Most people have an idea of what “metabolism” is but may find it difficult to clearly define. The concept of metabolism as it applies to food, weight control and energy balance encompasses the digestion, absorption and processing of food, and the storage or utilisation of the energy released from this.
Metabolic rate relates to the amount of energy required by the body to maintain a stable state over a fixed period of time. Rather like how much petrol a car requires to make a journey from Auckland to Wellington in 10 hours. Now not all cars are the same and bigger heavier cars will use more petrol than a small light car, however a poorly tuned car with dirty spark-plugs will use more petrol than a finely tweaked one. A similar concept may be applied to people. Not everyone has the same metabolic rate or efficiency, but the commonly held mis-belief that one’s slow metabolic rate is to blame for weight gain and obesity is unfounded, as I will try to demonstrate in this article.
Our food is made up of three main groups known as macronutrients. Each has fundamental building blocks: Carbohydrate made up of sugars, Protein of amino acids and Fat of fatty acids and glycerol – rather like petrol, diesel and LPG. Our bodies are able to digest and burn each of these to release their energy, but they require different metabolic pathways. For example, carbohydrates are broken down from complex molecules into simple sugars by the action of saliva and stomach and are absorbed through the upper part of the small bowel into the blood where they are first transported to the liver. This process stimulates the release of insulin from the pancreas which is a critical component of carbohydrate metabolism. Sugars are taken up by the liver and reassembled into a larger molecule called glycogen and stored or delivered to other body organs where they are taken up and stored or burnt to release energy for muscles or brain activity etc. In contrast, fat is digested by the action of bile acids and enzymes released into the upper small bowel from the pancreas, and broken down into fatty acids, formed into structureswhich are soluble in water, and are absorbed into the lymphatic system from where they are also delivered to the liver. From here they are further packaged and released into the blood where they are delivered to adipose (fat) tissue where they are taken up and stored or to muscle where they are taken up and burnt. The liver is the most important body organ in controlling the metabolism of ingested food and regulating the flow of energy within the body.
Efficient burning of these building blocks of food to release energy requires oxygen (aerobic metabolism), though less efficient release of energy can be achieved without oxygen (anaerobic metabolism) which occurs when inadequate oxygen is available. The relative contribution of each is partly determined by how well the body can deliver oxygen from the lungs to the body’s organs – this is related to cardiovascular fitness.
Another aspect of metabolism is that different organs in the body have different metabolic requirements. For example, the brain and parts of the kidney have a high metabolic demand and only use glucose for fuel under aerobic metabolism. The body goes to extraordinary lengths to maintain the delivery of glucose to these organs. In contrast, muscle is more flexible and will burn either fatty acids or glucose, depending on which is more readily available at any given time.
All of these aspects of “metabolism” can make it difficult to be clear about any potential metabolic differences between those who are lean and those who are overweight. But here are some “Metabolic Myths”.
Myth: “I am overweight because I have a slow metabolism.” Like it or not, our body weight is determined by the laws of thermodynamics. Energy intake must equal energy expenditure, else weight will increase or decrease accordingly. This principle can be proven through rather drastic means by locking a person in a tightly sealed room (known as a calorimeter) and precisely measuring everything that goes into and out of that room. By manipulating energy intake, it may be demonstrated that there is a highly predictable response in body weight. In contrast to this misbelief, those who are overweight have a higher metabolic rate than those who are lean. This is because much of the metabolic demand of the body is from muscle, and overweight people have more muscle to carry around the weight.
Metabolic rate can be broken down into three components (figure…). Basal metabolic rate refers to the amount of energy our bodies require simply to tick over in a steady state and makes up 60-80% of energy expenditure in sedentary adults. Diet induced thermogenesis (6-10%) is the amount of energy required to process ingested food to release its energy. Finally, a variable component which is due to physical activity and is the only controllable factor varying from 10-15% in sedentary individuals to 50% in active individuals or higher in athletes. As is shown in figure 1, there may be very different energy requirements between lean sedentary people and lean athletic people. Similarly, the energy requirement of a lean active person may be the same or greater than a sedentary person with obesity, due to differences in basal metabolic rate and energy demands of activity. Therefore, yes it is true that different individuals may have different metabolic requirements, but in the vast majority it is not true that people who are overweight have a slow metabolism.
Myth: “I am fat and hardly eat anything but my friend is skinny and can eat everything in sight.” Again this is a myth which can be dispelled by putting both people into a calorimeter, and is partly due to the issues described above such as physical activity. However, it is also an issue of perception. If the actual energy intake of these two people is measured over several days, the person with obesity will be consuming greater energy, but this may be disguised by issues of energy density of the food, frequency of consumption (snacking) or unrecognised consumption (self denial).
Myth: “If you eat too much carbohydrate this will get converted to fat”. It is true that one type of fuel can be converted to another in the body; ie carbohydrate can be converted to fat - a process known as de-novo lipogenesis- but this is an extremely inefficient and energy demanding pathway. Therefore, almost without exception under normal conditions, fat is either burnt for energy or stored as fat, and carbohydrate is burnt or stored as glycogen. In a situation where there are inadequate levels of glucose or stored glycogen as may occur after a period of starvation, then protein or fat can be converted to glucose in the liver – a process known as gluconeogenesis. In a state of excess energy intake as contributes to weight gain, where there is plentiful carbohydrate, little ingested fat is burnt for energy as the body switches to carbohydrate metabolism in preference and the ingested fat is then stored for a later famine!
In addition to that described above, there are other factors that have an influence on metabolic rate. For example stages of life. During the first couple of years of life and during puberty when there is accelerated growth, and during pregnancy, there is a dramatic increase in metabolic demand. In contrast, in the elderly where there is a reduction in muscle mass, energy requirements decline. External environment has an influence. In extremely cold environments, such as the antarctic, energy requirements dramatically increase, and can be double that for the same individual in a temperate climate. This is simply to maintain the core temperature of the body by generating heat.
Our genetic make up is often also blamed for a person being overweight, and for this there is a little more evidence. There are many ways that genetic differences may explain the differing tendency of two individuals to becoming overweight. However there is relatively little evidence to date for this in terms of metabolic pathways and common obesity. There is more evidence for genetic factors affecting appetite regulation and energy intake.
There are a small number of disease states which can affect metabolic rate too, such as an underactive (very seldom the cause of weight gain) or overactive thyroid, cancers, excessive cortisol levels (natural steroid) known as Cushings syndrome, excessive adrenaline production, insulin or sex hormones may all influence metabolic demand. However, despite these influences, the laws of thermodynamics still apply – though the degree of cognitive control over energy intake required to maintain a stable weight may be greater.
Are there ways I can boost my metabolism?
We are constantly bombarded by advertisements for foods or supplements claiming to have the critical ingredient in promoting weight loss or improving athletic performance, often by enhancing metabolism. As for most things in life, if it sounds too good to be true, it probably is! This highlights the issue of anecdotal support, placebo effect, and individual responses on which recommendations and advertising is all too often based, compared with that of well conducted scientific studies which are seldom performed in the area of specific foods or nutritional supplements. Such claims are made for components like L-carnitine, an amino acid, which if truly deficient (very rare) can impair fat metabolism and is reversed by replacement. This leads to the plausible notion that supplementation of carnitine will enhance fat metabolism and promote fat loss. However, there is no scientific evidence to support a benefit of carnitine supplementation in people who are not deficient. One example of scientific evidence supporting popular theory is for creatine, an amino acid derivative, which enhances performance in high intensity activities such as sprinting. Critical analysis of the wide range of products is beyond the scope of this article.
Even small amounts of weight loss are associated with reductions in lean body mass, which reduces basal metabolic rate. This is why when people reduce their energy intake, they lose weight initially and then reach a plateau as their intake matches their lean mass and new basal metabolic rate. Increasing physical activity can maintain lean mass and help to offset this. There is also some evidence that maintaining protein intakes can reduce the loss of lean mass.
So my best advice if you want to be able to eat anything you want to and not put on weight, is to have skinny parents, be a pregnant teenager on an arctic expedition travelling on foot with inadequate thermal clothing. Otherwise I’m afraid you need to join the rest of us and be constantly vigilant about your energy intake and energy expenditure. Yes, there are variations between individuals in metabolic activity and energy requirements, and yes certain disease processes influence metabolic demands, but these still follow the laws of thermodynamics. Like it or not “Energy In must Equal Energy Out” over a prolonged period else if intake is in excess weight gain will occur. As certain as death and taxes!
Dr Jeremy Krebs
The material in this article is based on a wide range of references, but covered and supported in the following texts.
Essentials of human nutrition. 2002 Second edition. Mann and Truswell. Oxford University Press. ISBN 0 19 850861 1
Clinical Sports Nutrition. 2000 Second edition. Burke and Deakin. McGraw Hill. ISBN 0 074 70828 7
Metabolic Regulation. A Human Perspective. 1996. Frayn. Portland Press. ISBN 1 85578 048 8