associated with a greater weight gain that mice that received microbes from healthy weight humans (7). This suggests that metabolic rate, and maintenance of a steady healthy weight, is more than the simple balance of energy in, equalling energy out. Instead it is also impacted by your gut microbiota. Most studies of people that were obese have found that their gut microbiota is characterised by a narrower range of organisms (lower diversity) (7). Long term weight gain (over 10 years) in humans is correlated with low microbiota diversity (narrow range of species of microorganisms), and this association is worsened by low dietary fibre intake (7).
How single celled microorganisms have such a profound effect on our weight and general health is astonishing, but it is probably mediated by a number of different routes. Gut microbiota imbalance probably promotes weight gain and metabolic complications by a variety of mechanisms including immune dysregulation, altered energy regulation, altered gut hormone regulation, and proinflammatory mechanisms (such as lipopolysaccharide endotoxins crossing the gut barrier and entering the portal circulation) (7).
Metabolic Rate Boosting Supplements
Dietary supplements are not required unlike medicines to prove that their products are safe or effective. Many of the items marketed as ‘metabolic boosters’ are ergogenic aids, meaning a performance enhancer that gives you a mental or physical improvement while exercising. As discussed, exercise does increase your BMR but improved performance doesn’t equate with further alteration of the BMR. Some items naturally found in our diet though have been found to alter performance, and caffeine is an example. Caffeine improves both aerobic and anaerobic performance and can marginally increase thermogenesis (8).
Your basal metabolic rate is determined by your genes and your body composition. Exercise can increase your metabolic rate and is important for so many factors of general health. While maintaining a steady weight used to be thought of in terms of simple energy balance, evidence of the important role of the gut microbiota, might explain why some people can appear to eat much more than the energy they expend, without gaining weight.
Ultimately the best way to support your metabolism, metabolic rate and a healthy weight is by regular exercise and a healthy diet that follows these principles:
• Eat lots of different coloured fruit and vegetables
• Choose wholegrains (bulgur wheat, millet, sweet potatoes, brown rice, brown bread)
• Eat a handful of nuts a day and add seeds to your food
• Drink water
• Eat oily fish twice a week
• Chose lean or plant based protein
• Enjoy healthy unsaturated fats such avocados, rapeseed and extra virgin olive oils.
• Support your microbiota to flourish by eating fermented foods, kefir, and 30g of fibre a day.
This post was written by Dr Harriet Holme who studied medicine at the University of Cambridge and has over a decade of experience as a paediatric doctor. Harriet also has a PhD in genetics from University College London. Harriet now uses her skills for the benefit of her clients and students, exclusively consulting as a Registered Nutritionist with the Association of Nutrition and lecturing in culinary science and nutrition. You can find Harriet @healthyeatingdr and on her website Healthy Eating Dr.
1. Horgan GW, Stubbs J. Predicting basal metabolic rate in the obese is difficult. Eur J Clin Nutr. 2003 Feb;57(2):335–40.
2. Martin M, Krystof S, Jiri R, Martina D, Renata V, Ondrej M, et al. Modulation of Energy Intake and Expenditure Due to Habitual Physical Exercise. Curr Pharm Des. 2016;22(24):3681–99.
3. Singh M, Dureha DK, Yaduvanshi S, Mishra P. Effect of aerobic and anaerobic exercise on basal metabolic-rate. British Journal of Sports Medicine. 2010 Sep 1;44(Suppl 1):i26–6.
4. Connolly J, Romano T, Patruno M. Selections from current literature: effects of dieting and exercise on resting metabolic rate and implications for weight management. Fam Pract. 1999 Apr;16(2):196–201.
5. Visconti A, Le Roy CI, Rosa F, Rossi N, Martin TC, Mohney RP, et al. Interplay between the human gut microbiome and host metabolism. Nature Communications. 2019 Oct 3;10(1):4505–10.
6. Furlow B. Gut microbe composition and metabolic syndrome. Lancet Diabetes Endocrinol. 2013 Aug;1 Suppl 1:s4–5.
7. Valdes AM, Walter J, Segal E, Spector TD. Role of the gut microbiota in nutrition and health. BMJ. 2018 Jun 13;361:k2179.
8. Grgic J. Caffeine ingestion enhances Wingate performance: a meta-analysis. Eur J Sport Sci. 2017 Oct 31;18(2):219–25.
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