How does your metabolism really work?

What it is, how it works and its relationship to food and exercise

If I had a penny for the number of times people tell me that they put on weight because their metabolism is too slow or that their friend has a fast metabolism because they never seem to put on weight no matter what they eat, I would be a very rich man. But I cant help thinking, how do they really know they have a slow metabolism and do they even know what metabolism is?

Its all about energy

Metabolism is a bit of a catch-phrase these days and it seems to have lost all its original meaning. Metabolism is derived from the Greek language for change.1 The scientific definition for metabolism is it …involves all the chemical reactions of biological molecules within the body, including both synthesis (anabolism) and breakdown (catabolism).2 So, in your bodys terms, metabolism is the change created by chemical reactions such as the times when your body chooses to store fats (anabolism) or burn fats for fuel (catabolism).

The history of metabolism

Chemical reactions occur inside any living creature and these reactions either use or create energy. That energy is measured as heat, either in joules or calories.3 The joule is the internationally accepted scientific unit of measure and the calorie is now an outdated empirical unit. Metabolism was founded in classic experiments by Atwater and Rosa in 1899.

These two pioneers discovered there was an almost perfect relationship between the heat given off by a person (calories) and the oxygen they used. They used information collated more than a century before them by the scientist, Antoine Laurent Lavoisier, in 1777. This data forms the foundation of the discipline calorimetry, which measures the energy used by metabolic chemical reactions and demonstrates that the history of metabolic research is more than two centuries old.

How to measure metabolism

What this discovery means to you is that the only method for measuring your metabolism is calorimetry. From the turn of the 19th century onwards, scientists have commonly used the indirect calorimetry method, which analyses gases from the breath instead of heat because of its portability, convenience and also because it measures carbon dioxide. Carbon dioxide allows an expert to determine the type of calorie that is being used as fuel by the body. By type of calorie, I mean fats or sugars. Proteins are an insignificant energy resource.

When fats are burnt, they give off 25 per cent less carbon dioxide than when carbohydrate is used. By using this information, a trained specialist can tell you how efficient you are at burning fats, which is, of course, very important in weight loss. Alternatively, the expert can also use the same evidence to determine how well sugars are burnt, which is very important information for diabetics and athletes.

When assessing metabolism its very important to be completely at rest; normally, you would have to fast for a minimum of five hours with water only.5 You would also have to avoid exercise, coffee, tea and cigarettes during the fast. An assessment room that is quiet and low in light with a comfortable bed is especially good at helping you reach a rested state.

Where is metabolism?

Metabolism occurs inside all living cells, so the digestive system the mouth, stomach and intestines are not your metabolism. They merely break down food into the most basic constituents, which are sugars (carbohydrates), amino acids (proteins) and fatty acids (fat). These fuels are evenly distributed throughout your body so metabolism can take place in every cell. Sugars are primarily used by the brain cells and fats are predominantly used by almost every other cell, including the muscles. Its only when you perform high-intensity exercise that you will need sugar to help muscles.

What influences metabolism?

Some people think the thyroid gland, found in the neck, is the centre of metabolism. Its true that up to 70 per cent of the calories we burn can be attributed to the stimulation of cells by the thyroid hormones. However, the thyroid is not the only hormonal influence on metabolism.

Testosterone, the masculine sex hormone, stimulates the working tissue of muscle and bone and, when these cells are energised, metabolism can increase by up to 20 per cent. The same can be said of the hormone progesterone, the partner to oestrogen, which has a positive effect on the thyroid gland. When under stress, adrenalin can also stimulate metabolic rate.

Exercise and food can also have a positive effect on metabolism. High-intensity exercise increases metabolic rate for a consistently longer period after exercise than low-intensity activity. All food increases metabolic rate, no matter what time of day (or night) it is. Some foods stimulate more than others, such as proteins, which can provide up to 25 per cent of an increase in metabolic rate. Overeating can also increase metabolic rate when carried out over a long period of time. This means that if you overeat, and gain weight, you will usually have a fast metabolism, not slow.

This fast metabolism demands more energy and so you eat more to feed the metabolism. You end up in a perpetual cycle of trying to feed (but necessarily overfeed) the demand for calories by your metabolism. The opposite is also true, that a very slim person will usually have a slow metabolism. Although most people say they eat like a horse, they are either expending a lot of energy through physical activity or they dont eat as much as they say they do. Very few people in the world have a fast metabolism, including athletes.

Metabolic food

While metabolic rate can be raised by eating food, carbohydrates and fats can also change the type of calorie you burn. Research studies show that a very high carbohydrate diet will cause your metabolic efficiency to adapt and preferentially burn sugar but not fat. This is because the stimulation of insulin by carbohydrates can prevent fats from being metabolised. Raised blood sugar concentrations can then be reduced by the cells that are burning sugar. In the short term, a high-carbohydrate diet will not gain weight, but over many years it will contribute to not only weight gain but also insulin resistance. Insulin resistance maintains fat stores, which make it harder to lose weight as we get older.

The opposite is also true: a high-fat diet will cause the efficiency of metabolism to drop and burn more fat than sugars. Weight gain through a high-fat diet may not only create a nutritionally unsound diet but possibly prevent sugar mechanisms from working well. Thats why a diet of moderation is recommended so we can burn both fuels. This moderate diet includes all three calorie types and is recommended for everyone who does frequent but moderate exercise and would like to maintain their weight.

There are no known common foods that directly stimulate the metabolism. In fact, its not recommended that you seek out any herbal, medication, supplement or food that purports to stimulate metabolism so you can lose weight. Most of them are harmful, dont work or make you reliant on that product to obtain weight loss. As an example, L-Carnitine is supposed to assist in the transfer of fats into cells so they can be metabolised. In theory, this is true; there is an L-Carnitine process within your cells that facilitates the transfer of fatty acids. However, research shows that the supplement does not reach the bloodstream, is digested or filtered out by the liver or, once in the bloodstream, is prevented from entering your cells.

When you do get weight loss from herbs, medications or supplements, have you thought about why that weight loss is occurring? Have you really hit on the best method of weight loss? Have you learnt anything about your lifestyle or your body that originally caused the weight gain?

How to get weight loss

Unfortunately, the quick weight loss fixes are often not sustainable. There is no magic bullet that will quickly solve your problems. Sustainable weight loss needs to be a slow, natural process and you should first find out the cause or causes of your weight gain. You do that by assessing your metabolism. Youll need to find a clinic that uses calorimetry equipment, which will be able to identify whether you have a dysfunctional metabolism. The metabolic results and a health history will provide you with the best indication of the influencing factor(s) of your weight gain. Once you know these influence(s), then you can work out a plan to combat it. A metabolic change usually occurs over two months, but for extreme cases, such as hormonal imbalances, the metabolic change can take years.

If you have been struggling with your weight, its very important to keep a vigilant eye on your metabolism so you can manage your weight. Theres no specific symptom of a dysfunctional metabolism since there are many causes and they overlap with other issues such as those seen in the list of causes of weight gain below. If you are unable to attend a metabolic clinic, the next best option is to go through each of the issues below one by one. Try to find patterns in your lifestyle to identify an influencing factor or multiple factors. Also note down your chronological history of weight gain and loss alongside events in your life such as illness, menarche (beginning of menstruation) and alcohol consumption. There could be clues to your weight gain conundrum in this list.


Metabolism information can also be used to obtain natural muscular weight gain without gaining excess body fat. Most bodybuilders simply eat more food and the accumulation of unwanted body fat is naturally accepted as par for the course. Piling on fat weight and stripping it off time after time may eventually cause liver damage by placing the body through extremes. The simplest way is to use science, and your body, in your favour. You need to find out what kind of metabolism you have. Is it within the average range, fast or slow? Do you burn more fats or sugars? Once that analysis is done, you can formulate the correct dietary and exercise plan.

First, understand that fat cannot become muscle and muscle cannot become fat. There are major chemical differences between the two types of tissue it just cant happen. Second, muscle does not develop without resistance. Even then you have to perform a specific style of weight-training program to obtain muscular growth. Essentially, you have to have plenty of variety of exercise, a large volume of work and large muscle groups using slow movements. Third, your diet will require some manipulation. You will, of course, need to eat more, but usually its no more than 500 calories extra. Again, you will only find this out with an assessment of metabolism. Timing of diet is important. Certain foods will need to be eaten at different times of day to stimulate growth. Foods should be as natural as possible, avoiding processed products, and try taking some vitamin and mineral supplements as you will go through some very palpable physical stress.

Finally, and probably most important, is rest and recovery. You must sleep at least eight hours per day. Growth hormone is primary for muscular growth, so stimulate it by sleeping! Professional bodybuilders usually sleep twice a day after their workouts and food to be able to make the most efficient use of their own natural hormone processes. This kind of natural weight gain does not require supplements unless you simply cannot physically eat the amount of food that may be required for weight gain. This regime has been personally proven to stimulate at least 0.5kg of body mass per week with no discernable or measurable body fat gain. This is done with no magic powders, herbs or pills. Some willpower is essential to turn up to the gym almost every day, although the results provide great motivation.

The WellBeing Team

The WellBeing Team

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